38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



typically they are vertical in position ; but they may comprise tergo- 

 pleural and sterno-pleural fibers (fig. 14 A, t-p, s-p) , as well as 

 tergo-sternal fibers {t-s) , and generally some of them are oblique. 

 A division of the lateral muscles into internal laterals and external 

 laterals (fig. 13 B, C, li, le) is not always apparent, often because 

 of the absence of the internal group, but it is of common occurrence. 



The internal lateral muscles, when present, are longer than the 

 external laterals because their upper attachments are at a higher 

 level on the dorsum than are those of the external muscles (figs. 

 13 C, li, 15 B, 775, 1/6). The position of the internal laterals along 

 the sides of a segment is variable. The muscles are usually situated 

 in the middle or anterior parts of the segments (fig. 16), but in 

 some cases they are limited to the extreme anterior regions, and in 

 certain holometabolous larvae they lie on the intersegmental folds. 

 The internal lateral muscles, however, do not in all cases constitute 

 a homogeneous group of muscles ; one or more sets of anterior fibers, 

 such as those forming the first internal lateral muscle of Dissosteira 

 (fig. 15 B, 1/3), lie internal to the lateral tracheal trunk, while the 

 more posterior fibers, as the second internal lateral of Dissosteira 

 (i/'6), may lie external to the tracheal trunk. In some insects, on 

 the other hand, the entire series of internal lateral fibers are internal 

 to the lateral tracheal trunk (fig. 22). 



An example of the limitation of the internal lateral muscles to the 

 intersegmental regions is well shown in the larva of Rhagoletis po- 

 monella (fig. 23), a trypetid fly, in which the muscles consist of 

 slender l^ands of fibers (//) lying laterally on the intersegmental folds 

 in both the abdomen and the thorax. Similar intersegmental muscles, 

 comprising each three groups of fibers, are described by Samtleben 

 (1929) in the larvae of Culicidae as " musculi dorsoventrales medi- 

 ales," the upper attachments of which are between the ends of the 

 dorsal longitudinal muscles, and the lower attachments between the 

 ends of the ventral muscles. In the larva of Tipitia ( fig. 22) the 

 internal lateral muscles consist of a series of approximately vertical 

 fibers (//') occu]:)ying the anterior half of the lateral wall of each seg- 

 ment, l)Ut the anterior fibers in each segmental group are attached 

 on, or close to, the intersegmental fold. All of these fibers lie internal 

 to the ventrolateral tracheal trunk (LTra) and a broad band of lon- 

 gitudinal paratergal fibers (p). In the caterpillars a group of several 

 internal lateral fibers (figs. 20. 21, /;') arise from the lateral extremity 

 of each ventral intersegmental fold and diverge posteriorly to the 

 dorsum, going internal to the lateral tracheal trunk (Tra) and the 

 paratergal muscles (p). 



