THE EXTERNAL STRUCTURE OF A FLEA

In comparing the structure of a flea with that of a man, or any other of the higher animals, it is of the utmost importance to understand that the one has an internal and the other an external skeleton. In either case the skeleton serves as an attachment for the muscles by which the animal moves itself. Everyone is familiar with the external skeleton of a lobster and can see for himself how the muscles are attached. The structure of a flea, though so much smaller, is somewhat similar, except that the skeleton is composed of a horny substance known as chitin instead of being calcified. The chitinous cuticle entirely covers the flea, but it varies in hardness and thickness on different parts of the body. The epidermis, or true skin, lies immediately beneath. On those parts of the body which are to the rear of the head the chitin forms a series of plates or shields which overlap one another somewhat like the tiles of a house. The segmented structure of a flea is there most clearly seen; this we may suppose is an inheritance from the segmented worms.

The chitin which forms the external skeleton of a flea is secreted by an outer layer of cells on the insect’s body. The deposit being thin at the joints, and thick on the plates, which serve for protection, the flea is encased in a suit of flexible armour. It is made of a fairly solid and dense substance, but, owing to the absence of carbonate and phosphate of lime, is much lighter than the familiar external skeleton of the lobster. Chitin is a very peculiar and durable substance which resists boiling in acids or alkalies. It is a structureless substance, in the sense that it does not consist of cells. Though horny in appearance it is, of course, in no sense true horn like that of the nails, hoofs, claws, and horns of vertebrates.

The different species of flea vary considerably in size. Some are smaller than the familiar human flea. Others are much larger. A very large flea (Hystrichopsylla talpÆ) is that found on the mole. The largest known flea (Dolichopsyllus stylosus) is found on small rodents in the United States where, as we know, all things are on a bigger scale than in the Old World. It is seven millimetres long.

The colour of the horny integument varies from a pale or light yellow to a ruddy or dark brown. It is plentifully sprinkled over with spines, bristles, or hairs, directed backwards so as not to impede progress. The presence or absence, the arrangement on the body, and the size of these serve, along with other features, to distinguish different species. They seem materially to help a flea in those wriggles to escape with which we are all familiar. The bristles are not always the same in size and arrangement in the two sexes of the same species of flea. As a rule the males are more bristly than the females.

These appendages of the flea’s integument are called by various writers either spines, bristles, or hairs. There is, however, no real distinction in the structure or nature of the appendages, and it is a question of degree which name is most appropriate.

In distinguishing species, very little reliance can be placed on the colour of the flea. An insect newly emerged from the pupa is always lighter in colour; and the difference between the appearance of an empty stomach and a stomach gorged with red blood is surprising.

The general external appearance of a typical flea belonging to the main family PulicidÆ is fairly familiar to most persons. This is well seen in the figure of the oriental rat-flea. The body is compressed or flattened from side to side, and this is a feature which is extremely rare among insects. It doubtless enables the animal to glide with greater facility through the hairs of its host. Like other insects, a flea is readily divided into a head, a thorax, and an abdomen. The head is rounded on the top and front and shows no obvious trace of segmentation; but what is known of the development of other insects leads one to think that it must properly be regarded as a number of segments closely fused together. On the under side of the front part of the head is a beak or proboscis for piercing and sucking, composed of the mouth-parts, whose structure is worthy of minute study. It will be best to examine them in detail in a subsequent chapter.

Some fleas have eyes, others have none. The common mouse-flea (Leptopsylla musculi) is blind. The bat-fleas are also destitute of eyes. The nocturnal habits of their hosts would render eyes of little or no use. If eyes are present they are large and placed on either side of the head. Each is a simple eye or ocellus; the compound eyes, divided into a great number of hexagonal facets, which are characteristic of many insects, are never found in fleas.

