I04 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



modifications. In the simpler or more generalised forms each 

 somite is protected by a simple annular sclerite, or by a dorsal 

 and a ventral semi-annulus. The consolidation of the cuticular 

 layers in the higher types commences in the inflected ridges 

 corresponding with the primary and secondary sutures, or in 

 relation with the insertion of the more powerful muscles; hence 

 numerous secondary sclerites occur. These are frequently 

 distinct in the adult nymph or young imago, but are often con- 

 cealed in the adult imago by large deposits of chitin between 

 them and the hypodermis. 



A very important fact, to which I would draw attention, 

 is the close resemblance which exists between an e.xternal 

 skeleton developed directly from the epiblast of the embryo 

 and an external skeleton formed entirely from the epiblast 

 of imaginal discs ; the secondary sutures in both are as closely 

 related as the primary ones. Whether they first appear 

 in the embryo or in the nymph, they represent the same 

 functional adaptation of the skeleton to the needs of the 

 organism. The same ridges and syndesmoses appear on the 

 wing-bearing segments, whilst no traces of them appear on 

 the wingless prothorax or abdominal somites. 



If, as I have already stated,* it is probable that the larva and 

 nymph states in the Metabola are interpolated stages of develop- 

 ment, we might expect to find a greater divergence of structure 

 from that of the primitive type in the nymph and young imago, 

 with a tendency to a return towards the primitive type in the 

 more mature insect ; and we actually see a more complex 

 condition of the skeleton in the young imago than in the adult 

 insect. In the vertebrate skeleton each bone, as a rule, is 

 formed from several ossific centres, which subsequently unite 

 with each other. In the young insects there are often 

 several sclerites, which are afterwards united to form a single 

 plate, but the process of union is different : the component 

 sclerites are not usually fused by the gradual growth of each 

 and their subsequent union, but by a continuous deposit of 

 chitinous laminae on the internal surface of the whole, so that 

 * Page 19. 



