stemmata) are common, they do not exist in all the orders of insects. They are generally round, and more or less convex and black, and there are three in the majority of cases. When there is this number they are most frequently placed in a triangle behind, and at a greater or less distance from the antennæ. Under the cornea, which varies in convexity, is found a transparent, rather hard, and nearly globular body, which is the true crystalline resting on a mass, which represents the vitreous body. This vitreous body is enclosed in an expansion of the optic nerve. Besides these, there is a pigment, most frequently red-brown, sometimes black, or blood-red. The organisation of these eyes is analogous to the eyes of fishes, and their refractive power is very great.

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 antennæ, commonly called horns, are two flexible appendages, of very variable form, which are joined to different parts of the head, and are always two in number. The joints of which they are made up have the power of motion, which enables the insect to move them in any direction.

The antennæ 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 antennæ, 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 antennæ of two beetles, one of the genus Asida, the other of the genus Zygia (Figs. 3 and 4).

Fig. 3. Antenna of a species of Asida.		Fig. 4. Antenna of Zygia oblonga.

Insects, for the most part, while in repose, place their antennæ on their backs, or along the sides of the head, or even on the thorax. Others are provided with cavities in which the antennæ repose either wholly or in part.

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

What is the use of the antennæ, 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 antennæ 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 liquids.

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).

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

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 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 or 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 phalanges.

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

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.