CHAPTER IV 



ADAPTATIONS OF AQUATIC INSECTS 



Ease, versatility and perfection of adaptation are beauti- 

 fully exemplified in aquatic insects. 



Systematic Position. Aquatic insects do not form a sepa- 

 rate group in the system of classification, but are distributed 

 among many orders, of which Plecoptera, Ephemerida, Odo- 

 nata and Trichoptera are pre-eminently aquatic. One third of 

 the families of Heter^bptera and less than one fourth those of 

 Diptera are more or less aquatic. One tenth of the families 

 of Coleoptera frequent the water at one stage or another, but 

 only half a dozen genera of Lepidoptera. A few Collembola 

 live upon the surface of water, and several Hymenoptera, 

 though not strictly aquatic, are known to parasitize the eggs 

 and larvae of aquatic insects. 



The change from the terrestrial to the aquatic habit has been 

 a gradual change of adaptation, not an abrupt one. Thus at 

 present there are some tipulid larvae that inhabit comparatively 

 dry soil; others live in earth that is moist; many require a 

 saturated soil near a body of water and many, at length, are 

 strictly aquatic. Among beetles, also, similar transitional 

 stages are to be found. 



Food. Insects have become adapted to utilize with re- 

 markable success the immense and varied supply of food that 

 the water affords. Hosts of them attack such parts of plants 

 as project above the surface of the water, and the caterpillar 

 of Paraponyx (Fig. 171) feeds on submerged leaves, espe- 

 cially of Vallisneria, being in this respect unique among Lepi- 

 doptera. Hydrophilid beetles and many other aquatic insects 

 devour submerged vegetation. The larvae of the chrysomelid 

 genus Donacia find both nourishment and air in the roots of 

 aquatic plants. Various Collembola subsist on floating algae, 



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