104 MOLLUSC A, INSECTS, ETC. 



bivalve shell. The numerous eggs of both these com- 

 mon bivalves will furnish the fish with abundant food. 

 The aquatic insects that every pond usually swarms 

 with, are not less attractive and interesting than the 

 fishes and mollusca. Indeed, some of them surpass 

 the latter in interest, on account of the life-stages 

 through which they pass ; such as the larvae of the 

 dragon-flies, caddis-worms, water beetles, &c. What 

 could be more astonishing than the fact that the early 

 life of many aerial and winged insects is passed in 

 water, under conditions which are as contrasted as 

 possible with those which affect them in the adult 

 condition ? Even in their individual life-histories these 

 creatures furnish a sufficient answer to those who 

 demand "missing links!" And it is not a little 

 suggestive that the insect orders which appeared first, 

 during the carboniferous epoch, were those whose 

 members now pass through less differentiated larval 

 stages than those which were introduced later on, such 

 as the butterflies (Lepidoptera) and Neuroptera. More- 

 over, the Orthoptera (a fossil species of which is the 

 first to appear of all known kinds of insects) is an 

 order which perhaps even yet furnishes a larger 

 number of species whose lives are passed under 

 aquatic as well as aerial conditions, than any others. 

 Even the caddis-worms, which are the larvae of in- 

 sects in some respects nearly related to the Lepi- 

 doptera, have an enormous geological antiquity. We 

 have seen limestone beds 5 and 6 feet thick, com- 



