Insect identification > Insect external structure

Insect external structure

Insect external structureBringing together the facts already stated about insects we find that an adult insect is (1) a bilaterally symmetrical animal; (2) consisting of a series of segments one behind another; (3) that these segments are grouped into three regions, the head in front, followed in order by the thorax and the abdomen as shown in Fig. 10; (4) that the animal is covered by a skeleton, shell-like in that it is on the outside of the body, but horny in its nature; (5) that attached to the thorax are three pairs of jointed legs; (6) that a pair of antennae, (7) mouth parts (8) and usually two pairs of wings are present; (9) that it breathes through air tubes; and (10) that the reproductive organs open near the hinder end of the body.

In the body of the adult insect there are apparently from 7 to 15 segments; 1 in the head, 3 in the thorax and from 3 to 11 in the abdomen. In the egg, however, the embryo shows the presence of 6 (some investigators think 7) segments in the head, 3 in the thorax and 12 in the abdomen, indicating that the original total number was 21 (or 22). This change to the adult has been brought about in the head by an extreme amount of fusion and condensation and in the abdomen partly by fusion, partly by a sort of telescoping or gradual shifting of one segment within another until it has been partly or entirely concealed.

The skeleton of the body or cuticula, as it is called, supports and protects the soft, living tissues within. It rests on the outer surface of a layer of living cells, the hypodermis. It seems to be formed in part from a fluid poured out from the hypodermis and in part by a transformation of a portion of the hypodermis cells themselves. The cuticula hardens quickly after its production and consists quite largely, at least, of a nitrogenous substance called chitin (pronounced ky-tin). It varies in thickness and is flexible where movement is needed, but thicker, more rigid and usually darker colored elsewhere. The hypodermis along certain lines forms folds which project inward, carrying its chitinous secretion along with it. Where these infoldings occur at places which need to be movable the chitin there remains flexible, thus forming the movable joints; along the other infoldings it hardens to become rigid and such places show on the surface only as slight grooves or scratches and are called sutures. These sutures and joint lines divide the cuticula into areas called plates or sclerites.
The sutures and joint lines are sufficiently regular in position in insects to make them convenient landmarks in descriptions. The sclerites on the back of each segment are usually together termed the notum; those on each side, the pleuron; and those below, the sternum. In the head the sutures are few in number, and only a few plates or sclerites are generally in evidence. In the thorax they are more numerous, while in the abdomen often only a dorsal and ventral sclerite for each segment are found. Occasionally the weakly chitinized areas are quite large (queen white ant, Fig. 74a) and elastic. Usually the elasticity of these places, as, for example, the portions connecting the segments, is rather slight. Spines, hairs, scales or other structures are often present on the chitin, sometimes entirely concealing its surface and its sutures.
The heads of different insects vary much in form and in the location of the mouth. In some cases this is on the underside (see Fig. 10), while in others (Fig. 12) it is practically on the front. Heads with the mouth beneath are called hypognathous; those with it in front are prognathous.

Structures found on the head are a pair of antennae, the two compound eyes, ocelli and the mouth-parts. On the thorax are the wings and legs; and on the abdomen are various organs such as the ovipositor, sting, cerci, styli, present in some cases, absent in others. Antennae are nearly always present. They are usually slender, jointed and therefore more or less flexible organs, varying greatly in the number of segments composing them. They are sometimes very short; sometimes long; often thread-like; sometimes enlarged near the tip; in many cases with fine branches either on one or both sides, so that they resemble feathers or plumes; rarely they fork; in fact are of many forms (Fig. 13a, and b). Sense organs are present on them for the sense of touch, and probably also for smell and hearing, at least in some cases.

