ADAPTATIONS OF AQUATIC INSECTS 



157 



means of undulatory movements of the body, and the larvae and pupae 

 of most black flies (Simuliidae, Fig. 231) secure a continuous supply of 

 fresh air simply by fastening themselves to rocks in swiftly flowing 

 streams. 



Rectal respiration is highly developed in odonate nymphs. In these, 

 the rectum is lined with thousands of tracheal branches, which are bathed 

 by water drawn in from behind, and then expelled. 



All these kinds of respiration cutaneous, branchial and rectal 

 occur in young ephemerid nymphs; while mosquito larvae have in ad- 

 dition spiracular respiration. 



With the arrival of imaginal life, tracheal gills disappear, except in 

 Perlidae, and even in these insects the gills are of 

 little, if any, use. 



Marine Insects. Except along the shore, the 

 sea is almost devoid of insect life, the exceptions 

 being a few chironomid larvae which have been 

 dredged in deep water, and fifteen species of Halo- 

 bates (belonging to the same family as our familiar 

 pond-skaters), which are found on warm smooth 

 seas, where they subsist on floating animal re- 

 mains. 



Between tide-marks maybe found various beetles 

 and collembolans, which feed upon organic debris; 

 as the tide rises, the former retreat, but the latter 

 commonly burrow in the sand or under stones and 

 become submerged, for example the common A nurida 

 maritima. 



Insect Drift. Seaweed or other refuse cast upon the shore harbors 

 a great variety of insects, especially dipterous larvae, staphylinid scaven- 

 gers and predaceous Carabidae. On the shores of inland ponds and lakes 

 a similar assemblage of insects may be found feeding for the most part 

 on the remains of plants or animals, or else on one another. During a 

 strong wind, the leeward shore of a lake is an excellent collecting ground, 

 as many insects are driven against it. On the shores of the Great Lakes 

 insects are occasionally cast up in immense numbers, forming a broad 

 windrow, fifty or perhaps a hundred miles long. Needham has described 

 such an occurrence on the west shore of Lake Michigan, following a gale 

 from the northeast. In this instance, a liter of the drift contained nearly 

 four thousand insects, of which 66 per cent, were crickets (Nemobius), 

 20 per cent. Acridiidae, and the remainder mostly beetles (Carabidae, 



B 



FIG. 231. Simu- 

 lium; A, larva; B, 

 pupa, showing respira- 

 tory filaments. 



