We boast of our candles, our wax-lights, and our Argand lamps, and pity our fellow-men who, ignorant of our methods of producing artificial light, are condemned to pass their nights in darkness. We regard these inventions as the results of a great exertion of human intellect, and never conceive it possible that other animals are able to avail themselves of modes of illumination equally efficient; and are furnished with the means of guiding their nocturnal evolutions by actual lights, similar in their effect to those which we make use of. Yet many insects are thus provided. Some are forced to content themselves with a single candle, not more vivid than the rush-light which glimmers in the peasant's cottage; others exhibit two or four, which cast a stronger radiance; and a few can display a lamp little inferior in brilliancy to some of ours. Not that these insects are actually possessed of candles and lamps. You are aware that I am speaking figuratively. But Providence has supplied them with an effectual substitute—a luminous preparation or secretion, which has all the advantages of our lamps and candles without their inconveniences; which gives light sufficient to direct their motions, while it is incapable of burning; and whose lustre is maintained without needing fresh supplies of oil or the application of the snuffers.

Of the insects thus singularly provided, the common glow-worm (Lampyris noctiluca) is the most familiar instance. Who that has ever enjoyed the luxury of a summer evening's walk in the country, in the southern parts of our island, but has viewed with admiration these "stars of the earth and diamonds of the night?" And if, living like me in a district where it is rarely met with, the first time you saw this insect, chanced to be, as it was in my case, one of those delightful evenings which an English summer seldom yields, when not a breeze disturbs the balmy air, and "every sense is joy," and hundreds of these radiant worms, studding their mossy couch with mild effulgence, were presented to your wondering eye in the course of a quarter of a mile,—you could not help associating with the name of glow-worm the most pleasing recollections. No wonder that an insect, which chiefly exhibits itself on occasions so interesting, and whose economy is so remarkable, should have afforded exquisite images and illustrations to those poets who have cultivated Natural History.

If you take one of these glow-worms home with you for examination, you will find that in shape it somewhat resembles a caterpillar, only that it is much more depressed; and you will observe that the light proceeds from a pale-coloured patch that terminates the underside of the abdomen. It is not, however, the larva of an insect, but the perfect female of a winged beetle, from which it is altogether so different, that nothing but actual observation could have inferred the fact of their being the sexes of the same insect. In the course of our inquiries you will find that sexual differences even more extraordinary exist in the insect world.

It has been supposed by many that the males of the different species of Lampyris do not possess the property of giving out any light; but it is now ascertained that this supposition is inaccurate, though their light is much less vivid than that of the female. Ray first pointed out this fact with respect to L. noctiluca[685]. Geoffroy also observed that the male of this species has four small luminous points, two on each of the two last segments of the belly[686]: and his observation has been recently confirmed by MÜller. This last entomologist, indeed, saw only two shining spots; but from the insect's having the power of withdrawing them out of sight so that not the smallest trace of light remains, he thinks it is not improbable that at times two other points still smaller may be exhibited, as Geoffroy has described. In the males of L. Splendidula and of L. hemiptera the light is very distinct, and may be seen in the former while flying[687].—The females have the same faculty of extinguishing or concealing their light—a very necessary provision to guard them from the attacks of nocturnal birds: Mr. White even thinks that they regularly put it out between eleven and twelve every night[688]: and they have also the power of rendering it for a while more vivid than ordinary.

Authors who have noticed the luminous parts of the common female glow-worm, having usually contented themselves with stating that the light issues from the three last ventral segments of the abdomen[689]; I shall give you the result of some observations I once made upon this subject. One evening, in the beginning of July, meeting with two of these insects, I placed them on my hand. At first their light was exceedingly brilliant, so as to appear even at the junctions of the upper or dorsal segments of the abdomen. Soon after I had taken them, one withdrew its light altogether, but the other continued to shine. While it did this it was laid upon its back, the abdomen forming an angle with the rest of its body, and the last or anal segment being kept in constant motion. This segment was distinguished by two round and very vivid spots of light; which, in the specimen that had ceased to shine, were the last that disappeared, and they seem to be the first parts that become luminous when the animal is disposed to yield its light. The penultimate and antepenultimate segments each exhibited a middle transverse band of yellow radiance, terminated towards the trunk by an obtusely-dentated line; a greener and fainter light being emitted by the rest of the segment.

