As the maxillae and labium are largely used by taxonomists in the systematic arrangement of the mandibulate Insects, we give a figure of them as seen in Coleoptera, where the parts, though closely amalgamated, can nevertheless be distinguished. This Fig. 52 should be compared with Fig. 51.
In speaking of the segments of the body we pointed out that they were not separate parts but constituted an uninterrupted whole, and it is well to remark here that this is also true of the gnathites. Although the mouth parts are spoken of as separate pieces, they really form only projections from the great body wall. Fig. 51, B, shows the intimate connexion that exists between the maxillae and labium; the continuity of the mandibles with the membrane of the buccal cavity is capable of very easy demonstration.
The head bears, besides the pieces we have considered, a pair of antennae. These organs, though varying excessively in form, are always present in the adult Insect, and exist even in the majority of young Insects. They are very mobile, highly sensitive organs, situate on or near the front part of the head. The antennae arise in the embryo from the procephalic lobes, the morphological import of which parts is one of the most difficult points connected with Insect embryology.
The eyes of Insects are of two sorts, simple and compound. The simple eyes, or ocelli, vary in number from one to as many as eighteen or twenty; when thus numerous they are situated in groups on each side of the head. In their most perfect form, as found in adult aculeate Hymenoptera, in Orthoptera and Diptera, ocelli are usually two or three in number, and present the appearance of small, perfectly transparent lenses inserted in the integument. In their simplest form they are said to consist of some masses of pigment in connexion with a nerve.
Fig. 53.—Two ommatidia from the eye of Colymbetes fuscus, × 160. (After Exner.) a, Cornea; b, crystalline cone; c, rhabdom; d, fenestrate membrane with nerve structures below it; e, iris-pigment; f, retina-pigment.
The compound, or facetted, eyes are the most remarkable of all the structures of the Insect, and in the higher and more active forms, such as the Dragon-flies and hovering Diptera, attain a complexity and delicacy of organisation that elicit the highest admiration from every one who studies them. They are totally different in structure and very distinct in function from the eyes of Vertebrata, and are seated on very large special lobes of the brain (see Fig. 65), which indeed are so large and so complex in structure that Insects may be described as possessing special ocular brains brought into relation with the lights, shades, and movements of the external world by a remarkably complex optical apparatus. This instrumental part of the eye is called the dioptric part in contradistinction from the percipient portion, and consists of an outer corneal lens (a, Fig. 53), whose exposed surface forms one of the facets of the eye; under the lens is placed the crystalline cone (b), this latter being borne on a rod-like object (c), called the rhabdom. There are two layers of pigment, the outer (e), called the iris-pigment, the inner (f), the retinal-pigment; underneath, or rather we should say more central than, the rhabdoms is the fenestrate membrane (d), beyond which there is an extremely complex mass of nerve-fibres; nerves also penetrate the fenestrate membrane, and their distal extremities are connected with the delicate sheaths by one of which each rhabdom is surrounded, the combination of sheath and nerves forming a retinula. Each set of the parts above the fenestrate membrane constitutes an ommatidium, and there may be many of these ommatidia in an eye; indeed, it is said that the eye of a small beetle, Mordella, contains as many as 25,000 ommatidia. As a rule the larvae of Insects with a complete metamorphosis bear only simple eyes. In the young of Dragon-flies, as well as of some other Insects having a less perfect metamorphosis, the compound eyes exist in the early stages, but they have then an obscure appearance, and are probably functionally imperfect.
In the interior of the head there exists a horny framework called the tentorium, whose chief office apparently is to protect the brain. It is different in kind according to the species. The head shows a remarkable and unique relation to the following segments. It is the rule in Insect structure that the back of a segment overlaps the front part of the one following it; in other words, each segment receives within it the front of the one behind it. Though this is one of the most constant features of Insect anatomy, it is departed from in the case of the head, which may be either received into, or overlapped by, the segment following it, but never itself overlaps the latter. There is perhaps but a single Insect (Hypocephalus, an anomalous beetle) in which the relation between the head and thorax can be considered to be at all similar to that which exists between each of the other segments of the body and that following it; and even in Hypocephalus it is only the posterior angles of the head that overlap the thorax. Although the head usually appears to be very closely connected with the thorax, and is very frequently in repose received to a considerable extent within the latter, it nevertheless enjoys great freedom of motion; this is obtained by means of a large membrane, capable of much corrugation, and in which there are seated some sclerites, so arranged as to fold together and occupy little space when the head is retracted, but which help to prop and support it when extended for feeding or other purposes. These pieces are called the cervical sclerites or plates. They are very largely developed in Hymenoptera, in many Coleoptera, and in Blattidæ, and have not yet received from anatomists a sufficient amount of attention. Huxley suggested that they may be portions of head segments.
Fig. 54.—Extended head and front of thorax of a beetle, Euchroma: a, back of head; b, front of pronotum; c, chitinous retractile band; d, cervical sclerites.