NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 5 



sometimes found in the pleural membrane of the abdomen appear to 

 be detached parts of tergites or sternites and hence to be latero- 

 tergites or laterosternites rather than true pleurites (see Abdomen). 



Segmental plates: Sclerotization of the body wall cuticle is highly 

 variable in different parts of the insect according to the functional 

 requirements. On the abdomen typically the sclerotization forms a 

 back plate or tergmn and a ventral plate or sternum separated on the 

 sides by membranous areas to allow for the movements of respira- 

 tion. On the thorax the support of the wings above and the legs 

 below necessitates the presence of a strong lateral or pleural sclero- 

 tization on each side. The head, though it includes at least four 

 primary body segments, is continuously sclerotized above and on the 

 sides to form a rigid cranium for the support of the antennae and 

 the mouthparts. 



Since the skeleton of each section of the insect's body is adapted 

 to the functions of the particular part, it is difficult to deduce what 

 the sclerotization of a primitive segment may have been. The centi- 

 pedes with their undifferentiated bodies have on each segment a dis- 

 tinct dorsal and a ventral plate with the legs arising from flexible 

 pleural areas between. This condition, however, is simply an adap- 

 tation to the centipede's way of locomotion and is not necessarily 

 primitive. On the other hand, in the lower Crustacea, such as Anas- 

 pides, the back plates are continuous over the dorsum and down on 

 the sides to the leg bases attached on the tergal margins. There are 

 here no differentiated pleural plates. Among the Malacostraca, in 

 the Mysidaceae the carapace covers only a part of the thorax, the 

 segments behind it carry the legs on the lower margins of the terga, 

 but where the carapace cuts through the back, the leg-carrying parts 

 of the terga are cut off and are called pleural plates. The so-called 

 pleural plates are here, therefore, only lateral parts of the tergal 

 plates. Finally, in the diplopods the segments are continuous rings. 

 It is clear, therefore, that there is no primitive basic pattern of seg- 

 ment sclerotization, nothing comparable to the evolution of the bony 

 skeleton of vertebrates, among the arthropods. An original wormlike 

 creature probably had a soft cuticle which has been variously sclero- 

 tized according to the needs in each group and according to the func- 

 tional demands in each segment of the body. 



The sclerotized cuticle also becomes variously reinforced by linear 

 inflections that form strengthening ridges on the inner surface. On 

 the external surface these appear as narrow grooves or sulci, long 

 erroneously called "sutures" in entomological terminology. The sulci 



