Having laid before you what observations I thought might sufficiently explain all the principal features of the Anatomy of insects both external and internal, you will next expect to be informed whether, like the higher animals, they are subject to have the admirable order observable in their frame interrupted by Disease; and you will perhaps imagine, from the multiplicity of their organs and vessels, that they must be peculiarly exposed to derangements of the vital and other functions. That they have their diseases is certain; but, except in the case of their appropriate parasitic assailants, which is a part of their economy, it does not appear that their maladies are more numerous and frequent than those of other animals. The same Almighty Power which endowed them with so complex a structure, generally upholds them in health during their destined career, until they have fulfilled the purpose of their creation, when they die and return again to their dust[918].

But perhaps I may seem to you as making too great a parade about these little insignificant creatures if I assign a separate letter to the consideration of their diseases: but when you recollect that Aristotle has a chapter on this subject[919], and that the learned Willdenow has devoted a distinct portion of his excellent introductory work on Botany to the diseases of Plants[920],—you will perhaps be of a different mind: indeed, some facts I shall have to communicate are so remarkable and interesting, that I am sure, when you have read this letter, you will not think the subject one that deserves to be slighted.

Insect diseases may, I think, be divided into two great classes; those resulting, namely, from some accidental external injury or internal derangement, and those produced by parasitic assailants.

I. Under the first head we may begin with wounds, fractures, mutilations, and other extraneous causes of disease. To these—insects are peculiarly subject; and though they are not, like the Crustacea and Arachnida[921] and some other invertebrate animals, endowed with the power of reproducing a mutilated limb, yet their wounds appear to heal very rapidly, and at the time they are inflicted to produce little pain[922]. But if those important members, their antennÆ, are mutilated, insects seem to suffer a kind of derangement; the great organ of their communication with each other, and in various respects with the external world, being removed, all their instincts at once fail them. I formerly related how the amputation of these affects the queen-bee[923]. A similar result, as Huber tells us[924], follows, when the same experiment is repeated on the workers or drones: they immediately become unable to take any further part in the labours of the hive; they can no longer guide themselves except in the light; if they petition one of their fellow-citizens for honey, they are unable to direct their tongue to its mouth to receive it; they remain near the entrance of the hive, and when the light is intercepted they rush out of it to return no more.

Insects occasionally are subject to tumours or a preternatural enlargement of their parts and organs. The antennÆ of bees sometimes swell at their extremity so as to resemble the bud of a flower ready to open, becoming at the same time very yellow, as does the fore part of the head[925]. I once saw a specimen of a Hydrobius—agreeing with H. fuscipes in every other respect even to the most minute punctum—which had a large tumour on each side of the prothorax, evidently accidental, occasioned probably by the stoppage of the pores by which the superfluous moisture and air escape when it undergoes its last change. The converse of this I have observed to take place sometimes in the same part of Geotrupes foveatus, the ordinary lateral foveÆ becoming very considerably enlarged;—this was the case with the specimen from which Mr. Marsham made his description of that insect. The species is, however, very distinct in other respects, and may always be known by its small size. It happens now and then also, that these tumours represent blisters. I saw one once on one elytrum of a beetle and not on the other. Those of Serropalpus (as Mr. MacLeay, on the authority of M. Clairville, informs me) are particularly subject to this disease. But, of all the organs, the wings are most exposed to derangements of this kind. De Geer, in a specimen of Pieris CratÆgi just excluded from the chrysalis, observed that one of these was distended by a considerable quantity of extravasated green fluid—two or three large drops following an incision. This disease appeared to arise from the lower membrane not adhering to the upper; so that the nervures—which are rather longitudinal channels, being open below, than tubes—were not closed to confine the fluid to its proper course. The malady, which might be called a dropsy of the wing, carried off the insect the day after its exclusion[926]. Reaumur observed that the wings of some flies were affected by an air-dropsy, as he calls it, which appeared to arise from the air escaping from its natural channels, and thus separating, the two membranes that form the wing, and filling the cavity produced by their separation[927].

Sometimes also monstrosities are to be met with in these animals, or variations from a symmetrical structure in organs that are pairs. I have a beetle in which the terminal joint of one of the maxillary palpi is short, ovate, and acute; and that of the other, long, semiovate, and rather obtuse. A specimen of Blaps mortisaga in my cabinet, taken by Mr. Denny, besides the terminal mucro of the elytra, has a long diverging lateral one. Goeze had the larva of a Semblis brought to him in which one of the two fore-legs, though perfect in all its parts, was only half the length of the other[928]; which he regarded as a reproduction, but it seems rather a malformation. MÜller mentions a most extraordinary fact of one of the NoctuidÆ, which when disclosed from the pupa retained the head of the larva[929]. One of the most remarkable instances of this kind that have fallen under my own observation, may be seen in a specimen of Chrysomela hÆmoptera in the cabinet of our friend Curtis; in which one of the thighs produces a double tibia, but only one of these is furnished with a tarsus.

The diseases of insects which arise from some internal cause are not very numerous. The first that I shall mention is a kind of vertigo. "Ants have also their maladies," says M. P. Huber: "I have noticed one extremely singular; the individuals attacked by it lose their power of guiding themselves in a straight line, they can walk only by turning round in a circle of small diameter and always in the same direction. A virgin female shut up in one of my glasses was seized on a sudden with this distemper; she described a circle of an inch in diameter, and made about a thousand turns in an hour, or not quite seventeen in a minute. She continued constantly turning round for seven days, and when I visited her in the night I found her still in motion. I gave her honey—and I think that she ate some of it." He observed that some workers were attacked by a similar disease: one of these, however, had the power of walking from time to time in a straight line; when placed upon its head it continued its gyrations[930]. Similar motions of a little moth, mentioned on a former occasion[931], may perhaps have been produced by the same cause. Bees are also subject to vertigo, which has been attributed to their eating poisonous honey[932]—but may not this disease in all these cases arise from some derangement of the nervous system? One of the ants which was so affected had lost one of its antennÆ; but as this was not the case with the others, no great stress is to be laid upon the circumstance. Huber does not inform us whether those attacked by this disease recovered or not.