Though many of the females of the LampyridÆ are without wings and even elytra, (in which circumstance they differ from all other apterous Coleoptera,) this is not the case with all. The female of Pygolampis[690] italica, a species common in Italy, and which, if we may trust to the accuracy of the account given by Mr. Waller in the Philosophical Transactions for 1684, would seem to have been taken by him in Hertfordshire, is winged: and when a number of these moving stars are seen to dart through the air in a dark night, nothing can have a more beautiful effect. Sir J. E. Smith tells us that the beaus of Italy are accustomed in an evening to adorn the heads of the ladies with these artificial diamonds, by sticking them into their hair; and a similar custom, as I have before informed you[691], prevails amongst the ladies of India.

Besides the different species of the genus Lampyris, all of which are probably more or less luminous, another insect of the beetle tribe, Elater noctilucus, is endowed with the same property, and that in a much higher degree. This insect, which is called the fire-fly, and is an inch long, and about one-third of an inch broad, gives out its principal light from two transparent eye-like tubercles placed upon the thorax; but there are also two luminous patches concealed under the elytra, which are not visible except when the insect is flying, at which time it appears adorned with four brilliant gems of the most beautiful golden-blue lustre: in fact, the whole body is full of light, which shines out between the abdominal segments when stretched. The light emitted by the two thoracic tubercles alone is so considerable, that the smallest print may be read by moving one of these insects along the lines; and in the West India islands, particularly in St. Domingo, where they are very common, the natives were formerly accustomed to employ these living lamps, which they called Cucuij, instead of candles in performing their evening household occupations. In travelling at night they used to tie one to each great toe; and in fishing and hunting required no other flambeau[692].—Southey has happily introduced this insect in his "Madoc" as furnishing the lamp by which Coatel rescued the British hero from the hands of the Mexican priests.

Pietro Martire tells us that the Cucuij serve the natives of the Spanish West India islands not only instead of candles, but as extirpators of the gnats, which are a dreadful pest to the inhabitants of the low grounds. They introduce a few fire-flies, to which the gnats are a grateful food, into their houses, and by means of these "commodious hunters" are soon rid of the intruders. "How they are a remedy," says this author, "for so great a mischiefe it is a pleasant thing to hear. Hee who understandeth he hath those troublesome guestes (the gnattes) at home, diligently hunteth after the Cucuij. Whoso wanteth Cucuij goeth out of the house in the first twilight of the night, carrying a burning fire-brande in his hande, and ascendeth the next hillock that the Cucuij may see it, and hee swingeth the fire-brande about, calling Cucuius aloud, and beating the ayre with often calling out Cucuie, Cucuie." He goes on to observe, that the simple people believe the insect is attracted by their invitations; but that, for his part he is rather inclined to think that the fire is the magnet. Having obtained a sufficient number of Cucuij, the beetle-hunter returns home and lets them fly loose in the house, where they diligently seek the gnats about the beds and the faces of those asleep, and devour them[693].—These insects are also applied to purposes of decoration. On certain festival days in the month of June, they are collected in great numbers, and tied all over the garments of the young people, who gallop through the streets on horses similarly ornamented, producing on a dark evening the effect of a large moving body of light. On such occasions the lover displays his gallantry by decking his mistress with these living gems[694]. And according to P. Martire, "many wanton wilde fellowes" rub their faces with the flesh of a killed Cucuius, as boys with us use phosphorus, "with purpose to meet their neighbours with a flaming countenance," and derive amusement from their fright.

Besides Elater noctilucus, E. ignitus and several others of the same genus are luminous. Not fewer than twelve species of this family are described by Illiger in the Berlin Naturalist Society's Magazine[695].