The eyes are of two kinds. There is a pair of compound eyes, each of which is a group of similar structures which usually are like tall, slender pyramids in form. Only the bases of these pyramids show on the surface, and their outlines can often be easily seen with a magnifying glass. They are called facets, and the eyes themselves are sometimes termed the facetted eyes. The other kind of eyes, called ocelli, may be absent or, if present, may vary in number in different insects, three being perhaps the most usual. Each, as seen from the surface, is a nearly circular, convex spot about the size of one of the facets of a compound eye. It may be larger than this but is never equal to an entire compound eye in size. In some cases a cluster of ocelli or of the pyramids of the compound eyes is found, not closely pressed together but somewhat separated, and such groups are called agglomerate eyes. The chitin of the surface of the body is transparent where it covers the surface of an eye, permitting access of light to the sensory structures within; elsewhere it is usually pigmented and rather opaque.
The mouth-parts of insects vary extremely in their structure. Apparently the original mouth-parts were for biting and chewing, and this type is very common. In some groups, however, they have been transformed into a sucking apparatus. Biting mouth-parts, being the more primitive and simple, are described here, while sucking mouth-parts having been differently transformed in different groups will be taken up in connection with those groups.

In front of (in hypognathous heads) or above the mouth opening (prognathous heads) is the front lip or labrum. It is a thin flap, hinged to the skeleton of the head, and moves forward and backward. It is often more or less divided by a central notch at the middle of its free edge. Its inner surface, forming the roof of the mouth, is often called the epipharynx. At the sides of the mouth opening, immediately behind the labrum, is a pair of jaws, the mandibles. These differ greatly in form in different insects (Fig. 14). They are often stout, heavy structures with crushing faces bearing blunt projections or teeth; sometimes they are long, curved and rather slender. In general their form is adapted to the feeding habits of the insect.
Immediately behind each mandible at the side of the mouth is a second appendage, the maxilla. This differs markedly from the mandible, being much weaker, and composed of a number of pieces (Fig. 15). The tips and outer internal margins of the maxillae usually bear numerous spines or hairs, but this condition varies according to the nature of the food of the insect. Attached on the outer side of each maxilla, not far from where the latter articulates with the head, is a sort of tiny antenna-like structure consisting of from one to six (usually five) segments, which is called the maxillary palpus. The function of the maxillae appears to be to hold and retain the food in the mouth while it is being worked upon by the mandibles, and also to aid these in breaking it up. The presence of sense organs on the maxillary palpi suggests that these are possibly concerned with the sense of smell. Both mandibles and maxillae move sideways.

Behind the maxillae, and closing the mouth opening behind, is the hinder lip or labium (Fig. 16). This was evidently once a pair of jaws somewhat similar to the maxillae, but with no mouth cavity between to separate them, their inner edges have grown together to varying degrees in different insects. In some, only one or two of the pieces nearest the head have fused; in others, fusion all the way to the tip has been accomplished, and all intermediate stages also occur, thus producing a structure which now moves forward and backward like the front lip, but which may be complete, or partially or almost entirely cleft in the middle line.

Like the maxilla the labium has a palpus on each side arising from near its base, and composed of three (rarely four) segments. The function of these labial palpi appears to be similar to that of the maxillary palpi. Near the base of the labium on its inner or mouth side there is frequently a fleshy swelling more or less covered by bristles or hairs, which is called the hypopharynx, lingua or tongue. It varies greatly in size and form. The thorax has its three segments usually quite clearly marked. Each segment bears a pair of legs, but the prothorax, or first of the three behind the head, bears no wings. On the second, or mesothorax, and on the third, or metathorax, both wings and legs occur in the majority of insects.

There is a tendency in some groups, carried furthest in the higher Hymenoptera, for the first segment of the abdomen to consolidate more closely with the metathorax than with the second abdominal segment, which in such cases is often slender (Figs. 366, 376) and gives thereby a semidetached appearance to the rest of the abdomen, as though the line of division between thorax and abdomen were at that place instead of farther forward. The first abdominal segment, when seemingly more a part of the thorax than of the abdomen, is called the median segment or propodeum. The three pairs of legs may be quite similar or differ widely, according to the uses to which they are put. In running and walking insects they are usually most similar; but when, for example, the forelegs are used for capturing other insects, their form will depart greatly from that of the others. The jumping power of the grasshopper is due to the great development of its hind legs as compared with its others. Different types of legs are shown in Fig. 17. Whatever may be the variations in form and details of the legs, all are composed of a definite number of pieces or segments, connected by hinge joints so arranged that, by combining the motions of these, a leg can be placed in nearly any position desired.