I have observed more than once, that the flesh-fly and some others of the same tribe are subject in particular seasons to a kind of convulsions. When thus attacked, they kick and struggle, and seem unable to fly. Sometimes they lie upon their backs without motion, but if a finger be placed near them their convulsive motions are renewed. When thrown into the air, instead of flying, they fall to the ground. Had this distemper occurred earlier or later in the year I should have attributed it to the benumbing effects of cold; but as my observations were made one year (1816) in May, and in another (1811) in the latter end of June, this could scarcely be the case. In the year last mentioned I observed that many flies died under its influence. In wet seasons this tribe is subject to another disease, which proves fatal to many of them, and indeed to other Diptera. A white crust appears to be formed upon the abdomen both above and below, of a granular appearance, much resembling fine moist sugar. On the back of that part this crust does not cover the margins of the segments, which gives it the appearance of white bands; so that deceived by it, I have often at first flattered myself that I had met with some new species. The under-side of the abdomen is wholly covered by it, divided in the middle into two longitudinal masses, the anal segment being bare. De Geer has noticed this or a similar disease, which, when flies are attacked by it, causes the abdomen to swell so as even to burst, and the segments become dislocated. Upon opening the abdomen it is found filled with a white unctuous substance, which often accumulates (as above described) on its external surface[933]. Dr. Host says that in this disease when the animal is dead, the wings, which were before incumbent, become extended, and its almost invisible pubescence grows into long hairs[934]. De Geer seems to think that these flies are thus affected in consequence of having eaten some poisonous food[935]; but I rather suspect, as I have observed it become prevalent chiefly in wet seasons, that it arises from a superabundance of the nutritive fluid, or of the fat, so that it seems to be a kind of plethora. I once observed a fly fixed to a pane of glass, round which was a semicircle of what appeared to be merely vapour, whose radius was nearly three-fourths of an inch. Taking it for an aqueous fluid that had transpired from the dead animal, I paid no further attention to it at that time: but observing from day to day that the moisture did not evaporate, after two or three months had elapsed, I had the curiosity to examine it more closely, and, upon scraping some of it off with a penknife, I found it was a white substance of a fatty nature. In this case, then, the fat must have exploded on all sides with considerable violence from half the body or the abdomen. Probably this was a more intense degree of plethora. When I examined this appearance the fly had fallen off, and I could not find it.

Mr. Sheppard once brought me a panicle of grass, the glumes of which were rough with hairs, or small bristles, to which several specimens of a fly related to Xylota pipiens adhered by their proboscis. At first I thought that having been entrapped by the bristles, and unable to extricate themselves, they had perished from want of food; but since when touched they readily dropped from the glumes, some other cause, perhaps disease, probably occasioned this singular suspension of themselves.

The maladies to which bees and silkworms are subject are more interesting to us than those of flies, on account of their utility as cultivated insects. One of the worst distempers which attacks the first of these animals is a kind of looseness or dysentery: this happens early in the year, when they are fed with too much honey without any portion of bee-bread[936], and sometimes destroys whole hives. Their excrements, instead of a yellowish red, then become black, and the odour they emit is insupportable; the bees no longer observe their usual neatness, inducing them to leave the hive when they void their excrements, but they defile it, their cells, and each other. Several remedies have been prescribed for this disease. To prevent it, a syrup made by an equal mixture of good wine and honey is recommended; and as a cure, to place in the hive combs containing cells filled with bee-bread[937]. But one of the worst maladies to which these useful animals are subject, is that called by Schirach Faux Couvain. It originates with the larvÆ; and is caused either by their being fed with unwholesome food, or when the queen, as sometimes happens, lays her eggs so that the head of the grub is not in a proper position for emerging from the cell when the period for its disclosure is arrived:—the consequence is, that in both cases it dies and becomes putrid, which sometimes produces a real pestilence in a hive. The remedy for this evil is to cut away the infected combs, and to make the bees undergo a fast of two days[938]. The hive should be cleaned and fumigated, by burning under it aromatic plants.

The cultivators of the silkworm in France have given names to several diseases to which that animal is subject. One is called La Rouge, and is supposed to be occasioned either by too great heat, or by too sudden a transition from cold to heat. It takes place when the caterpillar is first hatched; which lives perhaps, but in a very sickly state, till it should spin its cocoon and assume the pupa, when it expires. Another degree of the same disease is called Les Harpions or Passis. A second distemper of this animal is Des Vaches, Le Gras or La Saune: this is a mortal disease, supposed to be of a putrid nature, and produced by mephitic air; it shows itself after the second moult, but rarely after the subsequent ones. When a caterpillar is first attacked, changing the air may prove a remedy; but when the disease has made progress, it is best to burn or bury them, since if the poultry pick them up they might be poisoned by them. A third disease of silkworms is called Les Morts Blancs, or Tripes, which is also occasioned by impure air, when the leaves the animal feeds upon are heaped so as to produce fermentation. The caterpillars attacked by it die suddenly, and preserve after their death the semblance of life and health. Too great heat, whether artificial or natural, occasions La Touffe, a fourth, which, when the heat continues long, destroys all those that are arrived at their last stage of existence in their larva state. Black points scattered over different parts of the body, or livid and blackish spots in the vicinity of the spiracles, followed by a yellowish or reddish tint, are symptoms of a fifth malady, called La Muscardine. After this the animal soon dies, and becomes mouldy, but does not stink. This disease is not contagious, and is thought to be caused by a moist heat, attended by pernicious exhalations. La Luzette, Luisette, or ClairÈne, is another malady, which shows itself most commonly after the fourth moult. It seems to arise from some original defect in the egg. The caterpillars attacked by it may be known by their clear red and afterwards dirty white colour; their body becomes transparent, and the matter of silk exudes in drops from their spinnerets; consequently, though as voracious as the rest, they are never able to construct a cocoon, and should be destroyed. Les DragÉes is the name given to cocoons which include a larva that never becomes a pupa. The cause of this disorder has not been ascertained, and whole broods are sometimes subject to it, which, as in the last, seems to imply some defect in the eggs. But as the caterpillar spins its cocoon, and the silk is as good as usual, it is a malady of no great importance. Lastly, sometimes the mulberry leaves have a gummy rather acrid secretion, which purges the silkworms; their excrement is no longer solid; they become weak and languid; and if the secretion is abundant, their transpiration is impeded, and at the time of moulting they are become so feeble as to be unable to cast their skin[939].

