40 PROC. ENT. SOC. WASH., VOL. 20, NO. 3, MAR., 1918 



their many invaluable suggestions which have helped to clear 

 up certain of the more obscure phases of the present investigation. 

 I would also use this opportunity of expressing my gratitude to 

 Dr. N. Banks for the loan of Panorpid and Raphidian larvae 

 which have been of great value from the standpoint of a compara- 

 tive study of the sclerites, and without which the present paper 

 would have been very incomplete. As it is, the lack of material 

 has made it impossible to treat of certain larvae, but enough of 

 the principal types have been discussed, to furnish a basis for a 

 further study of the various modifications met with in the different 

 orders of insects. 



Sternal Subdivisions 



In the prothorax of the more primitive representatives of the 

 Plecoptera, such as Capnia (fig. 14) there occur six sternal scler- 

 ites, or sternites, is, ps, bs, fs, pfs, and ss, and traces of certain 

 of these sternites are found in a number of insects including 

 both adults and larval forms. 



The anteriormost sternal plate is of figure 14, called the inter- 

 sternite, is situated very close to the labium, and may possibly 

 be a posterior plate of the labial segment, rather than the ante- 

 riormost plate of the prothorax a question which can be best 

 settled by a study of the embryonic development of this region. 

 It is usually small and unimportant, and is retained in but few 

 larvae such as those of the Neuropteroid insects Corydalis (fig. 4, is) 

 and the Raphididae (fig. 6, is), and in certain rather primitively 

 organixed Coleopterous larvae such as Pyrochfoa (fig. 7, is), etc. 



The second sternite ps of figure 14, called the presternite, is 

 usually situated between the lateral cervical plates Ic. In most 

 cases, the lateral cervicals Ic remain distinct, as in figures 14, 



16, 17, 23, 24, etc., but in the Pyrochroid larva shown in figure 



17, they have apparently united with the presternite to form the 

 plate ps of figure 7, which is thus composed of plates ps and 

 Ic of figure 14. At any rate, plate ps of figure 7 includes a much 

 greater area than plate ps of figure 14. Similarly, in the Neurop- 

 teroid larvae such as Corydalis (fig. 4) and the Raphididae (fig. 

 6) the jugular plate, ps, or narrow transverse band in the neck 

 region contains a much greater area than the plate ps of figure 

 14, and apparently also includes the lateral cervicals (Ic of fig. 

 14) extending, as it does, well around into the dorsal region. 

 This narrow transverse jugular plate is characteristic of the 

 Neuropteroid larvae; and although the sclerite ps of figure 7 

 also very probably includes the lateral cervicals, it has not the 

 outline of the jugular plate ps of figures 4 and 6, nor does it ex- 

 tend so far around into the doisal region. In the prothorax of 

 the larva of Dytiscus (fig. 3) there occurs a large anterior plate 



