NO. 2 DRAGONFLY LARVA SNODGRASS 1 7 



paratus of the anisopterous larva, which involves a modification of 

 the head, modifications of the labium such as have occurred in no 

 other insect, the development of a hypopharyngeal apodeme support- 

 ing the labium, and the elaboration of a hydraulic mechanism with a 

 muscular diaphragm in the abdomen. Evolution by natural selection 

 is relatively easy to imagine when it pertains to a single organ that 

 might take on a new structure by the gradual addition of small modi- 

 fications guided to a functional end by the elimination of unfit varia- 

 tions. The matter is quite different, however, when several primarily 

 unrelated parts must all be modified in a correlated manner to work 

 together as a functional unit. The question is further complicated by 

 the necessary condition that a complex of parts can become operative 

 as a whole only when each of the elements is perfected and all are 

 simultaneously adapted to their subsidiary roles. 



Metamorphosis of the labium. — At an early stage in the larval trans- 

 formation the forming imaginal labium begins to retract within the 

 cuticle of the larval labium. At first the imaginal labium has essen- 

 tially the larval structure (fig. 6 A, iLb), but as it continues to retreat 

 and to decrease in size it takes on more of the adult form, until finally 

 it occupies only the narrow basal half of the larval postmentum, where 

 it is compactly rolled upon itself (3, iLb). When removed at this 

 stage (C) and spread out (D) the imaginal labium is seen to have 

 completed most of its development, so that little further change is 

 needed to give it the definitive structure (E). 



In the transforming labium there takes place, along with the ex- 

 ternal changes in shape, a reconstruction of the musculature. Mun- 

 scheid (1933) gives a detailed account of the histolysis and histo- 

 genesis of the labial muscles in Aeschiia cyanea, and a description of 

 the formation of tonofibrillae by which retained muscles are newly 

 attached on the imaginal cuticle. Degeneration of the larval muscles, 

 she says, begins with the dissolution of the myoplasm, but a small part 

 of it, together with a few muscle nuclei, remain as myoblasts for the 

 imaginal muscles, the formation of which takes place relatively early 

 and is quickly completed. Munscheid observes that the transformation 

 of the dragonfly labium gives an example of holometabolism affecting 

 one organ in an insect that is otherwise hemimetabolous, but she evi- 

 dently did not consider the extreme degree of metamorphosis that 

 takes place also in the skeleton and musculature of the abdomen. 



In the transformation of the labium of Aeschna tyanea, Munscheid 

 says that two pairs of larval muscles are lost and not replaced in the 

 imago. These muscles are the abductors of the palpi and the tentorial 

 extensors of the prementum. Whedon (1927) says that in Ana.v 



