AQUATIC INSECTS 



prey and on which they lay their eggs. They frequent still 

 pools strewn with plant litter and overspread with algae; 

 shore waters populated with insects, and with young water 

 animal? of all kinds (PL X). The few larvse such as the water 

 pennies which live in rapid water are very unlike their rela- 

 tives. 



Among the mayflies and dragonfiies the young are thorough- 

 ly aquatic while the adults are thoroughly eerial ; the nymphs 

 are the long-lived forms in which these insects winter over 

 and get through the difficult crises of their lives. 



On the other hand, most water beetles, except the water 

 pennies, winter over as adults. Whirligig beetles hide in 

 the mud and come out with the first spring days, sometimes 

 on warm days in midwinter, and they abound in the open 

 waters all summer as well and into the late fall. Water 

 pennies, larvae of Psephenus, forage through the winter on 

 diatom-covered stones in swift riffles (PI. I). 



Food. — Diving beetles (Dytiscidce) and whirligigs {Gyrinidce) 

 are predacious all their lives, the crawling water-beetles, 

 HaliplidcB, and water pennies, PsephenidcB, are vegetarian, 

 while the Hydrophilidcz are apt to be predacious as larv^ 

 and vegetable eaters when they become adult. Larvae of 

 the dytiscids, gyrinids, and haliplids suck their food through 

 mandibles which are grooved for the passage of juices; the 

 others chew their food. 



Associates. — Plants and animals which are the food supply 

 of beetles will be mentioned in the discussions of the dif- 

 ferent families. 



They have many enemies, counting themselves among the 

 worst, for all of the predacious beetle larvae are cannibals from 

 the day they are hatched. Needham and Williamson worked 

 with one beetle, Hydroporus, which was so blood-thirsty that 

 they had to construct compartments in which each larva 

 could be isolated during their study of its habits. The parti- 

 tions projected well above the water but the larvae climbed 



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