NO. 6 



INSECT ABDOMEN SNODGRASS 



the cuticula in which other substances than chitin predominate. It 

 has been shown by Campbell (1929), for example, that the exocuticula 

 of Periplancta contains only about 22 per cent of chitin, while the 

 soft endocuticula contains about 60 per cent ; and according to 

 Kunike (1926) the wing covers of a May beetle contain by weight 

 75 per cent of nonchitinous substances, and those of a grasshopper 

 as much as 80 per cent. The writer, therefore, follows the suggestions 

 of Ferris and Chamberlin (1928) in designating the sclerites as areas 

 of sclerothation rather than of " chitinization." 



Sclerotization of the body wall usually produces definite plates in 

 the several segmental regions. According to the scheme of nomen- 

 clature adopted in this paper, a major segmental plate of the dorsum 



Fig. I. — Diagrams illustrating the theoretical primitive structure of the ap- 

 pendages and their relation to the body wall. 



A, cross section of a segment, showing the basis of each appendage (LB) 

 movable antero-posteriorly on the body segment by an axis (a-b) in a vertical 

 plane, and the telopodite' (Tlpd) movable dorso-ventrally at the coxo-trochan- 

 teral joint (ct). 



B, the basis of each appendage subdivided into a subcoxa (Sex) and coxa 

 (Cx) by a secondary joint with a vertical axis (c-d) ; the upper part of the 

 subcoxa forming the " pleuron " of the body segment. 



is a tergum (fig. i B, T), a corresponding plate of the venter is a 

 sternum (Stn), and a single plate or group of plates in the pleural 

 region is a pleuron (PI). Subdivisions or component elements of 

 these principal segmental plates then become tergites, sternites, and 

 pleurites, respectively, since the suffix ite grammatically can mean 

 only *' a part of " some larger unit designated by the stem of the word 

 to which it is appended. 



The plan for distinguishing and naming the segmental regions, the 

 major sclerites, and the subdivisions of major sclerites given above 

 is not generally followed ; bvit the writer has not found any nomen- 

 clatural scheme for these parts that is consistently applied, or that 

 adequately meets the situation. Some writers define the terms " terga " 

 and " sterna " as the segmental dorsal and ventral regions, and then 

 designate the principal plates of these regions the " tergites " and 