The leg (Fig. 18) is composed of a coxa, a trochanter (two in a few cases), a femur, a tibia and a tarsus. The last is really not a single segment but a row of from one to five, small, and on the whole rather resembling each other. The coxa is the segment which articulates with the body, frequently partly lying in a more or less cup-shaped hollow of the latter. It may be short or long and is generally freely movable on the body and powerful. The trochanter is usually small and may not be visible on all sides of the leg. It is followed by the femur, generally the largest and stoutest, but not often the longest leg segment. The tibia is in most cases quite long, more slender than the femur and often provided with downwardly projecting spines or other structures which are of assistance to the insect in climbing plant stems and other objects, to help prevent slipping. The tarsal segments are generally rather small, short, tend to be broadest at their outer ends, and vary greatly in details of structure. At the end of the last a pair of claws is generally found, and between them a sort of pad or cushion, the pulvillus. Sometimes there are three of these, in which case the outer ones are called the pulvilli and the middle one the empodium. Where the reduced to a small number of segments, only one claw may be tarsi are present.

The wings are chitinous outgrowths from the body which vary much in size and form in different insects. Each consists of two delicate membranes in contact with each other except along certain lines (Fig. 19). Along these lines each membrane thickens and also rises above the general surface, so that if the two membranes could be separated and examined from the inner surface, they would appear uniform except for grooves with thickened sides and bottoms, running here and there. When the membranes are brought together again, these grooves combining form hollow rods which, being stronger than the rest of the membrane, serve as its support and hold it stiff. These hollow rods are usually called veins or nerves, though they are nothing of the sort. The main and largest veins arise at the base of the wing and extend outward, diverging as they go, and some branch several times before they reach the wing margin (Fig. 20). Cross veins also occur, connecting the radiating main veins or their branches. Areas of membrane between veins are termed cells and where entirely surrounded by veins are called closed cells. These may be relatively few or many, according to the number of veins and their branches present. The arrangement and number of the chief veins and their branches are of importance in identifying insects.

There is usually a point or tip called the apex, somewhere along the margin of the wing, though frequently the outline is so rounded that the exact apex is uncertain. The front margin of the wing from where it joins the body to where the edge begins to turn backward (in an extended wing) is called the costa Wings are entirely absent in some groups of insects, and it is probable that these are the direct descendants of the earliest forms, before wings were developed. In other cases where they are absent this is associated with a parasitic life where wings might be a distinct disadvantage, or with peculiar habits which would render them useless or even inconvenient, and in such cases they appear gradually to have become lost. In the flies the hinder pair is modified, forming small structures not wing-like, called halteres.

The abdomen does not usually show great differences in its segments except those near the hinder end, which may be modified for various purposes. Generally a dorsal plate and a ventral plate are the only two skeletal plates evident in a segment. Small openings, usually a pair in each, or at least in most, of the segments are the openings of the breathing organs, and these also occur on some of the thoracic segments where they are ordinarily less noticeable than on the abdomen.

Legs are very rarely present on the abdomen in adult insects but are often found in the earlier stages (Fig. 21). At the end of the abdomen in the females of those insects which lay their eggs within objects is a combination of pieces known as an ovipositor. It usually consists of about three pairs of parts, long or short, slender or stout, as the case may be, for the purpose of making a hole or sawing a slit in the object in which the eggs are placed and in guiding the eggs into the hole thus made. In one group which has apparently changed its habits and no longer needs to make holes for egg laying, the ovipositor being unnecessary for this purpose, has been transformed into a sting. A pair of many-segmented, antenna-like structures, sometimes short, sometimes long, may occur at the end of the abdomen, and these are called cerci. They probably serve as organs of touch, and possibly also of smell in some cases.