242 TJie Study of Animal Life PART in 



The wings are very characteristic. They are flattened sacs of 

 skin, into which air-tubes, blood-spaces, and nerves extend. It is 

 possible that they had originally a respiratory, rather than a 

 locomotor function, and that increased activity induced by bettered 

 respiration made them into flying wings. 



The breathing is effected by means of the numerous air-tubes 



{ v or tracheae which open externally on the sides and send branches 



/ to every corner of the body. As the air is thus taken to all 



' [ the tissues, the blood-vascular system has little definiteness, though 



there is (as in other 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 

 development 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 differ- 

 ence 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 previous 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 rebuilt, wings bud 



