54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



genital segments of the abdomen likewise signifies nothing more than 

 the retention of organs useful in the larval stage. In short, the vi^orm- 

 like form of the caterpillar and of other holometabolous larvae has 

 no phylogenetic significance. It is a secondary adaptation, derived 

 from the normal adult pterygote structure, accompanied by numerous 

 specializations peculiar to the larva, and later discarded. The com- 

 plex musculature of the caterpillar is only one of the features in the 

 larval organization that have been specially evolved from the gener- 

 alized adult structures of the immediate ancestors of the Lepidoptera 

 to enable the caterpillar to perform more efficiently the duties that have 

 devolved upon it from the apportionment of the life processes between 

 the immature and adult stages of the individual. 



Diptcra. — The musculature of the larvae of Diptera shows a 

 unique type of specialization in its highest development, but at the 

 other extreme it has a pattern corresponding entirely with that of the 

 generalized plan of abdominal musculature in adult Pterygota. 



The simpler forms of dipterous larval musculature, known in the 

 Tipulidae, Psychodidae, Chironomidae, Culicidae, and Tabanidae, con- 

 sist of dorsal and ventral bands of longitudinal fibers, and of lateral 

 dorsoventral muscles. A primitive type of musculature occurs in the 

 Psychodidae, where, as described by Dirkes (1928) for Psychoda 

 alternata, the dorsal and ventral muscles are mostly longitudinal and 

 attached on the intersegmental folds, though a few in each set are 

 shorter than segmental length. In the first abdominal segment there 

 are five dorsoventral laterals and two oblique laterals on each side. 

 The first of the dorsoventral muscles is attached on the intersegmental 

 fold between metathorax and abdomen, the others follow along the 

 side of the segment. A similar condition exists in the Culicidae, as 

 described by Samtleben (1929), except that here some of the inner 

 muscles of the dorsal and ventral series in each segment cross ob- 

 liquely over the outer muscles, and the inner lateral muscles are con- 

 fined to the anterior parts of the segments, where they are attached on 

 the intersegmental folds between the ends of the dorsal and ventral 

 muscles. In the larva of Tipiila (fig. 22) the musculature is compli- 

 cated by a great increase in the number of fibers in all the principal 

 groups, and by a diversification in their points of attachment, but 

 there are few fibers taking an oblique course. In both the dorsal and 

 ventral groups certain sets of fibers are attached regularly on the 

 intrasegmental transverse folds of the body wall, and some of the 

 median ventral fibers form somewhat oblique interlacing bundles. The 

 internal lateral muscles (//') comprise a series of dorsoventral fibers 



