The Insect World: Being a Popular Account of the Orders of Insects

With these eyes insects can only see such objects as are at a short distance. Of what use then can stemmata be to insects also provided with compound eyes? It has been remarked that most insects having this arrangement of eyes feed on the pollen of plants, and it has been surmised that the stemmata enable them to distinguish the parts of the flowers. The antennae, commonly called horns, are two flexible appendages, of very variable form, which are joined to different parts of the head, and are iUways two in number. The joints of which they are madt

up have the power of motion, which enables the insect to move them in any direction. The antennae consist of three parts: the basal joint, commonly distinguished by its form, length, and colour; the club, formed by a gradual or sudden thickening of the terminal joints, of which the number, form, and size present great variations; lastly, the stalk, formed by all the joints of the antennae, except the basal, when no club exists, and in case of the existence of a club, of all those between it and the basal one.

We give as examples the antennae of two beetles, one of the genus Asida, the other of the genus Zygia (Figs. 3 and 4). Insects, for the most part, while in repose, place their antennae on their backs, or along the sides of the head, or even on the thorax. Others are provided with cavities in which the antennae repose either wholly or in part.

During their different movements, insects move their antennae more or less, sometimes slowly and with regularity, at other times in all directions. Some insects impart to their antennae a perpetual vibration. During flight they are directed in front, perpendicular Fig. 3. Fig. 4. to the axis of the body, or else they Antenna of a Antenna of repose on the back. species of Asida. Zygia oblonga.

What is the use of the antennae, resembling as they do, feathers, saws, clubs, &c.? Everything indicates that these organs play a very important part in the life of insects, but their functions are imperfectly understood. Experience has shown that they only play a subordinate part as feelers, and have nothing to do with the senses of taste or smell. There is no other function for them to fulfil, except that of hearing.

On this hypothesis the antennae will be the principal instruments for the transmission of sound-waves. The membrane at their base represents a trace of the tympanum which exists among the higher animals. This membrane then will have some connection with an auditory nerve. The mouth of insects is formed after two general types, which correspond to two kinds of requirements. It is suited in the one case to break solid substances, in the other to imbibe hquids.

At first sight there seems no similarity between the mouth of a biting insect and of one living by suction. But on examination it is found that the parts of the mouth in the one are exactly analogous to the same parts in the other, and that they have only modifications suiting them to the different purposes which they have to fulfil.

The mouth of a biting insect is composed of an upper lip, a pair of mandibles, a pair of jaws, and a lower lip (Fig. 5).

The lower lip and the jaws carry on the outside certain appendages or filaments which have received the name of palpi.

When speaking of sucking insects, and in general of the various

Fig. 5. — Mouth of a masticating insect. Fig. 6. — Thorax of Acrocinus longimanus (a beetleV

orders of insects, we shall speak more in detail of the various parts of the mouth. The thorax (Fig. 6), the second primary division of the body of insects, plays almost as important a part as the head. It consists of three segments or rings, which are in general joined together — the prothorax, the mesothorax, and the metathorax, each of which bears a pair of legs. The wings are attached to the two posterior segments.

All insects have six true legs. There is no exception whatever to this rule, though some may not be developed.

From the segments to which they are attached, the legs are called anterior, posterior, and intermediate. The legs are composed of four parts: the trochanter, a short joint which unites the thigh to the body; the thigh ox femur; the tibia, answering to the shank in animals; and the tarsus, or foot, composed of a variable number of pieces placed end to end, and called the p/ia/anges.

We take as examples the hind leg of a Heterocerus (Fig. 7). and the front leg of a Zophosis (Fig. 8) (genera of beetles).

INTRODUCTION.

We shall not dwell on the different parts, as they perform functions which will occupy us later, when speaking of the various species of the great class of insects.

The functions which the legs of insects have to perform consist in walking, swimming, or jumping. In walking, says M. Lacor daire, insects move their legs in different ways. Some move their six legs successively, or only two or three at a time without distinction, but never both legs of the same pair together, consequently one step is not the same as another. The walk of insects is sometimes very irregular, especially when the legs are long; and they often hop rather than walk. Big- Others have one kind of step, Fig. 7. and walk very regularly. They HindHeterocerus.leg of a Front Teg of a commence by moving the pos- Zophosis. terior and anterior legs on the same side and the intermediate ones on the opposite side. The first step made, these legs are put down, and the others raised in their turn to make a second.

