THE INTEGUMENT OF THE THORAX. 61 



The lateral plates of this segment support the head, I hare named 

 them condyles (Plate V, Fig. 9). The condyle is very hard and opaque. 

 When seen from beneath it is almost triangular, with the anterior angle 

 prolonged into a rounded protuberance, which fits into tho cup-shaped 

 depression at the side of the occipital foramen, Seen from within it has 

 an irregular form, due to the turning in of its edges. A process projects 

 from its posterior external angle. Numerous muscles are inserted into it, 

 which regulate the movements of the head. 



The remainder of the thorax is composed of three segments, 

 called the Pro-thorax, the Meso-thorax, and the Meta-thorax, 

 They are all visible externally beneath, but above the whole 

 pro-thorax and meta-thorax are concealed by the meso-thorax ; 

 laterally a small part of the pro-thorax and meta-thorax only is 

 visible. Each consists of four plates, except the meso-thorax, 

 in which the lateral plate is divided vertically into two. The 

 inferior plates are called sterna, the lateral plates episterna, and 

 the dorsal plates the terga, of their respective segments. The 

 edges of each plate are turned in and form ridges and processes 

 in the inside of the thorax, and the terga of both the meso- and 

 meta-thorax are divided by similar ridges into four parts called 

 respectively from before backward, the pree-scutum, the scutum, 

 the scutellum, and the post-scutellum. 



It is a fact worthy of remark that, although the tendency of 

 the coalesence of segments is, that they unite first in the dorsal 

 region, so in the dorsal region we find the greatest tendency for 

 a single segment to become split up into several distinct parts. 

 This is doubtless, in both cases, due to the fact that development 

 commences in the embryo at the ventral surface, and the number 

 of segments is there first determined by the number of distinct 

 spots from which development proceeds. As the segments grow 

 up towards the dorsum, they either split or coalesce with each 

 other. 



The back of the thorax is almost entirely formed by the tergum 

 of the meso-thorax. That of the pro-thorax is reduced to little 

 more than a mere rim around the superior part of the anterior 