In the case of many caterpillars of Lepidoptera that died, Bonnet found by dissection that the disease was remotely occasioned by a diarrhea, which taking place immediately before they became pupÆ, prevented the inner membrane of their intestines from being rejected, as it would have been if no extraordinary cause had prevented it, attached to the hard excrement. He found this membrane converted into a jelly occupying great part of the stomach, which he conjectured was the proximate cause of their death[940].

To conclude this head—spiders are reputed to be subject to the stone: I do not say Calculus in Vesica; but we are informed by Lesser that Dr. John Franck having shut up fourteen spiders in a glass with some valerian root, one of them voided an ash-coloured calculus with small black dots[941].

II. I now come to that class of diseases which appears to prevail almost universally amongst insects—I mean those resulting from the attack of parasitic enemies. Thus millions and millions annually perish before they have arrived at their perfect state. Diseases of this kind proceed either from vegetable or animal parasites. I shall begin with the first, which will not occupy us long.

i. As insects pass often no small portion of their life in a state of torpidity, in which they remain chiefly without motion, it will not seem wonderful, should any partial moisture accidentally accumulate upon them, that it affords a seed plot for certain minute fungi to come up and grow in. Persoon observes with regard to his genus Isaria, that one species grows upon the larvÆ of insects (I. truncata), and another upon pupÆ (I. crassa[942]):—as he does not say upon dead larvÆ and pupÆ, as upon a former occasion[943], perhaps in these cases these plants may constitute an insect disease; but I lay no stress upon it, and only mention the circumstance here as connected with the history of these animals. Mr. Dickson has described a SphÆria under the name of entomorhiza that grows upon dead larvÆ; it has a slender long stipes and spherical granulated head: on the pupa of a species of Cicada in my cabinet, another kind of SphÆria, with a twisted thickish stipes and oblong head, springs up in the space between the eyes. I observed something similar but longer, in the grub of some large beetle in M. Du Fresne's museum at Paris; and I have a memorandum of having noticed something of the kind on the rostrum of a Calandra. Bees and humble-bees have been sometimes thought to have some species of mucor or other Fungilli occasionally growing upon them; but Mr. Brown is of opinion that stamina which they have filched from flowers have been mistaken for these Fungilli, since he has detected those of OrchideÆ in some of this tribe, and upon a beetle shown to him by Mr. MacLeay, one which he knew to be the stamen of an Aristolochia. I once observed a bunch of what I mistook for a singular mucor that adorned the vertex of a humble-bee, between the antennÆ, which doubtless were of the same description; and I even saw one upon its wing. Upon a former occasion I mentioned a parallel circumstance with respect to a species of Xylocopa[944].

ii. The animal parasites that infest insects are either themselves insects; or worms.

1. Their insect infesters, as far as we know at present, are confined to the Orders Strepsiptera, Hymenoptera, Diptera, and Aptera: they attack them sometimes in their egg state, most frequently when they are larvÆ, occasionally when pupÆ, and very rarely in their perfect state. Upon many of these I have formerly enlarged[945], and I shall now add such further circumstances as I then omitted. The Strepsiptera Order, as at present known, consists only of two genera, Stylops and Xenos; the first being appropriated to the imago of Andrena, a kind of bee, and the latter to that of the wasps. Their eggs appear to be deposited in the abdomen of these insects in which they feed, till having attained their full growth they perforate the membrane that connects its segments; and at the proper time their pupa-case bursts, they emerge, and take their flight. Sometimes four or five infest a single bee. Whether the latter dies upon their quitting it I have not been able to ascertain, but from their flying, when the little parasite is very near leaving them, with their usual activity, it should seem that this disease is not mortal; but it probably prevents their breeding: I do not recollect observing the exuviÆ of one in a male bee[946].

The great body of insect parasites, however, belong to the Hymenoptera Order, and chiefly to the Linnean genus Ichneumon. The insects of this order have been denominated Principes, because of the wonderful instincts of ants, wasps, bees, and other gregarious tribes that belong to it; and they merit a name of honour not less for the benefits that they confer upon mankind, by keeping within their proper limits the various insect-destroyers of the produce of the globe. It deserves notice that when these latter increase to a degree to occasion alarm, their parasites are observed to increase in a much greater, so as to prevent the great majority of them from breeding[947]. Though these benefactors of the human race constitute numerous genera, at present not well ascertained, I shall speak of most of them under the common name of Ichneumon.

The appearance of these little four-winged flies puzzled much the earlier naturalists:—that a caterpillar usually turning to a moth or butterfly should give birth to myriads of flies, was one of those deep mysteries of nature which they knew not how to fathom[948]: even the penetrating genius of our great Ray, though he ultimately ascertained the real fact[949], was at one time here quite at fault; for he seems at first to have thought, when from any defect or weakness nature could not bring a caterpillar to a butterfly, in order that her aim might not be entirely defeated, that she stopped short, and formed them into more imperfect animals[950].