Nothing is known about the flea’s powers of vision, but there is no reason to suppose that they are at all acute. The eyes are marked with pigment. Ocelli appear to be primitive types of insect eye which are, perhaps, an inheritance from a wormlike ancestor. Presumably all the fleas of long ago had eyes and those that are now blind have lost their organs of sight from disuse. In their simplest condition, the eyes of the lower invertebrates only enable the creature, so far as one can judge, to distinguish light from darkness. Entomologists believe that the power of vision of ocelli is probably confined to very near objects and that this simple form of eye is more useful in dark places than the compound eyes. There is no reason for believing that fleas can distinguish colours or can discern any object which is more than a few inches away. It is enough for their purpose to perceive from which point light comes upon them and to make all despatch to escape in the opposite direction.

In blind fleas there is often a spine where the eye should be. In one species the spine is rudimentary and there is some black pigment beneath it. It is not impossible that this is the vestige of a once functional eye. In one genus, however, the eye and the spine are both present. Of the fleas belonging to this genus one species is South American and the other European. The latter (Typhloceras poppei) is confined to the long-tailed field-mouse.

The organs by which fleas keep in touch with the outward world, and with other fleas, are their antennÆ. All fleas have antennÆ; but unlike those of a moth, a beetle, or a grasshopper, each fits neatly into a groove at the side of the head and can be protruded when desired. This is another adaptation to enable the insect to creep swiftly through a forest of hairs.

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The combs which are found on the heads of many fleas are organs of exceptional interest (Fig. 2). They are toothed and horny appendages, which are connected with parasitic habits, for somewhat similar combs are found on several unrelated groups of parasitic insects, as, for instance, on parasitic beetles (Platypsyllus) found on the beaver, on insects allied to bed-bugs (Polyctenes) found on bats, and on wingless flies (Nycteribia) which infest Egyptian and South American bats. The majority of the PulicidÆ have one or more combs with comparatively long teeth. These combs reach their maximum development in the bat-fleas which have no less than eight. Some Australian and South American fleas (Stephanocircus) have a helmet-like comb extending all round their heads. These combs are by some supposed to be of service in holding on to the hairs of the host; and, if one may judge from experiments made on live fleas in cotton wool, they are also used in moving forwards through the fur.

All the chigoes (SarcopsyllidÆ) have a large triangular post-oral process which is more or less curved and probably prevents the flea slipping back as it pushes forward. Bat-fleas (CeratopsyllidÆ) have lobes or flaps placed two on each side of the head, which may possibly serve an identical purpose, but whether they do so is not known.

These combs may be divided into three groups according to the part of the flea’s body on which they are found. Those found on the head are called genal combs and take the form shown in Fig. 2. There are also combs found on the thorax. The fossil flea (PalÆopsylla klebsi) described in the previous chapter has one of these thoracic combs. A certain number of fleas also have combs on the abdominal segments.

There are really two types of toothed organs on fleas to both of which the name of comb may be given. One is composed of a sheet of chitin with a number of slits and teeth and resembles a true comb. The other consists merely of a number of highly chitinised bristles arranged in a row. They probably both serve the same function. Apart from their use as organs to assist movement onwards, they may also serve as hair-tight joints and protect the flea from the inconvenience of getting the tips of the host’s fur into the joints of its horny armour.

The size of the head compared to the thorax and abdomen varies considerably in different species. Some fleas have what may be called by comparison large heads and others very small ones.

A small head is never found in a flea with powerful mouth-parts. The head being the bearer of piercing and sucking organs, which require strong muscles, there must be room not only for the organs but for their extensors and retractors.

There are normally three rows of bristles on a flea’s head which divide the head into four sections. It is possible that these correspond to the four segments of the ancestral insect which are now fused together.

The head of a flea is closely applied by the whole of its back surface to the body and that slender and conspicuous neck which is characteristic of the Diptera, or flies, is not to be found in any fleas. For this reason a flea cannot turn its head in any direction without at the same time following it round with its body.

