The mouth is constructed for biting, and consists of three pairs of jaws: the mandibles, the maxillae, and the labium. Each mandible is of one piece, triangular, attached by two condyles to the head. Its inner edge has at the base a grinding surface or mola, and in front of the mola and at the tip strong teeth. Neither teeth nor mola are fashioned alike in the two mandibles. They appear to interlock more or less perfectly. Each maxilla is composed of (1) a basal part, the cardo, placed horizontally and articulating with the head; (2) a stipes, which is placed vertically, and bears (3) a five-jointed palpus on its outer edge articulated to a basal piece representing the palpiger of some Insecta; (4) a hood-shaped galea in front, which is articulated by a distinct joint to the stipes; and (5) a lacinia. This lacinia is attached to the stipes by an imperfect joint; it ends with two stout conical teeth, at the base of which there rises on the internal edge a finger-shaped process, terminated by three or four recurved blunt teeth. Its inner edge is beset with stout hairs. The labium closes in the mouth posteriorly.
It consists (1) of a large basal sub-mentum, to the fore-edge of which is moveably articulated (2) a mentum; (3) of two three-jointed palpi attached each to a basal prominence, representing a palpiger borne on the external angle of the mentum; and (4) a ligula. The ligula is divided almost completely by a median cleft, at the base of which is a small triangular piece. Each half of the ligula bears two processes articulated to it: an outer, the paraglossa or lamina externa of Gerstacker (= galea ?), and an inner, the smaller of the two, the lamina interna of the same author (= lacinia?). A labium of this character is to be regarded as primitive. It is found in many Orthoptera, in Termesi Perla, Aeschna, and the incomplete stages of Ephemeridae, and shows clearly the origin of the labium from a pair of limbs fused medianly. The antennae are processes of the pro-cephalic lobes: while the mandibles, maxillae, and labium belong to three fused segments distinct in the embryo. The head of the insect is consequently often regarded as formed of four segments, one prae-oral and three post-oral
The labrum (supra) is formed in some Insecta by the fusion of two processes. They are, however, hardly to be regarded as limb-rudiments. The antennae are not modified limbs like the remaining appendages of the head, and are compared by Balfour with the paired processes of the prostomium in the Chaetopod Polychaeta. The embryonic rudiments of the labium are extremely large, and in some Insecta are turned backwards parallel to the thoracic limbs.
The epipharynx, which lies on the internal surface of the labrum, is not traceable in Periplaneta as a distinct process. The hypopharynx (= lingua), on the contrary, is very large. It lies on the inner or oral surface of the labium, and the salivary duct opens towards the base of its posterior surface.
The thorax is composed of three limb-bearing segments - pro-, meso-, and meta-thorax. A distinct neck1 intervenes between the thorax and the head. The pro-thorax has a large tergal (dorsal) plate, the pro-notum, which overlaps the head in front and the meso-thorax behind. Its sternal element is small, as are the two lateral elements, epi-merum, and episternum, which lie in front of the articulation of the limb. These parts are larger in the two segments behind. The tergal elements (meso- and meta-notum) of the meso- and meta-thorax are sub-equal in the Cockroach. In most Orthoptera, some Neuroptera, the meta- is larger than the meso-thorax, a condition the reverse of what obtains in most Insecta. The limbs increase in length progressively from before backwards. Each limb is made up of ten joints - a large coxa articulating with the thoracic ring, a small trochanter followed by a femur, a tibia, and a tarsus of six joints. The last tarsal joint is minute, and furnished with two claws, as is usual in Insecta that possess claws. The meso- and meta-notum carry wings. Those of the first pair in the male are stiff, semi-opaque, and coriaceous in texture. They act as wing-covers or tegmina, and extend so far as to cover the fifth abdominal somite.
Those of the second pair are membranous and large: each wing has an anterior triangular area of stiff texture and a thin posterior area. In repose, the base of the posterior portion closes like a fan, and then the remainder of the wing is folded once lengthwise, the edge of the fold being internal, the anterior firmer area of the wing lying uppermost and protecting the thinner posterior area. In some Cockroaches the tip of the wing is folded transversely. The base of attachment of both pairs of wings is broad. In the female there is a pair of short tegmina, while the hind pair of wings is represented only by two small triangular areae of the meta-notum marked by a few ridges. Certain Cockroaches, e.g. Polyzosteria, are wingless in both sexes.
1 In the neck there are certain chitinous pieces, or cervical selerites, one dorsal and median, with a longitudinal depression, two ventral and transverse, and two lateral. The first and second elements are inconspicuous, the third large. They are placed obliquely. Their significance is doubtful. See Huxley, Anatomy of Invertebrated Animals, p. 403.
The abdomen is flattened dorso-ventrally. Its outline is somewhat different in the two sexes. It is made up of a number of distinct segments or somites, composed each of a dorsal tergum and a ventral sternum connected at their margins by a soft pleural membrane, hidden by the projecting free edges of tergum and sternum alike. The portions of the terga and sterna exposed to light and air are hard and dark, but the membranes which connect successive terga and sterna are colourless and pliable. The first seven terga in both sexes are well developed, the 8th and 9th very narrow, and generally hidden by the 7th; the 10tn is triangular and projecting. The 18t sternum is rudimentary, and represented by a narrow band at the base of the 2nd sternum. This and the succeeding sterna to the 7tn inclusive are well developed in both sexes. In the male the 8th and 9th sterna are external, and the last named has articulated to its free margin a pair of unjointed setose styles, while the 10th sternum is internal, and has developed in connection with it variously shaped copulatory processes, which surround the aperture of the vas deferens.