386 ENTOMOLOGY 



Depth. Depth is mostly important indirectly, on account of its 

 influence on other conditions, such as circulation, temperature, light, 

 and pressure; but is in itself a determining condition in some instances. 

 Thus such larvae of horse-flies (Tabanidae) as have a posterior res- 

 piratory tube bearing the spiracles must have shallow water, where the 

 spiracles can be raised to the surface; though sometimes these larvae 

 are found in moist earth where there is no standing water. (R. C. 

 Osburn.) 



Bottom. "The character of materials and topography of the 

 bottom are very important to animals living on the bottom, but it has 

 its effect also on free swimming animals as a determining factor in the 

 amount of sediment. The kind of bottom is important because many 

 animals are dependent upon solid objects for attachment and are absent 

 from bottoms made up of fine materials. Others must burrow into 

 mud or creep on sand and gravel. " (Shelf ord.) 



Most Dytiscidae prefer clean live water, being averse to very muddy 

 bottoms. (J. D. Sherman.) On the other hand, nymphs of various dra- 

 gon flies bury themselves in the mud. Some of the caddis worms prefer 

 a clear sandy bottom; others, a bottom with slime, or one with sticks, 

 dead leaves or other debris. 



Vegetation. It goes without saying that vegetation, in its amount 

 and kind, is extremely important as a condition of aquatic existence. 

 The green plants give off oxygen. Plants are used for shelter, escape 

 from enemies, as places in which to wait for the prey (dragon fly 

 nymphs, Zaitha), as surfaces for locomotion (non-swimming larvae 

 of diving beetles, whirligig beetles and others), especially as a means of 

 reaching the surface for air or for the transformation (dragon fly nymphs, 

 etc.). Eggs may be laid on the plants (electric light bugs, back-swim- 

 mers, May flies, caddis flies, etc.) or inserted into plant tissues (diving 

 beetles, water scorpions). 



3. FOOD CONDITIONS 



The small beetles of the family Haliplidae are said to feed on algae. 

 The water-scavenger beetles (Hydrophilidae) feed mostly on decaying 

 vegetable matter, but are sometimes carnivorous, and some of j the 

 adults will eat green vegetation. The diving beetles (Dytiscidae) 

 are carnivorous; the whirligig beetles (Gyrinidae) feed on small insects 

 found on the surface of the water. The water-striders (Gerridae) 

 capture disabled or living insects for food; the water boatmen (Corix- 