Before I detail more particularly the proceedings of Ichneumons, I shall make a few general remarks upon them. The structure of the instrument by which they are enabled to deposit their eggs in their appropriate station has been before sufficiently described[951]; it is long or short according to the situation and circumstances of the larva which receives them: if this lives in the open air, and the access to it is easy, it is usually short and retracted within the body; but if it lies concealed in deep holes or cavities, or shuns all approach, it is often very long. Thus in Pimpla Manifestator, which commits its eggs to the grub of a wild bee inhabiting the bottom of deep holes bored in posts and rails, the ovipositor is nearly an inch and half in length, and in some extra-European species three inches. How the egg is propelled so as to pass in safety from the oviduct, along this extended and very slender instrument to the grub for which it is destined, has not been certainly ascertained: but from an observation of Reaumur's[952] it should seem that it is aided in its passage by some fluid ejected at the same time with it, or is so lubricated as to slide easily without being displaced. The flies we are speaking of, by some authors are called MuscÆ vibrantes, because when searching for the destined nidus of their eggs their antennÆ vibrate incessantly, and it is by the use of these wonderful organs that they discover it wherever it lurks. Bergman observed that Foenus Jaculator searches for the latent grub of certain bees and other Hymenoptera with its antennÆ[953]: and from Mr. Marsham we learn that Pimpla Manifestator, before it inserts its ovipositor in the nest of the grub of Chelostoma maxillosa, explores it first with one antenna and then with the other, plunging them all the while intensely quivering up to the very root[954]. With respect to their size, Ichneumons vary greatly; some being so extremely minute as to be invisible to the naked eye, unless moving upon glass; while others, as to their length, emulate the giants amongst insects. The former, unless appropriated to the eggs themselves, usually commit many eggs to a single larva, while the latter are directed by their instinct to introduce into them only one. Some of the former description are endowed with the faculty of leaping[955]. The food of Ichneumons, and indeed of other internal parasites, is chiefly the epiploon or fat of the larva, but they never touch any vital organ; so that it continues to feed, and probably more voraciously, grow, cast its skin, and often it changes to a chrysalis, although at the same time inhabited by an army of these little devourers.

Ichneumons, as far as has been at present ascertained, are parasitic upon other insects chiefly in their three first states, a solitary instance only having been observed of their inhabiting an imago; but from their first exclusion as eggs from the ovary till their assumption of that state they give them no rest. I shall therefore first treat of those that infest the eggs; next those appropriated to larvÆ; and lastly those that devour pupÆ.

Vallisnieri appears to have been the first naturalist who discovered that Ichneumons were appropriated to the eggs of other insects. He observed one proceed from those of the emperor-moth (Saturnia Spini): finding two holes in each egg, one larger than the other, he conjectured that one was made when it entered, and the other when it emerged. In this case the egg of the Ichneumon must be fixed on the outside of the egg it was to feed upon; though some appear to pierce it with their ovipositor, and consequently introduce their egg within: for he says afterwards; "I have seen with my own eyes a certain kind of wild flies deposit their eggs upon other eggs, and bore and pierce others with an aculeus—by which they have introduced the egg[956]." Count Zinanni, a correspondent of Reaumur's, saw an Ichneumon pierce the eggs with her ovipositor repeatedly; which in about fifteen days were filled with the pupa, and in six more produced the imago[957]. I. Ovulorum L. is the only known species of egg-devourers; but most likely there are many, varying in size, according to the size of the egg they inhabit. Probably I. Atomus L., and I. Punctum Shaw, are of this description[958]. It is wonderful what a number these little flies destroy:—out of a mass of more than sixty eggs which was brought to De Geer, not one had escaped the Ichneumon[959]. But the most extraordinary thing is, that even these little creatures we are told are destroyed by another still more minute[960].

Though the animals we are speaking of usually destroy only a single egg, yet some appear not so to confine themselves. Geoffrey informs us that the larva of one of the Ichneumons whose females are without wings (Cryptus) devours the eggs of the nests of spiders, and from its size—it is nearly a quarter of an inch long—it must require several of them to bring it to maturity[961]. One of those also which destroys the gnat infesting the wheat (I. inserens) appears to devour them in their egg state, and could not be brought to perfection by the food that a single one would furnish[962].

The Ichneumons that are parasitic upon larvÆ are the most numerous of all. Some of them are deposited by the parent fly on the outside of their prey, and others introduced into its interior. Ophion luteum is one of the former tribe; it plants its eggs in the skin of the caterpillar of the puss-moth (Cerura Vinula). Each egg is furnished with a footstalk terminating in a bulb[963], which is so deeply and firmly fixed that it is impossible to extract it without detaching a portion of the animal with it, and even when the caterpillar changes its skin it is not displaced. After it is hatched, the grub, while feeding, keeps its posterior extremity in the egg-shell, to which it adheres so pertinaciously, that it is scarcely possible to disengage it without crushing it. It fixes itself by its mandibles to the skin of the caterpillar, and keeps constantly sucking the contents of its body till it dies: sometimes nine or ten of these larvÆ inhabit a single caterpillar[964]. Reaumur has given an account of other external Ichneumons. Upon one caterpillar that he examined, they were so numerous as to render the poor animal quite a spectacle, and they underwent their metamorphosis attached to it[965]. One species of this description avenges the cause of insects upon their most pitiless foes, the all-devouring spider—for in the midst of her toils and lines of circumvallation it makes her its prey. De Geer, meeting one day with a young spider of a common kind, observed with surprise, engaged in sucking it, a small white grub, which was firmly attached to the abdomen near the trunk. Putting it by in a glass, after some days he examined it again; when he observed that it had spun the outline of a vertical web, had stretched threads from the top to the bottom of the glass and from one side to the other, and had also spun the radii that meet in the centre, and this was all;—but what was remarkable, the larva that had fed upon it was suspended in the centre of this web, where it was engaged in spinning its own cocoon, while the spider, exhausted by this last effort, had fallen dead to the bottom of the glass. It cannot be asserted positively that this suspension of the larva of the Ichneumon in the centre of the web always takes place; but if it does, as seems most probable, it shows that this little parasite is endowed with an instinct which causes it so to act upon the spider as may induce it to spin a web so nicely timed as to be sufficiently complete at the period of its death and of the change of the Ichneumon, for the latter to cast it down and assume its station[966].