The brilliant nocturnal spectacle presented by these insects to the inhabitants of the countries where they abound cannot be better described than in the language of the poet above referred to, who has thus related its first effect upon the British visitors of the new world:

The beautiful poetical imagery with which Mr. Southey has decorated this and a few other entomological facts, will make you join in my regret that a more extensive acquaintance with the science has not enabled him to spread his embellishments over a greater number. The gratification which the entomologist derives from seeing his favourite study adorned with the graces of poetry is seldom unalloyed with pain, arising from the inaccurate knowledge of the subject in the poet. Dr. Darwin's description of the beetle to which the nut-maggot is transformed may delight him (at least if he be an admirer of the Darwinian style) as he reads for the first time,

But when the music of the lines has allowed him room for pause, and he recollects that they are built wholly upon an incorrect supposition, the Curculio never inhabiting the nut in its beetle shape, nor employing its ivory or rather ebony beak upon it, but undergoing its transformation under ground, he feels disappointed that the passage has not truth as well as sound.—Mr. Southey, too, has fallen into an error: he confounds the fire-fly of St. Domingo (Elater noctilucus) with a quite different insect, the lantern-fly (Fulgora laternaria) of Madame Merian; but happily this error does not affect his poetry.

But to return from this digression.—If we are to believe Mouffet, (and the story is not incredible,) the appearance of the tropical fire-flies on one occasion led to a more important result than might have been expected from such a cause. He tells us, that when Sir Thomas Cavendish and Sir Robert Dudley first landed in the West Indies, and saw in the evening an infinite number of moving lights in the woods, which were merely these insects, they supposed that the Spaniards were advancing upon them, and immediately betook themselves to their ships[696]:—a result as well entitling the Elaters to a commemoration feast, as a similar good office the land-crabs of Hispaniola, which, as the Spaniards tell, (and the story is confirmed by an anniversary Fiesta de lÔs Cangrejos,) by their clattering—mistaken by the enemy for the sound of Spanish cavalry close upon their heels—in like manner scared away a body of English invaders of the city of St. Domingo[697].

An anecdote less improbable, perhaps, and certainly more ludicrous, is related by Sir J. E. Smith of the effect of the first sight of the Italian glow-worms upon some Moorish ladies ignorant of such appearances. These females had been taken prisoners at sea, and, until they could be ransomed, lived in a house in the outskirts of Genoa, where they were frequently visited by the respectable inhabitants of the city; a party of whom, on going one evening, were surprised to find the house closely shut up, and their Moorish friends in the greatest grief and consternation. On inquiring into the cause, they ascertained that some of the Pygolampis italica had found their way into the dwelling, and that the ladies within had taken it into their heads that these brilliant guests were no other than the troubled spirits of their relations; of which idea it was some time before they could be divested.—The common people in Italy have a superstition respecting these insects somewhat similar, believing that they are of a spiritual nature, and proceed out of the graves, and hence carefully avoid them[698].

