NO. 6 INSECT ABDOMEN — SNODGRASS 39 



Siinikii" groups of internal lateral muscles occur in both the abdomen 

 and the thorax of the larva of Dyfisciis. According to Speyer ( 1922} , 

 a two-branched internal lateral muscle, " musculus dorsoventralis 

 abdominis a," occurs in the anterior part of each of the first five seg- 

 ments of the abdomen in the larva of Dyfiscns marginal is (fig. 18, 

 ////). The lower ends of these muscles are inserted on the interseg- 

 mental folds, but their upper ends are attached in the anterior parts of 

 the segments following on the tergal plates between median and 

 lateral groups of fibers of the dorsal longitudinal muscles (di, p). 

 In the thorax, the upper ends of the corresponding muscles (//') are 

 more nearly intersegmental ; their lower attachments are on the inter- 

 segmental " furcillae " and on the sternal apophyses. These muscles 

 of the larval thorax possibly correspond with muscles of the adult 

 described by Bauer ( 1910), " uiuscitli levatorcs prothoracis and meso- 

 thoracis," extending from the sternal apophysis of the prothorax 

 and mesothorax to the first and second phragmata. respectively. Sim- 

 ilar muscles are sometimes present in the thorax of other adult in- 

 sects ; one such occurs in Acrididae attached ventrally on the prosternal 

 apophysis and dorsally on the intersegmental fold in front of the 

 mesepisternum (see Snodgrass. 1929, figs. 2^2^ 34' 59)- 



The fragmentary review of the position of the internal lateral mus- 

 cles given above suggests that the anterior fibers at least of each 

 segmental group represent lateral dorsoventral muscles that are 

 primarily intersegmental (fig. 13 A. //'). The fibers have a tendency 

 in the abdomen to migrate posteriorly, especially on the dorsum ( B ) , 

 though they may extend backward along the lateral edges of the 

 sternum also (fig. 22). Their upper ends thus cut ofi:' a lateral group 

 of fibers (fig. 13 B, p) from the longitudinal dorsals, which become 

 the paratergal muscles. These primarily intersegmental internal lateral 

 muscles run mesad to the lateral tracheal trunks in some insects 

 (Acrididae, lepidopterous larvae, tipulid larvae), and their homologues 

 presumably should do so in all insects, but this point has not been 

 determined. Other internal lateral muscles of the abdomen, lying 

 external to the lateral tracheae, are probably intrasegmental in their 

 origin. 



The external lateral muscles are typically dorsoventral and intra- 

 segmental (fig. 13 A, B, Ic). Some of them, however, are frequently 

 oblique (fig. 15 B, ijS, /70), and the latter may include an interseg- 

 mental muscle (fig. 15 A, //). The dorsoventral fibers are sometimes 

 attached on the pleural membrane or on " pleural " sclerites, form- 

 ing thus tergo-pleural and sterno-pleural muscles (fig. 14 A, i-p, s-p). 