But the great bulk of the parasitic Hymenopterous devourers of larvÆ have their assigned station within the body. As Entomologists in breeding insects have paid their principal attention to Lepidoptera, it necessarily follows that their Ichneumon infesters must be most generally known; but doubtless the larvÆ of the other Orders are not wholly liberated from this scourge: they also require to be kept within due limits, and have their appropriate parasites. Some, however, in most of them have been detected: of which I shall now proceed to state to you the most interesting examples, beginning with the Coleoptera.

Alysia Manducator[967], remarkable for having mandibulÆ that do not close, and toothed at the end, usually attends masses of dung, both of man and cattle, probably for the purpose of depositing its eggs in some of the Coleopterous larvÆ that inhabit it. Mr. Stephens, one of the most accurate observers as well as one of the best Entomologists of the present day, informs me that he once captured three specimens of Timarcha tenebricosa, from each of which forty or fifty minute Ichneumons emerged. An insect also of this Order, that is a great benefactor to mankind, as a destroyer of the plant-lice,—I mean the lady-bird (Coccinella), in its larva state is itself subject to the attack, as we learn from De Geer, of one of these small parasites[968]. He detected them also in that of two species of weevils: and in the pupa of some large grub of a beetle inhabiting the wood of the elm, perhaps that of the stag-beetle, he found the pupa of one of those Ichneumons that have an exserted ovipositor[969]. Doubtless, did we know their history, we should find that numberless species have their internal assailants belonging to this tribe.

Orthopterous larvÆ seem not to have been yet announced as affording a pabulum to these animals: but the late Dr. Arnold, whose tact for observation with regard to the manners and economy of insects has rendered his loss irreparable, discovered that the remarkable parasitic genus Evania was appropriated to the all-devouring Blatta. Whether it attacked it in its egg or larva state I have not been informed. This little benefactor is here extremely rare, at least in the country; perhaps in towns, where the cock-roach abounds, it may be more common.

The observations of naturalists have chiefly been confined to the Hemipterous genus Aphis; but these early attracted their notice. Leeuwenhoek has given a particular and entertaining account of the proceedings of I. Aphidum. As soon as the little flies approached their prey, they bent their abdomen, which is rather long, between their legs, so that the anus projected beyond the head; then with their ovipositor they pierced the body of the Aphis, at the same time carefully avoiding all contact with it in every other part: whenever they succeeded in their attempt, a tremulous motion of the abdomen succeeded. Only a single egg is committed to one Aphis: when hatched, the latter becomes very smooth and appears swelled; it is, however, full of life, and moves when touched. Those that are thus pricked separate themselves from their sound companions, and take their station on the underside of a leaf. After some days the inclosed grub pierces the belly of the Aphis, and attaches the margin of the orifice to the leaf by silken threads; upon this it dies, becomes white, and resembles a brilliant bead or pearl[970]. De Geer observed also an Ichneumon on the Coccus of the elm, I. Coccorum[971].

Amongst the Neuropterous tribes likewise, probably the IchneumonidÆ commit their usual ravages; but their exploits, as far as I recollect, have met with no historian. I have a small species related to Chelonus, which a memorandum made when I took it tells me was obtained from Æshna viatica; yet I do not remember ever tracing that species to its final change, so that I must have taken this Ichneumon from the perfect insect. It suffices, however, to prove that this tribe is also exposed to the attack of these parasites. Where larvÆ and pupÆ are aquatic, it seems probable, if any attack is made upon them, that it must take place after they have quitted the water.

In the Hymenoptera Order itself, almost every genus has been ascertained to have its Ichneumon parasites. Not even the fortified habitations of the gall-flies (Cynips) can escape them, almost every species becoming their prey; a circumstance which puzzled not a little some of the older naturalists, when they at one time saw a fly not remarkable for its colours or brilliancy emerge from the curious moss-like Bedeguar of the wild rose, and at another were struck by the appearance of one of those splendid minims of nature which almost dazzle the sight of the beholder[972]. Immunity, however, from this pest seems to have been granted to the gregarious Hymenoptera; at least none has yet been discovered to attack the ant, the wasp, the humble-bee, or the hive-bee; in which last, had there been one appropriated to it, it could never have escaped the notice of the Reaumurs and the Hubers. The solitary bees, however, as we have seen above[973], do not escape; and Epipona spinipes, a solitary wasp which feeds its own young with a number of green caterpillars[974], is itself, when a larva, though concealed in a deep burrow, the prey of the grub of an Ichneumon, which by means of a long ovipositor introduces its egg into its body[975]. Even these parasites, whose universal office it is in their first state to prey upon insects, are themselves subject to the same malady. Ichneumonidan devourers are kept in check by other Ichneumonidan devourers. These in some cases are so numerous as to destroy the tithe of the kinds they attack[976]. Thus an ever-watchful Providence prevents these parasites from becoming so numerous as to annihilate in any place the species necessary for the maintenance of the general economy and proportion of animal and vegetable productions. Amongst the assailants of the Hymenoptera, none seem to have a more laborious task assigned them than those that pierce the various galls in which the larvÆ of the Cynips tribe are inclosed. To look at an oak-apple, we should think it a work of difficulty, requiring much sagacity and address, for one of our little flies to discover the several chambers lurking in its womb, and to direct their ovipositor to each of them. Its Creator, however, has enabled it instinctively to discover this, and furnished it with an appropriate elongated instrument, which will open a way to the deep and hidden cells in which the grubs reside, penetrate their bodies, and to each commit an egg. When it prepares to perforate the gall, the Ichneumon begins by depressing this organ, that it may extricate it from its sheath; it next elevates its body as high as possible, and bending the instrument till it becomes perpendicular to the body and to the gall, so as to touch the latter with its point, it then gradually plunges it in, till it is quite buried[977]. A very remarkable Hymenopterous parasite (Leucospis), which when unemployed turns its ovipositor over the back of its abdomen, so that its end points to its head, is said to deposit its eggs in the nest of the mason-bee, most probably in the larva: but the curious observations that are stated to have been made by M. AmÉdÉe Lepelletier upon its history have not yet been given to the public[978].

