| Crayfish. | Cockroach. |
| Antennæ. | |
| —— | —— |
| Eyestalks. | |
| Antennules. | |
| Antennæ. | |
| Mandibles. | Mandibles. |
| Maxillæ (1). | Maxillæ (1). |
| Maxillæ (2). | Maxillæ (2). |
| —— | —— |
| Maxillipeds (1). | Thoracic Legs (1). |
| Maxillipeds (2). | Thoracic Legs (2). |
| Maxillipeds (3). | Thoracic Legs (3). |
Neck.
The neck is a narrow cylindrical tube, with a flexible wall strengthened by eight plates, the cervical sclerites, two of which are dorsal, two ventral, and four lateral. The dorsal sclerites lie immediately behind the head (fig. [14]); they are triangular, and closely approximated to the middle line. The inferior plates (fig. 27) resemble segments of chitinous hoops set transversely, one behind the other, rather behind the dorsal sclerites, and close behind the submentum. There are two lateral sclerites on each side of the neck (fig. 27), a lower squarish one, which is set diagonally, nearly meeting its fellow across the ventral surface, and an oblong piece, closely adherent to the other, which extends forwards and upwards towards the dorsal side.
Thorax.
Fig. 27.—Ventral Plates of Neck and Thorax of Male Cockroach. I, prosternum; II, mesosternum; III, metasternum. × 6.
The elements of the thoracic exoskeleton are simpler in the Cockroach than in Insects of powerful flight, where adaptive changes greatly obscure the primitive arrangement. There are three segments, each defended by a dorsal plate (tergum) and a ventral plate (sternum). The sterna are often divided into lateral halves. Of the three terga the first (pronotum) is the largest; it has a wide free edge on each side, projects forwards over the neck, and when the head is retracted, covers this also, its semi-circular fore-edge then forming the apparent head-end of the animal. The two succeeding terga are of nearly equal size, and each is much shorter than the pronotum, contrary to the rule in winged Insects.[67]