280 Adjustment to Conditions of Aquatic Life 



plates of many mayflies and damselflies are provided 

 with muscles, and these are used for swimming. 

 Caudal propulsion is also the rule in these same groups. 

 Among beetle and fly larvae locomotion is mainly 

 effected by wrigglings of the body, that are highly 

 individualized but only moderately efficient, if judged 

 by speed. 



It is worthy of note that the completest adaptations 

 to conditions of aquatic life do not occur in those groups 

 of insects that are aquatic in both adult and larval 

 stages. Beetle larvae and water-bug nymphs take air 

 at the surface, and in structure differ but little from 

 their terrestrial relatives. Fine developments of tra- 

 cheal gills occur in the nymphs of mayflies and stone- 

 flies, and in caddis worms; internal gill chambers, in the 

 dragonfly nymphs; attachment apparatus for with- 

 standing currents, in some dipterous larvae; the utmost 

 adaptability to all sorts of freshwater situations occurs 

 in the midges; and in adult life these insects are all 

 aerial. 



What then is the explanation of the dominance of 

 this remarkable insect group in the world to-day — a 

 dominance as noteworthy in all shoal freshwaters as it 

 is on land? What advantages has this group over 

 other groups? There is no single thing; but there are 

 two things that, taken together, may give the key to 

 the explanation. These are: 



i . Metamorphosis, the changes of form usually per- 

 mitting an entire change of habitat and of habits 

 between larval and adult life. The breaking up of the 

 life cycle into distinct periods of growth and reproduc- 

 tion permits development where food abounds. 



2. The power of flight in the adult stage permits easy 

 getting about for finding scattered sources of food supply 

 and for laying eggs. 