Dipterous insects, likewise, do not escape from these pests of their Class: but few observations, however, have been recorded as to the species assailed by them. We learn from De Geer, that a gnat (Cecidomyia Juniperi), which forms galls upon the juniper is devoured by an external Ichneumon[979]; that which injures the wheat in the ear, whose ravages I formerly mentioned to you[980], affords food to three of these parasites,—one I lately mentioned as probably devouring its eggs; another pierces the glumes of the floret, where its destined prey is concealed; and the third enters it. I once placed a number of the larvÆ of the gnat upon a sheet of paper, at no great distance from each other, and then set down one of these last Ichneumons in the midst of them. She began immediately to pace about, vibrating her antennÆ very briskly: a larva was soon discovered, upon which she fixed herself, the motion of her antennÆ increasing intensely; then bending her abdomen obliquely under her breast, she inserted her ovipositor, and while the egg was depositing these organs became perfectly motionless. The larva when pricked gave a violent wriggle. This operation was repeated with all that had not already received an egg, for only one is committed to each larva. I have often seen it mount one that was already pricked, but it soon discovered its mistake, and quitted it untouched[981]. The Hessian Fly also (Cecidomyia Destructor) related to the preceding, whose alarming ravages I formerly described to you[982], has a peculiar parasite attached to it, which keeps it in check. The only other Dipterous insects that I have seen mentioned as affording pabulum to an Ichneumon, are—one of the aphidivorous flies mentioned by De Geer, who does not note the species, to the larva of which the Ichneumon commits only a single egg, producing a grub that intirely devours its interior[983];—and two described by Scopoli, one, the larva of a fly frequenting hemp; and the other, which feeds on a Boletus, that of a gnat[984].

The Lepidoptera, however, is the Order over the larvÆ of which the Ichneumons reign with undisputed sway; attacking all indiscriminately, from the minute one that forms its labyrinth within the thickness of a leaf, to the giant caterpillar of the hawk-moth. The most useful of all, however, the silkworm, appears at least with us, exempted from this scourge. De Geer, out of fifteen larvÆ that were mining between the two cuticles of a rose-leaf, belonging to the first tribe here alluded to, found that fourteen were destroyed by one of these parasites, only one coming forth to display itself in all its brilliancy and miniature magnificence[985]. One of the most useful to us is that which destroys the clothes-moth, which the same writer also traced[986]. Another, equally serviceable, takes up its abode in the caterpillar that ravages our cabbages and brocoli (Pontia BrassicÆ) which perish by hundreds from its attacks. As this falls frequently under our notice, it will not be uninteresting to give a fuller account of it. Reaumur has traced and related its whole history. One of these little flies that he observed, was so intent upon the business in which she was engaged, that she suffered him to watch her motions under a lens, without being discomposed. She pursued nearly the same plan of proceeding with that of the Ichneumon of the wheat-gnat just described; except that she repeated her operations frequently on the same caterpillar in different parts, alternately plunging in and extracting her ovipositor. She seemed to prefer the spot where the segments of the body are united, particularly where the eighth meets the ninth, and the ninth the tenth. When the fly had completed its work and quitted the caterpillar, Reaumur gave it food, and it did not seem less lively and vigorous than others of its kind; in less than a fortnight it assumed the pupa; and in four days the whole of its interior being devoured, it died: but its parasites, perhaps not finding a sufficient supply of nutriment in it, never came to perfection[987]. Sometimes, however, these little grubs arrive at maturity before the caterpillar has become a chrysalis, when they pierce the skin and begin to emerge. First appears a little white tubercle, which gradually elevates itself in a direction perpendicular to the body; while this is doing, a second appears in another place; and so on, till fifteen or sixteen are seen on each side, giving the caterpillar a very grotesque appearance. By the alternate contraction and relaxation of their bodies the grubs effect their complete liberation, which takes place with respect to the whole in less than half an hour. When entirely disengaged, they place themselves close to the sides of the caterpillar: even before this they begin spinning, and draw unequal threads in different directions, of which they form a cottony bed, which serves as the base of the separate cocoon of each individual, which they next construct of a beautiful silk thread of a lovely yellow, which, if it could be unwound and in sufficient quantity, would yield a silk unrivalled in lustre and fineness[988].

De Geer has recorded a very singular fact which deserves your notice. An Ichneumon, appropriated to one of the Tortrices, had deposited its eggs in two of their caterpillars; each produced a considerable number; but those that emerged from one were all females, and those from the other, males[989]. He observed a similar fact take place with Misocampus Puparum[990]. One might conjecture from this circumstance, that as in the queen-bee[991], so in these Ichneumons, the eggs producing the two sexes were arranged separately in the ovaries. Reaumur has related, that in one instance three or four males were produced to one female; and in another four or five females to one male[992].

But though the great majority of insects are subject to this Scolechiasis[993] in their larva state, yet sometimes they are not attacked by the Ichneumon till they have become pupÆ. Of this kind is one just mentioned (M. Puparum), which commits its eggs to the chrysalis of the butterfly of the nettle, Vanessa UrticÆ: the moment this caterpillar quits its skin to assume that state, while it is yet soft they pierce it and confide to it their eggs[994]. De Geer and others have supposed that this same Ichneumon attacks the Cocci and CoccinellÆ[995]; but this probably is an erroneous supposition. Cryptus Compunctor also attacks the pupÆ of butterflies[996].

If we consider the great purpose of Providence in giving being to this tribe of destroyers—the keeping of insects within their proper limits,—we may readily conceive that this purpose is more effectually answered by destroying them in their preparatory than in their ultimate state, since at that time the laying of their eggs and a future progeny could not so effectually be prevented;—this will account for there being few or no Ichneumons appropriated to them in their latter state.

