AQUATIC INSECTS 909 



Many of the largest larvae are fiercely cannibalistic and will eat 

 their brethren even when other food offers. 



Some of the larvae are provided with swimming fringes on the 

 legs, some have them on the tail, and many have them in both 

 places. Some, like the larvae of Acilius, are exceedingly lithe and 

 graceful creatures. Others (Fig. 1377) scarcely swim at all, but 

 creep about among the trash at the shore line. 



FIG. 1377. The larva of Coptotomus interrogates. 



In the present state of our knowledge, it is not possible to give 

 keys that will determine genera of Dytiscid larvae, and the best 

 means of identifying native larvae is by comparison with the beau- 

 tiful figures of Schiodte, who long ago (1861) described the European 

 representatives of many of our genera. 



THE TWO-WINGED FLIES (Order Diptera) 



Of this immense order a considerable portion has taken to a 

 more or less aquatic life. A majority of the families have some 

 members that develop in the water, but only a few of the smaller 

 families are wholly aquatic. Those best fitted for life in the water 

 show adaptations of the most diverse sorts, so that here again the 

 families are best considered separately. 



Since nearly all the families of the Diptera have some aquatic 

 members, the determination of the adult flies must be left to the aid 

 of the keys in the entomological manuals that are everywhere avail- 

 able. Each of these families has a characteristic type of wing ven- 

 ation, and some aid may be had from comparison with the typical 

 wings shown in Fig. 1378. 



It is quite impossible in the space at command to give keys 

 to the genera of Dipterous families, these being very numerous 



