NO. 2 



DRAGONFLY LARVA — SNODGRASS 



21 



ture is clearly a functional adaptation for the benefit of the adult ; it 

 must have been early impressed upon the larva, and retained by the 

 larva because it had no disadvantage for the larval activities. By the 

 method of puncture-scarring the larval cuticle, it has been shown by 

 Sargent (1937) that the dorsal extension of the mesepisterna takes 



Fig. 8. — The thorax. 



A, thorax and base of abdomen of an Anax larva. B, first thoracic spiracle 

 of larva, left side, external. C, same spiracle of right side, inner view, main 

 tracheal trunk removed. D, thorax and base of abdomen of an adult libellulid. 



Atr, atrium; Epiih, epimeron of mesothorax, Epfih, epimeron of metathorax; 

 EpS2, upper plate of episternum of mesothorax ; Epsz, episternum of metathorax ; 

 Ni, pronotum; oancl, occlusor muscle of spiracle; Ptr, peritreme; iSp, first 

 spiracle. 



place by marginal growth, while the metepimera increase both by 

 general expansion and by growth of the ventral margins. The post- 

 coxal plate of the metasternum, however, moves bodily backward 

 from the legs as the epipermal plates intervene. 



In the adult dragonfly (fig. 8 D) the backward slant of the wing- 

 bearing segments is still more pronounced than in the larva, and a 

 reason for it is now evident. It brings the legs forward in a group 