The insects hitherto adverted to have been beetles, or of the order Coleoptera. But besides these, a genus in the order Hemiptera, called Fulgora, includes several species which emit so powerful a light as to have obtained in English the generic appellation of Lantern-flies. Two of the most conspicuous of this tribe are the F. laternaria and F. candelaria; the former a native of South America, the latter of China. Both, as indeed is the case with the whole genus, have the material which diffuses their light included in a hollow subtransparent projection of the head. In F. candelaria this projection is of a subcylindrical shape, recurved at the apex, above an inch in length, and the thickness of a small quill. We may easily conceive, as travellers assure us, that a tree studded with multitudes of these living sparks, some at rest and others in motion, must at night have a superlatively splendid appearance.—In F. laternaria, which is an insect two or three inches long, the snout is much larger and broader, and more of an oval shape, and sheds a light the brilliancy of which transcends that of any other luminous insect. Madame Merian informs us, that the first discovery which she made of this property caused her no small alarm. The Indians had brought her several of these insects, which by day-light exhibited no extraordinary appearance, and she inclosed them in a box until she should have an opportunity of drawing them, placing it upon a table in her lodging-room. In the middle of the night the confined insects made such a noise as to awake her, and she opened the box, the inside of which to her great astonishment appeared all in a blaze; and in her fright letting it fall, she was not less surprised to see each of the insects apparently on fire. She soon, however, divined the cause of this unexpected phenomenon, and re-inclosed her brilliant guests in their place of confinement. She adds, that the light of one of these FulgorÆ is sufficiently bright to read a newspaper by: and though the tale of her having drawn one of these insects by its own light is without foundation, she doubtless might have done so if she had chosen[699].—Another species (F. pyrrhorynchus) is figured by Mr. Donovan in his Insects of India, of which the light, though from a smaller snout than that of F. laternaria, must assume a more splendid and striking appearance, the projection being of a rich deep purple from the base to near the apex, which is of a fine transparent scarlet; and these tints will of course be imparted to the transmitted light.

In addition to the insects already mentioned, some others have the power of diffusing light, as two species of Centipedes (Geophilus electricus and phosphoreus), and probably others of the same genus. In these the light is not confined to one part, but proceeds from the whole body. G. electricus is a common insect in this country, residing under clods of earth, and often visible at night in gardens. G? phosphoreus, a native of Asia, is an obscure species, described by LinnÉ, on the authority of C. G. Ekeberg, the captain of a Swedish East Indiaman, who asserted that it dropped from the air, shining like a glow-worm, upon his ship, when sailing in the Indian ocean a hundred miles (Swedish) from the continent. However singular this statement, it is not incredible. The insect may either, as LinnÉ suspects, have been elevated into the atmosphere by wings with which, according to him, one species of the genus is provided; or more probably, perhaps, by a strong wind, such as that which raised into the air the shower of insects mentioned by De Geer as occurring in Sweden in the winter of 1749, after a violent storm that had torn up trees by the roots, and carried away to a great distance the surrounding earth, and insects which had taken up their winter quarters amongst it[700]. That the wind may convey the light body of an insect to the above-mentioned distance from land, you will not dispute when you call to mind that our friend Hooker, in his interesting Tour in Iceland, tells us that the ashes from the eruption of one of the Icelandic volcanos in 1755 were conveyed to Ferrol, a distance of upwards of 300 miles[701].—Lastly, to conclude my list of luminous insects, Professor Afzelius observed "a dim phosphoric light" to be emitted from the singular hollow antennÆ of Pausus sphÆrocerus[702]. A similar appearance has been noticed in the eyes of Acronycta Psi, Cossus ligniperda, and other moths. Chiroscelis bifenestrata of Lamarck, a beetle, has two red oval spots covered with a downy membrane on the second segment of the abdomen, which he thinks indicate some particular organ perhaps luminous[703]: and M. Latreille informs me that a friend of his, who saw one living which was brought from China to the Isle of France in wood, found that the ocelli in the elytra of Buprestis ocellata were luminous.

But besides the insects here enumerated, others may be luminous which have not hitherto been suspected of being so. This seems proved by the following fact. A learned friend[704] has informed me, that when he was curate of Ickleton, Cambridgeshire, in 1780, a farmer of that place of the name of Simpringham brought to him a mole-cricket (Gryllotalpa vulgaris, Latr.), and told him that one of his people, seeing a Jack-o'lantern, pursued it and knocked it down, when it proved to be this insect, and the identical specimen shown to him.