The next tribe of insect parasites are to be found in the Diptera Order. The species that has been particularly noticed as such is the Tachina Larvarum; its larva is polyphagous, laying its eggs upon the bodies of caterpillars of different kinds. Sometimes a pair is placed on the first segment, sometimes on the head itself, and sometimes near the anus. These eggs are very hard, convex, of an oval figure, polished and shining like a mirror. They are fixed so firmly that if you attempt to remove them with a penknife the skin comes off with them. When hatched, they enter the body and feed on the interior, and, undergoing their metamorphosis within it, do not emerge till they enter their perfect state. The caterpillar thus attacked lives long enough to spin its cocoon, when it dies[997]. Sometimes, however, these animals quit their prey sooner. Reaumur saw a grub of one of the MuscidÆ come out of a caterpillar, and then become a pupa, which was so large that he wondered how it could have been contained in the animal it had quitted[998].

We have now done with those parasites that produce in insects the disease I have called Scolechiasis[999]: the rest, which belong to the Aptera Order, will afford us examples both of Phthiriasis and Acariasis[1000].

I begin with the first. Mr. Sheppard once brought me a specimen of a bird-louse (Nirmus) which he took upon a butterfly (Vanessa Io): and should such a capture be more than once repeated, it would afford a certain instance of the first of these diseases amongst insects;—but most probably the specimen in question had dropped from some bird upon the butterfly. The only remaining animal belonging to the apterous hexapods that is parasitic on insects, is by many supposed to be the larva of a giant-beetle (Meloe ProscarabÆus). I have before alluded to this animal[1001], and shall now resume the subject. Goedart, Frisch, and De Geer, observed that it deposited in the earth one or two considerable masses, containing an infinite number of very minute orange-coloured eggs adhering to each other, which in about a month were hatched, and produced a number of small hexapods distinguished by two pairs of anal setÆ and a proleg, by means of which they could move readily upon glass, as I have myself seen: these little animals precisely corresponded with one found by the latter author upon Eristalis intricarius; and when that fly was placed amongst them, they immediately attached themselves to it, so as to leave no doubt of their identity[1002]. A congenerous species had been detected upon wild bees, and described by LinnÉ under the name of Pediculus Apis. De Geer is so thoroughly to be depended upon for his veracity and accuracy of observation, that we cannot suppose there is any incorrectness in his statement. If the mass of eggs be, as he represents it, of the size of a hazel-nut, it must have been the product of a very large insect: in confirmation of this opinion it may be further observed, that the larva of the kindred genus Cantharis agrees with it in having anal setÆ, though it appears to differ in having only two conspicuous segments in the trunk[1003]. Those which infest wild bees make their first appearance upon acrid plants, which the Meloe likewise feeds upon; from whence with wonderful agility they leap upon the AndrenÆ, &c. that visit these flowers. Strong, however, as all these facts appear, still we cannot help exclaiming with the illustrious Swede last named, Who could ever have imagined that the larva of this great beetle would be found upon the body of flies,—and we may add, or bees? Who could ever imagine that it would feed like a bird-louse and resemble it so closely? that in the insertion of its palpi it should exhibit a character exclusively belonging to that tribe[1004]? Another circumstance seems to indicate that these hexapods at the time that they take their station in bees or flies are perfect insects—they do not vary in size, at least not materially. Where, we may also ask, if they are to become large beetles, where do they take their principal growth? It cannot be as parasites on the little bees or flies that they are usually found upon; they must soon desert them, and like their kindred blister-beetles, as is most probable, have recourse to vegetable food. What an anomaly in rerum natura! It is much to be wished that some skilful insect-anatomist would carefully dissect the Meloe; or perhaps by digging round the roots of the ranunculuses and other acrid plants the larva of that beetle might be discovered in a later stage of growth, and so this mystery be cleared up. I should observe here, that Scopoli has described three parasites as Pediculi; viz. P. rostratus, coccineus, and Cerambycinus; the first of which Fabricius has adopted under the name of P. GryllotalpÆ, but which are all evidently hexapod Acarina[1005].

Acariasis seems a disease almost as universal amongst insects as Scolechiasis; with this difference however, that Acari most commonly take their station upon them in their perfect state. You have doubtless often observed the common dung-beetles (Geotrupes) covered on the underside of their body with small mites (Gamasus Coleoptratorum) which look as if they were engaged in suction—they are often so numerous that no part is uncovered; they also attack other beetles[1006], and are sometimes found on humble-bees. They are easily disturbed, run with great swiftness, and may often be seen in hotbeds and fermenting dung prowling in search of the stercorarious beetles. But the most remarkable insect of this kind is the Uropoda vegetans: it derives its nutriment from the insects it assails not by its mouth, but by means of a long anal pedicle by which it is attached to them. De Geer found these in such numbers upon a species of Leptura, that its whole body was almost covered with them; they hung from the legs and antennÆ in bunches, and gave the animal a most hideous and disgusting appearance. Under this load of vermin it could scarcely walk or move, and all its efforts to get rid of them were in vain: many were attached to its body and to each other by their anal pedicles, but others had cast them off and were walking about. When put into a glass with earth, they began to abandon their prey, so that in a few days it was quite freed from its plagues. He found that these parasites lived long in alcohol[1007].

If you inquire—How are these mites originally fixed by their pedicles? it seems most probable, that as the Hemerobii, when they lay their eggs, know how to place them upon a kind of footstalk, so the parent Uropoda has the same power; and this pedicle appears to act the part of an umbilical chord, conveying nutriment to the foetus not from a placenta, but from the body of the insect to which it is attached; till having thus attained a certain maturity of growth and structure, it disengages itself and becomes locomotive. Many eggs of the aquatic Acarina (Hydrachna, &c.) are also furnished with a short pedicle by which they are fixed to Dytisci and other water insects. De Geer found some of this description on the underside of the water-scorpion, so thickly set as to leave no void space: they were oval, of a very bright red, and of different sizes on different individuals; whence it was evident that they grow when thus fixed: when hatched or released—for perhaps they may be regarded as foetuses in their amnios rather than eggs—they cease to be parasitical. Let us admire on this occasion, (piously observes this great Entomologist,) the different and infinitely varied means by which the Author of Nature has endowed animals, particularly insects, for their propagation and preservation: for it is a most extraordinary sight to see eggs grow, and pump as it were their nutriment from the body of another living animal[1008]. As these mites are fixed to the crust as well as its inosculations, they must have some means of forcing their nutriment through its pores.

