CHAPTER IV 



ADAPTATIONS OF AQUATIC INSECTS 



Ease, versatility and perfection of adaptation are beautifully exempli- 

 fied in aquatic insects. 



Systematic Position. Aquatic insects do not form a separate group 

 in the system of classification, but are distributed among many orders, of 

 which Plecoptera, Ephemerida, Odonata and Trichoptera are pre-emi- 

 nently aquatic. One third of the families of Heteroptera 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 transi- 

 tional stages are to be found. 



Food. Insects have become adapted to utilize with remarkable 

 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. 172) feeds on submerged 

 leaves, especially of VaUisneria, being in this respect almost unique 

 among Lepidoptera. Hydrophilid beetles and many other aquatic in- 

 sects 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, and larvae of mosquitoes and 

 black-flies on microscopic organisms near the surface, while larvae of 

 Chironomus find food in the sediment that accumulates at the bottom of a 

 body of water. 



Predaceous species abound in the water. Notonecta (Fig. 225) ap- 

 proaches its prey from beneath, clasps it with the front pair of legs and 

 pierces it. Nepa and Ranatra likewise have prehensile front legs along 