This singular fact, while it renders it probable that some insects are luminous which no one has imagined to be so, seems to afford a clue to the, at least, partial explanation of the very obscure subject of ignes fatui, and to show that there is considerable ground for the opinion long ago maintained by Ray and Willughby, that the majority of these supposed meteors are no other than luminous insects. That the large varying lambent flames, mentioned by Beccaria to be very common in some parts of Italy, and the luminous globe seen by Dr. Shaw[705] cannot be thus explained, is obvious. These were probably electrical phenomena: certainly not explosions of phosphuretted hydrogene, as has been suggested by some, which must necessarily have been momentary. But that the ignis fatuus mentioned by Derham as having been seen by himself, and which he describes as flitting about a thistle[706], was, though he seems of a different opinion, no other than some luminous insect, I have little doubt. Mr. Sheppard informs me that, travelling one night between Stamford and Grantham on the top of the stage, he observed for more than ten minutes a very large ignis fatuus in the low marshy grounds, which had every appearance of being an insect. The wind was very high: consequently, had it been a vapour, it must have been carried forward in a direct line; but this was not the case. It had the same motions as a Tipula, flying upwards and downwards, backwards and forwards, sometimes appearing as settled, and sometimes as hovering in the air.—Whatever be the true nature of these meteors, of which so much is said and so little known, it is singular how few modern instances of their having been observed are on record. Dr. Darwin declares, that though in the course of a long life he had been out in the night, and in the places where they are said to appear, times without number, he had never seen any thing of the kind: and from the silence of other philosophers of our own times, it should seem that their experience is similar.

With regard to the immediate source of the luminous properties of these insects, Mr. Macartney, to whom we are indebted for the most recent investigation on the subject, has ascertained that in the common glow-worm, and in Elater noctilucus and ignitus, the light proceeds from masses of a substance not generally differing, except in its yellow colour, from the interstitial substance (corps graisseux) of the rest of the body, closely applied underneath those transparent parts of the insects' skin which afford the light. In the glow-worm, besides the last-mentioned substance, which, when the season for giving light is passed, is absorbed, and replaced by the common interstitial substance, he observed on the inner side of the last abdominal segment two minute oval sacs formed of an elastic spirally-wound fibre similar to that of the tracheÆ, containing a soft yellow substance of a closer texture than that which lines the adjoining region, and affording a more permanent and brilliant light. This light he found to be less under the control of the insect than that from the adjoining luminous substance, which it has the power of voluntarily extinguishing, not by retracting it under a membrane, as Carradori imagined, but by some inscrutable change dependent upon its will: and when the latter substance was extracted from living glow-worms it afforded no light, while the two sacs in like circumstances shone uninterruptedly for several hours. Mr. Macartney conceives, from the radiated structure of the interstitial substance surrounding the oval yellow masses immediately under the transparent spots in the thorax of Elater noctilucus, and the subtransparency of the adjoining crust, that the interstitial substance in this situation has also the property of shining—a supposition which, if De Geer and other authors be correct in stating that this insect has two luminous patches under its elytra, and that the incisures between the abdominal segments shine when stretched, may probably be extended to the whole of the interstitial substance of its body.—What peculiar organization contributes to the production of light in the hollow projections of Fulgora laternaria and candelaria, the hollow antennÆ of Pausus sphÆrocerus, and under the whole integument of Geophilus electricus, Mr. Macartney was unable to ascertain. Respecting this last he remarks, what I have myself observed, that there is an apparent effusion of a luminous fluid on its surface, that may be received upon the hand, which exhibits a phosphoric light for a few seconds afterwards; and that it will not shine unless it have been previously exposed for a short time to the solar light[707].

