GENERAL CHARACTERS OF THE EXO-SKELETON. 105 



with advancing maturity there is a tendency to return to a 

 simpler form of skeleton. 



A classif cation of the various sclerites indicative of their 

 morphological significance is not possible with our present 

 knowledge. It is not probable, for example, that any one of 

 the smaller plates which occur in the prosternal region of a 

 young imago in the Diptera represents the sternal plate of a 

 more simple somite, or the prosternum of an archaic insect, 

 nor is there any reason to regard the consolidated prosternum 

 of the adult imago, formed by the union of these several 

 sclerites, as homologous with the more simple prosternum 

 of a more generalised type. 



Generally, the sclerites which appear at the earliest period 

 of development are either superficial plates, which are sub- 

 sequently united by a continuous deposit of chitin on their 

 internal surface, or chitinized portions of inflected folds of 

 integument. In the former case, I shall term them exo-sclerites, 

 and in the latter endo-sclerites. The continuous deposit of 

 chitin by which these are united with each other I shall term 

 the scleral matrix ; and the sclerites formed of two or all these 

 elements compound sclerites; those which consist of only one 

 element simple ; and sclerites which are formed entirely from 

 the scleral matrix I shall term matrix sclerites. Lastly, certain 

 sclerites occur only in the more generalised forms of insects, 

 such, for example, are the bows of chitin which partially 

 surround the limbs of the Cockroach {Periplancta) ; these 

 are formed in folds of the syndesmotic integument between 

 the dorsal plate and the limb. I think it probable that 

 these belong to the limb rather than to the thorax, and that 

 they represent two limb joints between the coxa and the 

 trunk, corresponding to the two basal joints of the limb in some 

 Crustacea. If this be so, they are clearly indicative of an under- 

 lying archaic type. They are not present in the more highly 

 modified types of insect, and such sclerites may be termed 

 evanescent, as they tend to vanish in the higher forms. 



Apodemes. — This term was applied by Audouin to endo- 

 sclerites which form lever-like rods on which the muscles act. 



