NO. 2 DRAGONFLY LARVA SNODGRASS 35 



porting plate by a distinct circular groove (C, n) and is readily broken 

 off at the base. The median appendage (A, dl) must be admitted to 

 be a secondary outgrowth of the epiproct ; the presence of similar ap- 

 pendages (//) on the lateral basal plates is then no evidence that the 

 latter are not paraprocts likewise bearing secondary outgrowths. 



SUMMARY 



1. The larval and the adult dragonfly have evolved independently 

 along divergent lines, the larva in adaptation to life in the water, the 

 adult to life in the air, until the two have become so different that 

 they might pass for unrelated animals. The egg, however, retains 

 the dual potentiality of redeveloping both the larva and the adult, each 

 one in its latest evolutionary stage. 



2. The larval adaptation has affected the head, the labium, the ali- 

 mentary canal, the tracheal system, the muscular system, and the 

 abdomen. 



3. In correlation with the mechanism of protraction and retraction 

 of the highly modified larval labium, there has been developed from 

 the base of the hypopharynx a large T-shaped apodeme with its cross- 

 bar imbedded in the posterior lip of the base of the postmentum. This 

 apodeme controls the movements of the larval labium ; it is greatly 

 reduced in the adult or replaced by a ligament. 



4. In protraction and retraction the labium swings on anterior 

 points of its base in the ventral head membrane. Neither of the two 

 pairs of labial muscles from the head can produce the movement of 

 protraction, since they enter the postmentum behind the axis on which 

 the labium swings, but either pair or both pairs may cause retraction 

 of the labium. It is suggested, therefore, that the abdominal dia- 

 phragm between the fourth and fifth segments of the abdomen plays 

 an important part in the mechanism of labial protraction. Compres- 

 sion of the anterior part of the abdomen and the rear part of the 

 thorax of a freshly anesthetized larva at once protracts the labium 

 and inflates the neck behind the labium. If blood pressure extends 

 the labium of a live larva in this manner, the diaphragm at least pre- 

 vents the backward expenditure of the pressure, and probably con- 

 tributes something to the forward pressure. 



5. No evidence was observed that the diaphragm has any respira- 

 tory function in the aeschnid larva. Expansion and contraction of 

 the abdomen appear to be due entirely to the lateral tergosternal mus- 

 cles and the elasticity of the arched tergal plates of the abdominal 

 segments. 



