SURROUNDINGS OF GROWING INSECTS 189 



enemies, how it obtains against them some measure of pro- 

 tection ; all these aspects of the surroundings demand con- 

 sideration. Insects, on account of their immense numbers in 

 species and often in individuals, and the great variety in details 

 of structure which they display, are admirably suited as the 

 subjects for such a comprehensive study. We are thus 

 concerned with insects in relation to their surroundings during 

 the period of growth and change. 



Among the Exopterygota it frequently happens that the 

 young growing insect, resembling closely its parents in most 

 features of bodily structure, lives and feeds in much the same 

 way. Such is the case for example with grasshoppers, cock- 

 roaches, lice, bugs, green-fly, and many other groups the 

 life-history of which has been reviewed in previous chapters. 

 But in other Exopterygota, and in almost all orders of the 

 Endopterygota, the mode of life of the larva differs greatly 

 from that of the imago, and this sharp distinction of habit 

 and behaviour accompanying the distinction in structure 

 during the successive periods of an insect's existence is a 

 striking and suggestive feature in the developmental history 

 of the class. 



We have already traced the main features in the life-history 

 of dragon-flies (pp. 39-52) and mayflies (pp. 91-7). These afford 

 well-known examples of insects whose larval stages are passed 

 under water, and such a marked divergence between the 

 surroundings of the adult insect and those of its young is worthy 

 of attention. Insects, as a class, are pre-eminently aerial 

 creatures, stamped as such by their power of flight and the 

 system of complex branching air-tubes by means of which 

 they breathe. Yet many insects live habitually in water 

 during the perfect stage several families of beetles, for 

 example, besides such groups of bugs as the water " scorpions" 

 and water " boatmen ". Without exception these insects 

 that are aquatic in the winged state breathe atmospheric air, 

 having some provision for carrying down with them below the 

 surface a supply of air, stored for example beneath the wing- 

 cases of many water-beetles, or for keeping in touch with the 

 atmosphere by means of prolongations of the body, such as the 

 tail-processes of the water-scorpion, thus ensuring a sufficient 

 supply of fresh air to the tracheal system. Such arrangements 



