NO. I INSECT THORAX SNODGRASS 7 



rather than with the one before it, is decided by the fact that the 

 animal already possesses cephalization. In order to retain the power 

 of cephalic movement when the body wall becomes hardened, the 

 segments must remain capable of being drawn forward by the con- 

 traction of their muscles, and this can be accomplished best if the 

 flexible region of each segment is its posterior part. Hence, each 

 intersegmental, muscle-bearing ridge lias become continuous with the 

 segmental plate behind it, and usually only a narrow lip extends 

 forward of the ridge and its suture to connect with the preceding 

 membranous area. 



Each dorsal or ventral plate of the body wall, fcrginn or sternum. 

 now includes: (i) a segmental sclerite of varying extent, the pri- 

 mary tergum or sternum (fig. 2 B, T, 6") ; (2) an anterior inter- 

 segmental part, consisting of an internal, transverse, sub-marginal 

 ridge, the antecosta {Ac), marked externally by a corresponding 

 antecostal suture {ac) ; and (3) a narrow anterior marginal lip, or 

 precosta (Pc), belonging to the segment preceding. In the posterior 

 part of each segment, behind the tergum and sternum, is a circular 

 membranous area (Mb). In an animal thus constructed, the mem- 

 branous rings of the body wall are its movable joints, and they are 

 called the " intersegmental membranes " ; the longitudinal muscles 

 have become intersegmental in function, since they extend from the 

 antecosta of one plate backward to the antecosta of the plate fol- 

 lowing. But, a body division of this kind is clearly a secondary seg- 

 mentation. The antecostal ridges, still carrying the muscle attach- 

 ments, mark the limits of the primitive segments. All adult insects 

 with hard plates in their walls have a secondary segmentation ; the 

 soft-bodied larvae of some insects, such as caterpillars, grubs, maggots, 

 retain a primary segmentation. This difference in the segmental limits 

 between larval and adult insects, and the fact that the membranous 

 " intersegmental " rings of adult insects are the posterior parts of the 

 true segments, has already been noted by Janet (1898), who says: 

 " The name intersegmental membrane generally given to such a mem- 

 brane, justified by its physiological function, is, however, inexact 

 from a morphological standpoint." 



Arthropods in general are characterized by another feature of 

 their outer organization, and this is the telescoping of their segments, 

 each segment being partially retracted into the one before it (fig. 2 C) . 

 This condition follows naturally from the relation of the muscle 

 attachments to the segmental plates and to the flexible membranous 

 areas. As a result, each tergum or sternum usually ends posteriorly in 



