Packakd] tNTSECTS OF THE POND AND STREAM. 137 



most of the Syrphidae are Aphis-eaters in the larval state. 

 Of that immense family MuscitUe, of which the house fly is 

 a t3pe, which are almost purely terrestrial, Epln'dra (Fig. 

 98, fly and pupa case) is aquatic. It may be said on the 

 whole that the aquatic larvie of the Diptera were originally 

 terrestrial insects, which have adopted an aquatic life and 

 are exceptions to the rule. 



In the Neuroptera, however, there are whole families which 

 are aquatic both in the larval and pupal state, having exter- 

 nal gill-like appendages in both stages, and in the case of 



Fig. 99. 



Pteronarcys, which is closely allied to Perla (Fig. 99, from 

 Figuier), they are retained in the winged fly. The Neurop- 

 tera arc par excellence water insects, and yet it is doubtful 

 even whether they have not originally assumed this excep- 

 tional mode of life, and, while the earliest of all insects, were 

 not at first terrestrial. This is speculation and guesswork, 

 but facts seem to point to this conclusion. 



Now the changes in structure fitting the insect for a life 

 in ponds and streams are to be found in the organs of loco- 

 motion and the breathing apparatus, and in this essay I 



9 



