256 THE STUDY OF ANIMAL LIFE CHAP, xn 



Arthropods) a dorsal contractile heart. The larvae of 

 some insects, e.g. dragonflies, mayflies, etc., live in the 

 water, and the tracheae cannot open to the exterior (else 

 the creature would drown), but they are sometimes spread 

 out on wing-like flaps of skin (" tracheal gills "), or 

 arranged around the terminal portion of the food-canal in 

 which currents of water are kept up. 



The student should learn something about the different 

 mouth-organs of insects and the kinds of food which they 

 eat ; about the various modes of locomotion, for insects 

 " walk, run, and jump with the quadrupeds, fly with 

 the birds, glide with the serpents, and swim with the 

 fish " ; about the bright colours of many, and the develop- 

 ment of their senses. 



In the simplest insects the old-fashioned wingless 

 Thysanura and Collembola the young creature which 

 escapes from the egg-shell is a miniature adult. There 

 is no metamorphosis. So with cockroaches and locusts, 

 lice and bugs ; except that the young are small, have 

 undeveloped reproductive organs, and have no wings, 

 they are like the parents, and all the more when the 

 parents (e.g. lice) also are wingless. 



In cicadas there is a slight but instructive difference 

 between larvae and adults. The full-grown insects live 

 among herbage, the young live in the ground, and the 

 anterior legs of the larvae are adapted for burrowing. 

 Moreover, the larval life ends in a sleep from which an 

 adult awakes. But much more marked is the difference 

 between the aquatic larvae of mayflies and dragonflies 

 and the aerial adults, in which we have an instance of 

 more thorough though still incomplete metamorphosis. 



Different, however, is the life of all higher insects- 

 butterflies and beetles, flies and bees. From the egg- 

 shell there emerges a larva (maggot, grub, or caterpillar), 

 which often lives an active voracious life, growing much, 

 and moulting often. Rich in stores of fatty food, it falls 

 into a longer quiescence than that associated with pre- 

 vious moults and becomes a pupa, nymph, or chrysalis. 

 In this stage, often within the shelter of a silken cocoon, 

 great transformations occur ; the body is undone and 



