no THE BIOLOGY OF INSECTS 



plicated to be explained as due to one tropism only, for they 

 find their way to the sub-mucous coat of the gullet, where 

 they rest or wander to and fro for several weeks, and whence 

 they travel backwards by way of the dorsal muscles or the 

 diaphragm and the vertebral canal to a position just beneath 

 the skin of the host-animal's back. Their final resting-place 

 is never far from the line of the beast's backbone, which 

 suggests that negative geotropism may still be one factor 

 governing their behaviour, and not long after their arrival 

 there they perforate the beast's skin. This action ensures 

 a supply of fresh air to the spiracles at the tail-end of the 

 body directed towards the hole during the later weeks of 

 lars'al life, when the maggots have attained a considerable 

 size, and also provides for the maggot when it is ripe a way 

 out of the host's body ; it works through the skin, falls to 

 the ground, seeks shelter, and pupates (Plates II, III, XVI). 



Many more examples might be given, but these are 

 sufficient to indicate how often an insect's behaviour at 

 some period of its life has reference, not only to its present 

 need for food and possibly for shelter, but also to future 

 contingencies in its gro\\th and development which it 

 cannot possibly foresee. These actions are remarkably 

 purpose-like, but the creature that performs them can have 

 no knowledge of their purpose. The tendency to react in 

 certain ways to the environment and its stimulation is part 

 of the insect's inherited nature ; it is so bound up with 

 the stor}^ of the race, that many thinkers on these questions 

 who realise that the memor}- or foresight of the individual 

 can play no part in the appointed process, do not hesitate 

 to idealise that process by suggesting that it implies a 

 '' racial memor>^ " so impressed on the species that the 

 appropriate Unes of behaviour are followed by adult and 

 larva as one generation follows another. 



The behaviour of social insects is a subject of special 

 interest, and some details of this will be discussed in a later 

 chapter. In closing this general sketch of insectan behaviour 

 it may be noted how in many cases a large insect population 

 of the same or of allied species, not practising social life 