The thorax of a flea consists of three segments called respectively the prothorax, mesothorax and metathorax. The chitinous external skeleton which covers each of these three segments is primarily a hoop but each hoop is further subdivided into a number of complicated plates. Attached to the thorax are the three pairs of legs which are characteristic of all adult insects. The hind pair are very much the strongest (Fig. 3). They are the organs of hopping. It has often been pointed out that if men had the leaping powers of some fleas they would bound with ease backwards and forwards over the cross on the top of St. Paul’s Cathedral. Each leg consists of four segments beautifully articulated and plentifully supplied with bristles. At the end comes the foot with five very short segments. The last segment is provided with a pair of more or less formidable claws. Fleas use their legs for leaping, for running, and for clinging to their hosts. They also use their mouth-parts for the last purpose and it is worthy of note, as we shall see later on, that in those fleas in which the mouth-parts are shortest and weakest the legs are most liberally supplied with bristles and possess the stoutest claws. The legs of a flea are unique in the insect world owing to the enormous development of the segment nearest the body called the coxa. Most leaping insects rely for their activity on the muscles of the lower joints. In a grasshopper it is the third joint from the body (femur) which is so immensely enlarged. The three pairs of legs are each attached to a different thoracic segment.

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When fleas walk, they are so to speak plantigrades walking on the sole of the foot, and all the tarsal or foot joints are applied to the surface of the ground. The claws serve as grips so as to make the most of any unevenness; and thus the insect drags itself along with surprising rapidity when it moves through the hairy coat of a mammal. But on an open surface fleas are not really rapid movers compared with many other insects.

The two claws on the end of each ultimate foot segment are freely moveable and are in fact highly modified bristles or setÆ.

In all fleas one of the plates of the metathorax (or hindmost thoracic segment) called the epimeron, is large and prolonged towards the rear. It invariably bears a stigma. The epimeron is placed laterally to the first abdominal tergite. The older naturalists jumped to the conclusion that this was the remains of a wing. The best judges have, however, formed a decided opinion that no trace of the relic of a flying organ can be detected on the thorax of a flea. Heymons, a German entomologist, has also failed to detect any sign in dissections which he has made of the larvÆ and the pupÆ.

The epimeron is in fact neither a scale nor a wing but a portion of the thorax present in all insects. It is of no special service to the flea except as a portion of the thoracic armature which covers the body.

The larva of a flea has no legs; the adult insect has six. A study of other embryo insects shows that the ancestors of insects had many legs. It is an interesting problem why insects lost the legs on their abdomens, why legs should now invariably be restricted to the thorax, and why there should never be more than three pairs. In the earliest known insects which lived on the earth, before winged forms were evolved, the number of legs was already six. But our knowledge of fleas is too small to attempt, at present, to trace their exact line of ancestral descent.

The abdomen of a flea consists of ten segments. The horny plates which cover the dorsal side are called tergites; those on the ventral side sternites. In fleas, as in all holometabolous insects, that is those which pass through a complete metamorphosis, the sternite of the first abdominal segment is suppressed and has completely disappeared. The tergite which covers the dorsal part of the first abdominal segment nearest to the thorax is, however, always present.

The ultimate segments of the male and female flea are modified for reproductive purposes and of these segments more must be said later.

Having now given a rough outline of the external skeleton of a flea, it only remains to say something about the muscular system. Attached to the inside of the chitinous armature are an enormous number of muscles, whitish and almost transparent. They act as extensors, retractors, flexors, elevators and depressors. The joints and hinges of the skeleton allow of considerable, but not perfect, freedom. The muscles of locomotion are partly in the thorax and partly in the several joints of the legs. Our knowledge of the muscular system of fleas is very imperfect. But, as in other insects, the general arrangement of the muscles is based on the segmented structure of the body.

For the reader who can accurately picture to himself the external structure of a flea and of the typical insects belonging to other orders, a few words may be said on the probable ancestry of fleas and their relationship to other living insects. This vexed and much debated question is still, as the older naturalists would have said, tremendum mysterium. Very little light has yet been thrown upon it, and the most divergent views have been expressed by learned and competent entomologists. A historic survey of the various opinions that have been held since the days of LinnÆus would fill many pages; but a short summary of the different orders to which fleas have been referred by different zoologists will suffice.

