NO. 2 DRAGONFLY LARVA — SNODGRASS 2^ 



moulted larval instars only for the withdrawal of the tracheal linings, 

 but they all become functional in the imago. 



The principal internal changes that take place in the thorax pertain 

 to the completion of development in the wing muscles, though there 

 is also an elimination of some of the larval neck and leg muscles. The 

 muscles of the larval thorax have been shown by Maloeuf (1935) to 

 be of two types : large, striated functional muscles, which are mostly 

 the muscles of the legs ; and slender, unstriated nonfunctional muscles, 

 which are the prospective wing muscles. The number of muscles re- 

 mains the same throughout the life of the larva, but the number of 

 fibers in individual muscles increases with the larval growth. The 

 wing muscles, Maloeuf says, "grow greatest in fiber number and 

 diameter during the time of the final transformation." The muscle 

 attachments undergo little change during transformation, though "the 

 skeletal parts on which they are attached may become greatly modi- 

 fied." The elaborate endoskeletal system of the adult thorax is said 

 by Sargent (1937) to be developed entirely at the transformation to 

 the imago. The change in the position of the muscles from larva to 

 imago is correlated with the increased obliquity of the thoracic pleura 

 in the adult. 



The growth of the wing muscles during the larval life has been de- 

 scribed by Marcus (1920), and more fully by Cremer (1934). In 

 the very young larva the wing muscles are said by Cremer to be a 

 hyaline tissue in which are numerous nuclei, but as yet no cell walls 

 or fibrillae. Individual muscle cells appear later, and in the last stage 

 of larval growth the fibrillae are differentiated from the peripheral 

 sarcoplasm. The growth of the muscle cells and the multiplication of 

 fibrillae ends with the beginning of the imaginal stage. 



The thoracic musculature of the adult anisopterous dragonfly has 

 been shown by Clark (1940) to include essentially the same muscles 

 that are present in the thorax of other insects, including direct and 

 indirect wing muscles. The dragonflies, however, which do not flex 

 or extend the wings horizontally, appear to have used the direct wing 

 muscles and the tergosternal muscles more efficiently for the up-and- 

 down motion of the wing, rather than depending on vibrations of the 

 terga produced by the dorsal longitudinal and tergosternal muscles, 

 as do most other insects. The intersegmental dorsal muscles are very 

 small as compared with their great size in most other flying insects, 

 including the Ephemeroptera. In the cockroaches, mantids, and ter- 

 mites also, the dorsal muscles are small or absent, but these insects 

 have not developed from the other muscles a motor mechanism for 

 the wings in any degree comparable to that of the dragonflies. Clark 