Another insect, remarkable for its resemblance in some respects to the scorpion—called in this country the book-crab (Chelifer cancroides), from its being sometimes found in books—occasionally is parasitic upon flies, especially the common blue-bottle-fly (Musca vomitoria). They adhere to it very pertinaciously under the wings; and if you attempt to disturb them, they run backwards, forwards, or sideways, with equal facility.

Spiders also are infested by mites. Mr. Briggs once found a very small Theridion, to the thorax of which were attached four oblong bright scarlet mites, each of which was as large as the thorax itself. He afterwards met with another spider still smaller, attacked by two of these swoln parasites, one of which appeared to him nearly equal to the spider in size. This mite was probably either Leptus Phalangii, or Astoma parasiticum.

2. We now come to a perfectly distinct tribe of insect parasites, which belong to that section or order of intestinal worms which Rudolph has denominated Entozoa nematoidea, and Lamarck Vers rigidules[1009]. To this tribe belong the Gordius of LinnÉ and the Filaria of modern zoologists, which from the experiments and observations of De Geer, Dr. Matthey, &c. appear to have been too hastily separated, being really congenerous, and living indifferently in water and in the intestines of insects and other animals[1010]. To this genus belong the guinea-worm (Gordius medinensis[1011]), the Furia infernalis, and several others that are found in various vertebrate animals. These little worms have been discovered in insects of almost every Order; and their attack generally produces the death of the animal, though they appear not to devour those parts that are essential to life[1012]. I once took a specimen of Poecilus azureus, and upon immersing it in boiling water I was surprised to see what at first I mistook for an intestine, thrust itself forth; but upon a nearer inspection, to my great surprise I found it was one of these worms, thicker than a horse-hair and of a brown colour. Mr. W. S. MacLeay also once found one in Abax Striola. It still remains in my specimen, making it appear as if it had a long tail. De Geer long ago found these worms in grasshoppers[1012]; but Dr. Matthey has given the fullest account of one which infested Acrida viridissima. A friend of his noticing one of these insects which had not strength enough to leap and could scarcely even walk, being struck with the circumstance, caught the animal, upon which its hind legs were immediately detached from it. His surprise was greatly increased when he saw issue from its body a cylindrical worm about two feet and a half in length. Upon being called, Dr. M. soon recognised it for a Gordius or Filaria; and on his putting it into water, it moved in it with great velocity, twisting its long and slender body in all directions. Upon opening the body of the grasshopper, nothing appeared within it but the intestine shrunk up to a thread. A few days after, another was brought, which appeared in full vigour, but its abdomen was enormously distended, and from it another worm was extracted, which remained without motion rolled in a spiral direction: intending to preserve this in spirits of wine—as it had become flat he first immersed it in water, that it might recover if possible its cylindrical form. Upon immersion a movement took place in the animal, and it gradually recovered its plumpness; but it still remained without motion, as if dead, for nearly five days, when another living specimen being brought and placed with it, as soon as water was poured on them, the seemingly dead one began to show by a slight oscillation in its extremities that life was not extinct in it. Fresh water being poured upon it, at the end of the day it had recovered all its strength and agility. He afterwards often repeated the same experiment with a similar result[1013]. From this account it appears that the Gordius or Filaria has a property resembling that of the Vibrio Tritici, so well described and so beautifully figured by M. Bauer[1014], of apparently dying and being resuscitated by immersion in water. How long it can retain this property remains to be ascertained.

De Geer states that he had seen them of the length of two feet[1015]; but they vary considerably in this respect. In ants, in which Gould detected them, he states their length to be not more than half an inch[1016]. In caterpillars, which they sometimes infest, they are longer; in that of Notodonta Ziczac, De Geer found one three inches and a half long[1017]; and RÖsel three, of six inches, in that of Deilephila EuphorbiÆ[1018]; and in Phalangium cornutum, according to Latreille, they extend to more than seven inches[1019]. In the larva of a Phryganea L. the author first named found one which was more than a foot long, corresponding exactly with the Gordius aquaticus of LinnÉ; being forked at one extremity, brown above, gray below, and black at each end[1020]. These animals appear to die as soon as they leave the body[1021] they have preyed upon; except this happens in water, when their activity has no repose. In this element they give their bodies every possible inflexion, often tying themselves in knots in various places, interlacing and twisting themselves in a hundred different ways; so that when confined in the body of an insect, from their extreme suppleness and power of contortion they find sufficient space wherein to pack their often enormous length[1022]. LinnÉ makes one of their habitats clay; and Mr. W. S. MacLeay finds them very common at Putney in clay at the bottom of pools.

Dr. Matthey asks—How does the Gordius get into Acrida viridissima[1023]? And De Geer—Why do they die after having quitted a caterpillar? and where do they perpetuate their species[1024]? These questions, without further observations, cannot easily be answered. However, it may be supposed that carnivorous insects, such as Harpali, &c. may swallow them when found apparently dead in clay, where the water has been evaporated, or when they have been ejected by other insects; and they may revive in their bodies, as Dr. Matthey found them to do in water. It is not difficult to conjecture that the larvÆ of PhryganeÆ may meet with them when young in the water, and sometimes unluckily swallow them with their food. Why they become as dead when they emerge from their prey we cannot at present conjecture; but no doubt to answer some wise purpose;—in rainy seasons they probably revive and get into little hollows full of rain-water. Upon De Geer's last question—How they perpetuate their species—at present I can offer no conjecture.

I am, &c.