Some of them are suctorial, i. e. have a short and slender tube, with which they suck the blood of the animals of which they are parasites; while others are mandibulate, or have mandibles, and also maxillæ, their food consisting of portions of feathers, hairs, and scurf. The legs are usually short and stout, and the claws large and powerful, to enable them to hold firmly to the hairs, &c. The Anoplura are most abundant on dirty and diseased animals.
Suctor´ia.—The fourth Order of insects consists of the genus Púlex,—Pulex irrítans being the human flea. Other species are found upon different animals, as upon the dog, the rat ([Pl. X.] fig. 22), the fowl, the pigeon, &c.
The head and the dark eye (fig. 21, head of the human flea) are very evident. The antennæ or head-feelers are very minute and difficult to find, being sunk in a little pit or fossa behind the eye; they may, however, generally be recognized by the detection of the last joint, which is pectinate or cut like a comb. The body, including the head, consists of thirteen joints, one for the head, three for the chest or thorax, and nine for the belly or abdómen,—it being understood that by “joint” is meant a segment, and not the line of junction of two segments. The indication of these joints is afforded by the horny integument, which consists of a corresponding number of rings, forming in fact the skeleton of the animal. This in the flea, as in all insects and other Articulata, is external or cutaneous (cútis, skin), and consists of the hardened skin, the peculiar animal substance of which it is composed being called chitine (χιτὠν, tunic). These chitinous rings overlap, and are composed of a dorsal or upper, and a ventral or lower half; and near the middle of each is a row of hairs directed backwards. Along the sides of the body of the insect may be seen a row of dots; these are the spiracles or orifices of the breathing-tubes (tracheæ).
The legs are many-jointed, long, furnished with numerous spines, and terminated by two slightly curved claws, each with a little blunt tooth at its base. The claws are not perfectly smooth on the inside, but are covered with slightly raised lines, like a file, so that a better hold can be taken of bodies.
But the most interesting parts of the flea are those of the mouth, with which it punctures the skin and sucks the blood. There are nine of these; and they are best seen when the head of the flea is pulled off with the mounted needles, and the parts spread out and mounted in balsam, a high power being used to examine them. One of them forms a long and slender bristle (seta) or tongue, furnished with distant minute teeth. On each side of this is a flattened seta, with two rows of teeth on the edges; these are the lancets, and when not in use these organs are inclosed in two jointed sheaths. Outside these are two representatives of jaws, or maxillæ (b), each having a jointed feeler or palp (a) arising from it; the probable use of the palps being to feel the position of the skin, so that the animal may be able to adjust the lancets at a proper distance for puncture.
The eggs of the flea are often visible within the body of the parent; and when this is crushed, they are more distinctly seen, of various sizes, and contained within a long tube, which is the ovary or egg-bag. The eggs are laid by the animals upon carpets, woollen garments, or in the cracks of dirty floorboards; they are just perceptible to the eye as white oblong specks, and they may always be found on the rug when a cat is kept in the house. When hatched, they give rise to a minute white worm-like maggot, or larva (fig. 30), having a 12-jointed body, with two rudimentary antennæ and two slightly curved hooks appended to the last joint. The mouth-organs of the larva are adapted for biting, and not for sucking, as in the perfect animal, the jaws or maxillæ being distinctly toothed. When they have acquired full growth, which takes place in about twelve days in warm weather, they spin around themselves a little silky cocoon, and become transformed into a chrysalis or pupa; and from this, in about a fortnight, the perfect insect escapes.
Dip´tera (δἰς, twice, πτερὀν, wing).—This, which forms the fifth Order of Insects, consists of the two-winged insects, or flies, as the house-fly, the blue-bottle, the gnats, &c.
Mus´cidæ.—The house-fly, and the blue-bottle or meat-fly, are both species of the genus Mus´ca, belonging to this family, the former being Musca domes´tica, the latter Musca vomitor´ia. Both these insects are seen to be wonderfully constructed when minutely examined, and they possess considerable resemblance in general structure.
On examining the head of the house-fly ([Pl. X.] fig. 17) under a low power, and as an opake object, the observer will be struck with the remarkable appearance presented by the two eyes, which are large, placed one on each side of the fore part of the head, and composed of very numerous little eyes closely packed together, or they are compound, as it is called. The use of this compound structure is evidently to enable the little animal to see in all directions without moving the head and eyes. Each little eye has a lens to bring the rays of light emanating from objects to a focus upon a nerve. The packing of the eyes together gives rise to their angular form or their straight sides, each of the little surfaces or facets being hexagonal, or bounded by six sides (fig. 33). In front of and between the eyes are seen the two small antennæ; these have three joints, the third of which is larger than the rest (fig. 17 a), and arising from near its base is a feathery bristle or seta; these structures are best seen when the antennæ are pulled off with a pair of forceps and mounted separately. Below the antennæ, and extending downwards and forwards is the proboscis, or tongue, as it is called, which can be entirely retracted within a pit in the fore part of the head, or protruded at the will of the animal. This is a very beautiful and complicated instrument, and is best examined when spread out and separately mounted ([Pl. X.] fig. 29). It consists of a fleshy tube, dilated at the end into two lobes, which are flattened beneath to form a sucking-disk. The end is furnished with two solid horny lateral branches to keep it expanded, and with two longitudinal tubes beneath, running parallel, from the outer sides of which arise numerous nearly parallel branches. These tubes and their branches are incomplete beneath, and consist of imperfect rings, otherwise greatly resembling tracheæ. On each side of the proboscis is a lip-feeler or labial palpus, for the organ represents the labium of other insects. All these parts are better seen in the proboscis of the blow-fly than in that of the house-fly, on account of their larger size. The head, the thorax, and the abdomen are very distinct in the fly, being separated from each other by well-marked constrictions.
The legs are composed of five parts, each having a separate name. The first piece or joint, which is that attached to the body, is called the hip, or coxa; the next is a very small, somewhat triangular piece, and is the trochan´ter; next comes the long and stout thigh, or fémur; this being succeeded by the tib´ia, which, as in most insects, is furnished with strong spines at the end; and lastly is the foot, or tar´sus, consisting of five joints, the three last of which are represented in the figure ([Pl. X.] fig. 32). At the end of the fifth or last joint of the tarsus (fig. 32) (for it must be noted that the joints of the limbs of insects are always numbered in order of distance from the body) are two soft little cushions, or pulvil´li, which are covered on the under surface with numerous hair-like bodies dilated at the end, acting as suckers in enabling the fly to adhere to smooth surfaces. In addition to these organs are two curved claws, and between them a sharp straight spine.