I 



ADAPTATIONS OF AQUATIC INSECTS 1 71 



ing 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. Locustidae, and the remainder mostly 

 beetles (Carabidae, Scarabaeidae, Chrysomelidae, Coccinellidae, etc.), 

 dragon flies, moths, butterflies (Anosia, Pieris, etc.) and various 

 Hemiptera, Hymenoptera and Diptera. A large proportion of the insects 

 were aquatic forms, such as Hydrophilus, Cybister, Zaitha, and a species 

 of caddis fly; these had doubtless been carried out by freshets, while the 

 butterflies and dragon flies had been borne out by a strong wind from 

 the northwest, after which all were driven back to the coast by a north- 

 east wind. While some of these insects survived, notably Coccinellidae, 

 Trichoptera, Asilidas, Locustidae and Gryllidae, nearly all the rest were 

 dead or dying, including the dragon flies, flies, bumblebees and wasps. 

 Foraging Carabidae were observed in large numbers, also scavengers of 

 the families Staphylinidae, Silphidae and Dermestidae. 



On the seashore and on the shores of the Great Lakes, the salient 

 features of insect life are essentially the same. Similar species occur in 

 the two places with similar biological relations, on account of the general 

 similarity of environment. 



Origin of the Aquatic Habit. The theory that terrestrial insects 

 have arisen from aquatic species is no longer tenable, for the evidence 

 shows that the terrestrial type is the more primitive. Aquatic insects 

 still retain the terrestrial type of organization, which remains unob- 

 scured .by the temporary and comparatively slight adaptations for an 

 aquatic life. Thus, the development of tracheal gills has involved no 

 important modification of the fundamental plan of tracheal respiration. 

 It is significant, moreover, that the most generalized, or most primitive, 

 insects Thysanura are. without exception terrestrial. Aquatic in- 

 sects do not constitute a phylogenetic unit, but represent various orders, 

 which are for the most part undoubtedly terrestrial, notwithstanding the 

 fact that a few of these orders (Plecoptera, Ephemerida, Odonata, Tri- 

 choptera) are now wholly aquatic in habit. Adaptations for an aquatic 

 existence have arisen independently and often, in the most diverse 

 orders of insects. 



