INTRODUCTION 13 



of the eggs, or the young may be fed like the helpless nest- 

 lings of birds. Thus family life of a kind is exemplified by 

 many insects, and it is well known how, among the wasps, 

 bees, and ants, for example, the size of the family becomes 

 enormously increased so that the assemblage may not un- 

 suitably be compared to a state with its " officers of sorts," 

 and the nest to a city whose streets and habitations swarm 

 with orderly and industrious crowds. Such " social " 

 insects have from early times aroused the admiration of 

 observant men who have sought to draw, for their own 

 guidance, lessons of wisdom from the apparently intelligent 

 behaviour of these small yet wonderfully organised fellow- 

 creatures. Later we may have opportunity to discuss how 

 far such comparisons between insectan and human societies 

 may justifiably be carried. 



In bringing to a close our short survey of the field which 

 it is proposed to cover in this volume, a summary — condensed 

 and therefore necessarily technical in mode of expression — 

 of the main features of insect structure in relation to life- 

 conditions may perhaps be of service. 



Insects, then, are a class of the Arthropoda : segmented 

 appendiculate animals, clothed with a chitinous cuticle 

 which forms an exoskeleton giving attachment to striated 

 muscle-fibres. The head, whose appendages are modified 

 as feelers (one pair) and jaws (usually three pairs), is sharply 

 distinct from the three-segmented thorax which bears six 

 legs and usually four wings, the latter being dorso-lateral 

 outgrowths of mesothorax and metathorax ; thus a variety 

 of precise and rapid motions, including flight, may be 

 possible. The breathing- organs are complex, branching 

 air- tubes, Hned with spirally- thickened chitinous cuticle, 

 facilitating direct gaseous exchange between tissues and 

 atmosphere wherewith this tracheal system typically com- 

 municates through a series of paired spiracles. The 

 digestive tube has extensive anterior and posterior tracts 

 lined with chitin. The perivisceral and pericardial spaces 

 are haemocoels, the latter receiving blood through its 