Running does not change the order of the movements, it only makes them quicker — very rapid in some species, and surpassing in proportion that of all other animals; but in others the pace is slow. Some insects rather Fig. 9. — Posterior leg of a jumping insect. crawl than walk. In swimming, the posterior legs play the principal parti; The other legs striking the water upwards or downwards, produce an upward or downward motion. The animal changes its course at will by using the legs on one side only, in the same way as one turns a rowing boat with one oar without the aid of a rudder. Swimming differs essentially from walking, for the foot being surrounded

by a resisting medium, the legs on both sides aie moved al the same time. The act of jumping is principally performed by the hind legs. Insects which jump have these legs very largely developed, as in

Fig. 9. When about to jump they bring the tibia into contact ^ith the thigh, which is often furnished with a groove to receive it, having on each side a row of spines. The leg then suddenly straightens like a spring, and the foot being placed firmly on the ground, sends the insect into the air, and at the same time propels forward. The jump is greater in proportion as the leg is longer.

To treat here in a general manner of the wings of insects would be useless. We shall refer to them at length in their proper place, when treating of the various types of winged insects.

In the perfect insect the abdomen does not carry either the wings or the legs. It is formed of nine segments, which are without appendages, with the exception of the posterior ones, which often carry small organs differing much in form and function. These are saws, probes, forceps, stings, augers, &c We shall consider these different organs in their proper places.

With vertebrate animals, which have an interior skeleton suited to furnish points of resistance for their various movements, the skin is a more or less soft covering, uniformly diffused over the exterior of the body, and intended only to protect it against external injury. In insects the points of resistance are changed from the interior to the exterior. The skin is altered by Nature to fit it to this purpose. It is hard, and presents between the segments only membranous intervals, which allow the hard parts to move in all directions.

We are examining a perfect insect; we have glanced at its skeleton, and the different appendages which spring from it. The principal organs which are contained in the body remain to be examined.

We will first study the digestive apparatus. This apparatus consists of a lengthened tubular organ, swollen at certain points, forming more or less numerous convolutions, and provided with two distinct orifices. This alimentary canal is always situated in the median line of the body, traverses its whole length, and is at first surrounded by, and then passes above, the nervous ganglia.*

In its most complicated form the alimentary canal is composed of an (esophagus, or gullet, of a crop, of a gizzard, of a chylific ventricle or stomach, a small intestine, a large intestine, divers appendages, salivary, biliary, and urinary glands. The oesophagus is often not

• Ganglion — a mass, literally a knot, of nervous matter.

INTRODUCTION.

wider than a hair, and part of it in many species is enlarged into a pouch, which is called the crop, because it occupies the same position, and performs analogous functions with that organ in birds. It is enough to say that the food remains there some time before passing on to the other parts of the intestinal canal, and undergoes a certain amount of preparation. It is in the gizzard, when one exists, that the food, separated by the masticatory organs of the mouth, undergoes another and more complete grinding. Its structure is suited to its office. It is, in fact, very muscular, often half cartilaginous, and strongly contractile. Its interior walls are provided with a grinding apparatus, which varies according to the species, and consists of teeth, plates, spines, and notches, which convert the food into pulp. It only exists among insects which live on solid matters, hard vegetables, small animals, tough skin, &c. This apparatus is absent in sucking insects and those which live on soft substances, such as the pollen of flowers, &c.

The chylific ventricle or stomach is never absent \ it is the organ which performs the principal part in the act of digestion. Fig. lo. — Digestive apparatus of Carabus auratus.

Two kinds of appendages belong to the chylific ventricle, but only in certain families. The first are papillae, in the form of the fingers of a glove, which bristle over the exterior of this organ, and in which it is believed that the food begins to be converted into chyle. The second are casca, and larger and less numerous. They have been considered as secretory organs, answering to the pancreas in vertebrate animals.

58*

Fig. 10, which represents the digestive apparatus of Carabus aurattis, a common beetle, presents to the eyes of the reader the different organs of which we are speaking.

A is the mouth of the insect, b the oesophagus, c the crop, d the gizzard, e the chylific ventricle, f and g the small and large intestines, and h the anus. It is not necessary to consider the other parts of the alimentary canal in insects, but only to refer to some of the appendages of this apparatus. The salivary glands pour into the digestive tube a liquid, generally colourless, which, from the place where it is secreted, and its alkaline nature, corresponds to the saliva in vertebrate animals. It is this liquid which comes from the tongue of sucking insects in the form of drops. These glands are always two in number. Their form is as variable as complicated. The most simple is that of a closed flexible tube, generally rolled into a ball, and opening on the sides of the oesophagus. At the posterior extremity of the chylific ventricle are inserted a variable number of fine tubes, usually elongated and flexible, and terminating in culs-dt-sac at one end. Their colour, which depends on the liquid they may contain, is sometimes white, but more frequently brown, blackish, or green. They appear to be composed of a very slight and delicate membrane, as they are very easily torn, and nothing is more difficult than to unroll and to disengage them from the fatty or other ti':sues by which they are enveloped. The function of these vessels is uncer- Fig. II. — Posterior extremity of the j-oin Cuvier and Leon Dufour supposed chylific ventricle,Malpighiansurroundedvessels. by the them'■"•^^^* to be analogous to the liver, and on