The mandibles of the Cockroach are powerful, single-jointed[50] jaws, each of which is articulated by a convex “condyle” to the lower end of the epicranial plate, and again by a concave “ginglymus” to the clypeus. The opposable inner edges are armed with strong tooth-like processes of dense chitin, which interlock when the mandibles close; those towards the tip of the mandible are sharp, while others are blunt, as if for crushing. Each mandible can be moved through an angle of about 30°. A flexible chitinous flap extends from its inner border to the labrum. The powerful flexor of the mandible arises within the epicranial vault; its fibres converge to a chitinous tendon, which passes outside the central plate of the tentorium, and at a lower level through a fold on the lower border of the clypeus, being finally inserted near the ginglymus. A short flexor arises from the crus of the tentorium. The extensor muscle arises from the side of the head, passes through the fold formed by the lower end of the gena, and is inserted close to the outer side of the condyle of the mandible.
Fig. 20.—The Jaws, separated. Mn, mandible, seen from behind (to left) and front (to right); Mx' maxilla (first pair); Mx" labium, or second pair of maxillæ. The other letters as before. × 20.
The anterior maxillæ lie behind the mandibles, and like them are unconnected with each other. They retain much more of the primitive structure of a gnathite than the mandibles, in which parts quite distinct in the maxillæ are condensed or suppressed. The constituent pieces are seen in fig. [20]. There is a two-jointed basal piece, consisting of the cardo (ca) and the stipes (st). The cardo is a transverse plate bent upon itself, and enclosing muscles; it is attached to the outward-directed pedicel of the occipital frame, and carries the vertical stipes. To the side and lower end of the stipes is attached the five-jointed palp (pa), a five-jointed limb used in feeding and in exploration, while the lacinia (la) and galea (ga) are articulated to its extremity. The lacinia is internal and posterior to the galea; it is broad above, but narrows below to a bifid tooth of dense chitin; its inner surface is beset with a cluster of strong setæ. The galea is more flexible, and forms an irregular three-cornered prism with an obliquely truncated end, upon which are many fine hairs. A flexible and nearly transparent flap connects the inner edges of the stipes and cardo, and joins both to the labium. The muscles which move the bases of the maxillæ spring from the crura, central plate, and keel of the tentorium.
On the posterior surface of the head, below the occipital foramen, we find a long vertical flap, the labium, which extends downwards to the opening of the mouth. It represents a second pair of maxillæ, fused together in their basal half, but retaining elsewhere sufficient resemblance to the less modified anterior pair to permit of the identification of their component parts. The upper edge is applied to the occipital frame, but is neither continuous with that structure nor articulated thereto. By stripping off the labium upwards it may be seen that it is really continuous with the chitinous integument of the neck. The broad shield-like base is incompletely divided by a transverse hinge into an upper and larger piece, the submentum, and a distal piece, the mentum. To the mentum are appended representatives of the galeæ (here named paraglossæ) and laciniæ, while a three-jointed palp with an additional basal joint (distinguished as the palpiger) completes the resemblance to the maxillæ of the first pair.[51] In front of the labium, and lying in the cavity of the mouth is a chitinous fold of the oral integument, the lingua, which lies like a tongue in the floor of the mouth. The common duct of the salivary glands enters the lingua, and opens on its hinder surface. The lingua is supported by the chitinous skeleton represented in the figures of the salivary glands. (Chap. [vii]., infra.)
The epipharynx, which is a prominent part in Coleoptera and Diptera, is not recognisable in Orthoptera.
Functions of the Antennæ and Mouth-parts.
We must now shortly consider the functions of the parts just described. The antennæ have long been regarded as sense-organs, and even the casual observer can hardly fail to remark that they are habitually used by the Insect to gain information concerning its immediate surroundings. Long antennæ, such as those of the Cockroach, are certainly organs of touch, but it has been much disputed whether they may not also be the seat of some special sense, and if so, what that sense may be. Several authors have found reason to suppose that in the Insect-antenna resides the sense of hearing, but no evidence worth the name is forthcoming in favour of this view. Much better support can be found for the belief that the antenna is an olfactory organ,[52] and some experiments which seem conclusive on this point will be cited in a later chapter.