NO. 6 



INSECT ABDOMEN — SNODGRASS 



49 



on the larval muscles of the Coleoptera. The complete account of the 

 muscles of the Dytiscus larva given by Speyer (1922) and by Kor- 

 schelt (1924), however, furnishes a basis for an understanding of 

 the relation between the larval musculature and the musculature of 

 adult insects. The structural changes which take place in the trans- 

 formation from the larval to the imaginal musculature have been 

 described by Breed (1903) in a trogositid, Thyinaliis mar gink ollis. 

 The abdominal musculature of the larva of Dytiscus marginaUs, 

 as described by Speyer, consists of four primary groups of fiber 

 bundles, namely, dorsal muscles, ventral muscles, and, on each side, 

 a set of lateral (dorsoventral) muscles. In the region of the first seven 

 abdominal segments, the dorsal muscles comprise an internal set of 



Fig. 18. — Body muscles in right half of thorax and first two abdominal seg- 

 ments of Dytiscus vmrginalis larva. (Outline from figure by Speyer, 1922, re- 

 lettered in accord with muscle nomenclature adopted in this paper.) 



d{, internal dorsal muscles ; //'. internal lateral muscles, upper ends of which 

 cut ofif paratergal muscles (/>) from the other dorsals; vi, internal ventrals. 



intersegmental longitudinal fibers of segmental length (fig. 18, dil, 

 dill), and outer sets of short fibers extending from the posterior 

 parts of the terga to the following intersegmental folds. The ventral 

 muscles consist likewise of internal (vil, vill) and external sets of 

 intersegmental fibers. In each side of the first five abdominal segments 

 Speyer distinguishes six lateral (dorsoventral) muscles. Five of 

 these are external laterals, three of which are tergo-sternal and two 

 tergo-pleural in their attachments. The other lateral muscle is an 

 internal lateral and consists of two bundles of fibers (lill), which 

 arise ventrally by a common base on the intersegmental fold. Dorsally 

 the two branches are inserted on the anterior lateral part of the 

 tergum between median and lateral sets of the dorsal longitudinal 

 fibers {di and p). In the thorax the muscles (//') corresponding with 