The older authors, LinnÆus, Geoffroy, Cuvier and DumÉril, and Gervais placed them among the Aptera because they were wingless. Kircher regarded them as Orthoptera, an order which includes grasshoppers and crickets; but he has had few followers. By Fabricius and by Illiger they were treated as Hemiptera or bugs. Lameere, a Belgian, has recently expressed a decided view that fleas are really a family of Coleoptera or beetles. Those who have held the once orthodox opinion that they belonged to the Diptera or flies are Roesel, Oken, Straus-DÜrkheim, Burmeister, Newman, Walker, von Siebold and Wagner.

The structure of an adult flea, however, differs from that of an adult fly in the following noteworthy respects: the mouth-parts are differently constructed, the head of the flea is closely joined to its thorax, the three divisions of the thorax are not joined and fused, the flea is wingless, the eyes of fleas are simple ocelli, and there are differences of lesser importance in the stigmata, which give access to the tracheal system by which all insects breathe.

The number of those who have regarded fleas as belonging to a distinct order of insects is considerable: they are Lamarck, De Geer, Latreille, Kirby and Spence, MacLeay, Leach, DugÈs, BouchÉ, van der Hoeven, Westwood, Landois, Brauer, Kraepelin, and Taschenberg. Modern opinion is all but unanimous on this point.

There remains, however, a second question. Even if it be agreed that there must be a distinct order for Suctoria, Aphaniptera, Siphonaptera, or fleas; where ought that order to be placed? In which other order of insects must we look for the nearest relations of fleas? For a time after the acceptance of the fact that insect forms have been evolved, and not separately created, the ancestors of fleas were searched for among some species of fly.

Then Kraepelin rejected the view that flies were as closely related to fleas as most entomologists thought and his followers could only find points of difference and no points of resemblance. Dahl (1899), a German, then took up the cudgels for the fly theory. Dahl pointed out the resemblance between fleas and a group of flies called PhoridÆ also parasitic on warm-blooded animals. During the ensuing years the debate was resumed afresh with much liveliness and sometimes with a little acrimony.

The fleas were placed by MacLeay and by Balbiani between the Diptera and Hemiptera; by Leach between the Hemiptera and Lepidoptera; by DugÈs between the Hymenoptera and Diptera; by Brauer between the Diptera and Coleoptera. Handlirsch thinks that fleas have no connection at all with beetles and Gross can find no signs of relationship with either Coleoptera or Diptera.

Embryology and the study of larval forms have thrown so much light on the ancestry of many animals, that it was hoped that a microscopic examination of the larvÆ of fleas, in various stages of development, would produce some facts of importance. In this hope entomologists have, to a great extent, been disappointed. There seems to be much similarity between the embryos of beetles, moths, flies, wasps and fleas. Those who have dwelt on the likeness of the larval flea to the maggot of a fly seem to forget that the resemblance to an embryo beetle is nearly as strong.

The young larva of the flea is very transparent and the digestive canal, heart and nervous system are easily recognised. The egg-shell breaker is an interesting example of the development of a temporary larval structure and it is the only known instance of such a structure in an insect. There are no traces of eyes. The antennÆ are three-jointed. They are rather long and slender, being about one-third as long as the head. The head is well-developed and the larva has no feet.

The biting mandibles are broad and triangular. Compared with those of other larvÆ they are said to be more like the mandibles of coleopterous than of dipterous larvÆ. The maxillÆ, or second pair of jaws, are somewhat reduced and rudimentary. The absence of eyes and of legs are points of similarity between the larvÆ of fleas and flies. The maggot of a fly has also two pairs of jaws, and a pair of antennÆ.

At the tail end of the larval flea’s abdomen are two small projections called caudal stylets (Fig. 1). They are strong, recurved, chitinous, structures which prop up the body of the larva when it creeps and wriggles. There are similar props in the larvÆ of certain beetles and no exactly similar organs are known in dipterous larvÆ. But caudal stylets are of small taxonomic importance.

In one respect the mature flea is certainly nearer to a beetle than to a fly: the three joints of the thorax are free as in a beetle and not fused as in a fly; but when one studies the mouth-parts, the true view seems to be that the mouth-parts of a flea are equally unlike those of a fly and those of a beetle. Such being the present state of our knowledge, one must wait for fresh light to be thrown on the matter by further researches. It seems unlikely that the immediate future will produce a solution of the problem.