In the adult Insect the integument or crust of the body is more or less hard or shell-like, sometimes, indeed, very hard, and on examination it will be seen that besides the divisions into segments and into dorsal, ventral, and pleural regions, there are lines indicating the existence of other divisions, and it will be found that by dissection along these lines distinct pieces can be readily separated. Each hard piece that can be so separated is called a sclerite, and the individual sclerites of a segment have received names from entomotomists. The sclerites are not really quite separate pieces, though we are in the habit of speaking of them as if such were the case. If an Insect be distended by pressure from the interior, many of the sclerites can be forced apart, and it is then seen that they are connected by delicate membrane. The structure is thus made up of hard parts meeting one another along certain lines of union—sutures—so that the original membranous continuity may be quite concealed. In many Insects, or in parts of them, the sclerites do not come into apposition by sutures, and are thus, as it were, islands of hard matter surrounded by membrane. A brief consideration of some of the more important sclerites is all that is necessary for our present purpose: we will begin with the head.
Fig. 49.—Capsule of head of beetle, Harpalus caliginosus: A, upper; B, under surface: a, clypeus; b, epicranium; c, protocranium; d, gula; e, facetted eye; f, occipital foramen; g, submentum; h, cavity for insertion of antenna.
The head is most variable in size and form; as a part of its surface is occupied by the eyes and as these organs differ in shape, extent, and position to a surprising degree, it is not a matter for astonishment that it is almost impossible to agree as to terms for the areas of the head. Of the sclerites of the head itself there are only three that are sufficiently constant and definite to be worthy of description here. These are the clypeus, the epicranium, and the gula. The clypeus is situate on the upper surface of the head-capsule, in front; it bears the labrum which may be briefly described as a sort of flap forming an upper lip. The labrum is usually possessed of some amount of mobility. The clypeus itself is excessively variable in size and form, and sometimes cannot be delimited owing to the obliteration of the suture of connexion with the more posterior part of the head; it is rarely or never a paired piece. Occasionally there is a more or less distinct piece interposed between the clypeus and the labrum, and which is the source of considerable difficulty, as it may be taken for the clypeus. Some authors call the clypeus the epistome, but it is better to use this latter term for the purpose of indicating the part that is immediately behind the labrum, whether that part be the clypeus, or some other sclerite; the term is very convenient in those cases where the structure cannot be, or has not been, satisfactorily determined morphologically.
In Figure 50 the parts usually visible on the anterior aspect of the head and its appendages are shown so far as these latter can be seen when the mouth is closed; in the case of the Insect here represented the bases of the mandibles are clearly seen (g), while their apical portions are entirely covered by the labrum, just below the lower margin of which the tips of the maxillae are seen, looking as if they were the continuations of the mandibles.
The labrum is a somewhat perplexing piece, morphologists being not yet agreed as to its nature; it is usually placed quite on the front of the head, and varies extremely in form; it is nearly always a single or unpaired piece; the French morphologist Chatin considers that it is really a paired structure.
Fig. 50.—Front view of head of field-cricket (Gryllus): a, epicranium; b, compound eye; c, antenna; d, post-: e, ante-clypeus; f, labrum; g, base of mandible; h, maxillary palpus; i, labial palpus; k, apex of maxilla.
The gula (Fig. 49, B d, and Fig. 47, z) is a piece existing in the middle longitudinally of the under-surface of the head; in front it bears the mentum or the submentum, and extends backwards to the great occipital foramen, but in some Insects the gula is in front very distant from the edge of the buccal cavity. The epicranium forms the larger part of the head, and is consequently most inconstant in size and shape; it usually occupies the larger part of the upper-surface, and is reflected to the under-surface to meet the gula. Sometimes a transverse line exists (Fig. 49, A) dividing the epicranium into two parts, the posterior of which has been called the protocranium; which, however, is not a good term. The epicranium bears the antennae; these organs do not come out between the epicranium and the clypeus, the foramen for their insertion being seated entirely in the epicranium (see Fig. 50). In some Insects there are traces of the epicranium being divided longitudinally along the middle line. When this part is much modified the antennae may appear to be inserted on the lateral portions of the head, or even on its under-side; this arises from extension of some part of the epicranium, as shown in Fig. 49, B, where h, the cavity of insertion of the antenna, appears to be situate on the under-surface of the epicranium, the appearance being due to an infolding of an angle of the part.
There is always a gap in the back of the head for the passage of the alimentary canal and other organs into the thorax; this opening is called the occipital foramen. Various terms, such as frons, vertex, occiput, temples, and cheeks, have been used for designating areas of the head. The only one of these which is of importance is the gena, and even this can only be defined as the anterior part of the lateral portion of the head-capsule. An extended study of the comparative anatomy of the head-capsule is still a desideratum in entomology. The appendages of the head that are engaged in the operations of feeding are frequently spoken of collectively as the trophi, a term which includes the labrum as well as the true buccal appendages.