With respect to the remote cause of the luminous property of insects, philosophers are considerably divided in opinion. The disciples of modern Chemistry have in general, with Dr. Darwin, referred it to the slow combustion of some combination of phosphorus secreted from their fluids by an appropriate organization, and entering into combination with the oxygene supplied in respiration. This opinion is very plausibly built upon the ascertained existence of phosphoric acid as an animal secretion; the great resemblance between the light of phosphorus in slow combustion and animal light; the remarkably large spiracula in glow-worms; and upon the statement, that the light of the glow-worm is rendered more brilliant by the application of heat and oxygene gas, and is extinguished by cold and by hydrogene and carbonic acid gases. From these last facts Spallanzani was led to regard the luminous matter as a compound of hydrogene and carburetted hydrogene gas. Carradori having found that the luminous portion of the belly of the Italian glow-worm (Pygolampis italica) shone in vacuo, in oil, in water, and when under other circumstances where the presence of oxygene gas was precluded, with Brugnatelli ascribed the property in question to the imbibition of light separated from the food or air taken into the body, and afterwards secreted in a sensible form[708]. Lastly, Mr. Macartney having ascertained by experiment that the light of a glow-worm is not diminished by immersion in water, or increased by the application of heat; that the substance affording it, though poetically employed for lighting the fairies' tapers[709], is incapable of inflammation if applied to the flame of a candle or red-hot iron; and when separated from the body exhibits no sensible heat on the thermometer's being applied to it—rejects the preceding hypotheses as unsatisfactory, but without substituting any other explanation; suggesting, however, that the facts he observed are more favourable to the supposition of light being a quality of matter than a substance[710].

Which of these opinions is the more correct I do not pretend to decide. But though the experiments of Mr. Macartney seem fairly to bear him out in denying the existence of any ordinary combination of phosphorus in luminous insects, there exists a contradiction in many of the statements, which requires reconciling before final decision can be pronounced. The different results obtained by Forster and Spallanzani, who assert that glow-worms shine more brilliantly in oxygene gas, and by Beckerheim, Dr. Hulme, and Sir H. Davy, who could perceive no such effect, may perhaps be accounted for by the supposition that in the latter instances the insects having been taken more recently, might be less sensible to the stimulus of the gas than in the former, where possibly their irritability was, as Brown would say, accumulated by a longer abstinence: but it is not so easy to reconcile the experiment of Sir H. Davy, who found the light of the glow-worm not to be sensibly diminished in hydrogene gas[711], with those of Spallanzani and Dr. Hulme, who found it to be extinguished by the same gas, as well as by carbonic acid, nitrous and sulphuretted hydrogene gases[712]. Possibly some of these contradictory results were occasioned by not adverting to the faculty which the living insect possesses of extinguishing its lights at pleasure; or different philosophers may have experimented on different species of Lampyris.

The general use of this singular provision is not much more satisfactorily ascertained than its nature. I have before conjectured—and in an instance I then related it seemed to be so—that it may be a means of defence against their enemies[713]. In different kinds of insects, however, it may probably have a different object. Thus in the lantern-flies (Fulgora), whose light precedes them, it may act the part that their name imports, enabling them to discover their prey, and to steer themselves safely in the night. In the fire-flies (Elater), if we consider the infinite numbers that in certain climates and situations present themselves every where in the night, it may distract the attention of their enemies or alarm them. And in the glow-worm—since their light is usually most brilliant in the female; in some species, if not all, present only in the season when the sexes are destined to meet; and strikingly more vivid at the very moment when the meeting takes place[714]—besides the above uses, it is most probably intended to conduct the sexes to each other. This seems evidently the design in view in those species in which, as in the common glow-worm (L. noctiluca), the females are apterous. The torch which the wingless female, doomed to crawl upon the grass, lights up at the approach of night, is a beacon which unerringly guides the vagrant male to her "love-illumined form," however obscure the place of her abode. It has been objected, however, to this explanation, that—since both larva and pupa, as De Geer observed[715], and the males shine as well as the females—the meeting of the sexes can scarcely be the object of their luminous provision. But this difficulty appears to me easily surmounted. As the light proceeds from a peculiarly organized substance, which probably must in part be elaborated in the larva and pupa states, there seems nothing inconsistent in the fact of some light being then emitted with the supposition of its being destined solely for use in the perfect state: and the circumstance of the male having the same luminous property, no more proves that the superior brilliancy of the female is not intended for conducting him to her, than the existence of nipples and sometimes of milk in man proves that the breast of woman is not meant for the support of her offspring. We often see without being able to account for the fact, except on Sir E. Home's idea, that the sex of the ovum is undetermined[716], traces of an organization in one sex indisputably intended for the sole use of the other.

I am, &c.