In an early stage every somite has its tergal region divided into lateral halves, owing to the late completion of the body on this side. Traces of this division may survive even in the imago. There is often a conspicuous dorsal groove in the thoracic terga, and at the time of moult the terga split along an accurately median line (see fig. [12]).
Additional pieces may be developed between the terga and sterna, and these have long been termed pleural.[38] There may be, for example, single stigmatic plates, as in the abdomen of the Cockroach, pieces to support the thoracic legs, and pieces to support the wings; but the number and position of these plates depends upon their immediate use, and their homologies become very uncertain when Insects of different orders are compared. In general, the pleural elements of the segment are late in development, variable, and highly adaptive.
Somites of the Cockroach.
The exoskeleton of the Cockroach is divisible into about seventeen segments, which are grouped into three regions, as follows:—
| Head |  | Procephalic lobes | 3 | |
| Post-oral segments | ||||
| Thorax | 3 | [39] | ||
| Abdomen | 11 | |||
| — | ||||
| 17 | ||||
| — | ||||
It is a strong argument in favour of this estimate that many Insects, at the time when segmentation first appears, possess seventeen segments.[40] The procephalic lobes, from which a great part of the head, including the antennæ, is developed, are often counted as an additional segment.[41]
The limbs, which in less specialised Arthropoda are carried with great regularity on every segment of the body, are greatly reduced in Insects. Those borne by the head are converted into sensory and masticatory organs; those on the abdomen are either totally suppressed, or extremely modified, and only the thoracic limbs remain capable of aiding in locomotion.
The primitive structure of the Arthropod limb is adapted to locomotion in water, and persists, with little modification, in most Crustacea. Here we find in most of the appendages[42] a basal stalk (protopodite), often two-jointed, an inner terminal branch (endopodite), and an outer terminal branch (exopodite), each of the latter commonly consisting of several joints. It does not appear that the appendages of Insects conform to the biramous Crustacean type, though the ends of the maxillæ are often divided into an outer and an inner portion.
We shall now proceed to describe, in some detail, the regions of the body of the adult Cockroach.