CHAPTER XXVII 

 AaUATIC INSECTS 



By JAMES G. NEEDHAM 



Professor of Limnology, Cornell University 



Insects are essentially terrestrial animals. Their organization 

 fits them for exposure to the air. On land they are numerically 

 dominant, and it is a comparatively small portion of the group 

 that is to be found in the water. But the lesser portion of a 

 group so large is in itself a host, including a very great variety of 

 forms. 



That insects are primarily terrestrial and that they have been 

 secondarily adapted to aquatic life is evidenced in many ways. 

 Their complete armor of impervious chitin and their respiratory 

 apparatus, consisting of internal branching chitin-lined air tubes 

 (tracheae), opening to the outside for the intake of air through 

 spiracles, speak strongly against an' aquatic origin. It would be 

 hard to imagine an organization more unsuited to getting air when 

 in the water. 



Furthermore, all adult insects, even those that live constantly 

 in the water, have preserved the terrestrial mode of respiration: 

 they all breathe air directly, instead of breathing the air that is 

 dissolved in the water. They have merely acqioired means of 

 carrying air from the surface down into the water with them for 

 use there. They are no more aquatic in their mode of respiration 

 than is a man in a diving bell. It is only the more plastic immature 

 stages that have acquired a strictly aquatic type of respiratory 

 apparatus. 



Again, it is only isolated and rather small groups of insects that 

 inhabit the water. A few of the smaller orders, Hke the stone- 

 flies. Mayflies, dragonflies and caddisflies are practically aU aquatic 

 in their immature stages; but the larger orders are not so. 



There is abundant evidence of the independent adaptation of 

 the various groups. Practically all the adult insects found in the 



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