PROC. ENT. SOC. WASH., X r OL. 23, NO. 8, \< ) \ K \1 B E R, 1921 175 



Limitation and Composition of Larval Segments. 



The larva of a sawfly is distinctly divided into a head anil 

 body. The body is further divided, more or less distinctly, into 

 prothorax, mesothorax and metathorax and ten abdominal seg- 

 ments and it is readily observed that in the abdomen each seg- 

 ment excepting the ultimate is subdivided into a constant num- 

 ber of areas and that the areas of one segment are recognizable 

 duplicates of the areas of any other abdominal segment. 1 lencc 

 between any two abdominal spiracles on the same species there 

 is a constant number of areas (see Plate XIV, figs. 2-5 and 6). 

 In each of the first nine larval abdominal segments of Ptcronidea 

 ribesii there are four tergal areas; three ornamented with spots 

 and hairs, and one, plain without markings or hairs. The 

 thoracic segments (see Plate XIV, figs. 1 and 2) are somewhat 

 similar to the abdominal ones especially in containing four 

 tergal areas but differ chiefly as follows: 



1. The position occupied by the spiracle. 



2. The possession of but two spiracles for the three segments, one very large 



within and apparently belonging to the prothorax ami one very small and 

 obscure, situated between the mesothorax and metathorax. 



3. Some change in the proportion and spotting of the areas. 



4. A considerable constriction of the anterior area of the prothorax where it 



unites with the head. 



5. The division of the pleural zone into four more or less distinctly defined 



areas termed preepipleurite and postepipleurite and prehypopleurite 

 and posthypopleurite. 



Thus, in Pteronidea ribesii, the segments, exclusive of the head 

 and the anal segment, are composed dorsally of four areas each. 

 It will be seen from a study of the drawings illustrating the com- 

 position of the body areas for both thorax and abdomen of saw- 

 flies belonging to different groups, that the number of folds per 

 segment is not the same for all sawfly larvae, but that although 

 some annulets may be divided, homologies do exist between 

 these areas from abdomen to thorax and from species to species. 

 (See Plate XIV, figs. 2, 4, 7 and 8.) The homologies between 

 the different segments of an individual larva are readily seen if a 

 definite area be followed throughout the segments from abdomen 

 to thorax. The homologies between widely different larvae 

 can be determined by comparison although it may sometimes be 

 necessary to assure oneself by an examination of the muscles. 



It now becomes necessary to decide with which of the trans- 

 verse tergal folds segments begin. In this connection the mus- 

 culature offers some suggestions and since the infoldings of the 

 skin, which determine the annulets, are used for muscle attach- 

 ments these muscle attachments undoubtedly add weight to the 

 value of these infoldings as intersegmental and segmental 



