Actions and How They Are Performed 131 



doubt it is the feel of the mother's breast, or any similar object, 

 which initiates the sucking. Similarly, young birds will gape, i.e. open 

 their mouths for food, whenever a small object approaches the nest. 

 The red-backed shrike has an innate tendency to impale its food on 

 sharp thorns. 



In all these cases the patterns of action are all present in the brain 

 — imprinted, as it were, among the nerve connections and only 

 requiring the appropriate stimulus to bring them into action. 



This method has reached its greatest development among the 

 insects, which carry out extraordinarily complex actions in their life 

 cycles, obviously without any 'understanding' of their object. Fabre 

 and other students of insect life have recorded many remarkable 

 examples. For example, the hunting wasp, Ammophila, lays its eggs 

 on the top of a caterpillar, which is alive and has merely been 

 paralysed by injecting a nerve poison into the nerve centre of each of 

 its segments. It is then placed in a hole in the ground which was 

 previously prepared, which is then sealed and the caterpillar remains 

 alive but immobile, ready to feed the larva when it hatches out. 



Another wasp, Eumenes, constructs a clay urn, the lower part of 

 which is filled with live caterpillars. Suspended from the top by a 

 thread and out of reach of the caterpillars, is a single egg. When the 

 egg hatches out the larva hangs on the thread and is able to eat the 

 nearest caterpillars without danger. When it gets bigger it can descend 

 from its thread and consume the rest. 



The insect performs a quite extraordinary series of actions without 

 any knowledge of what it is doing. It has been found that each series 

 of this kind is a sequence. One step forward cannot be taken until 

 the previous one is completed. Every step, when complete, provides 

 the stimulus for the next. The series of actions can, as a rule, be 

 only carried out in one order. 



Students of instinct, such as Tinbergen, Lorenz and Bierens de 

 Haan, have recognized that, to initiate an instinctive action, a par- 

 ticular sensory stimulus is required, which 'releases' the series of 

 actions which follow. What it really does must be to 'release' a 

 sequence of signals which pass along the nerves to the muscles and 

 so cause the necessary muscular movements. The ability to make 

 these nerve signals is obviously imprinted on the nervous system, 

 waiting only for the proper sense stimulus to bring it into action. 



Tinbergen has shown that the releaser is easily imitated and that 

 it has to have certain features while others are unimportant. 



There is some doubt as to how much latitude is possible in per- 

 forming the actions. In many cases an insect is quite at a loss if the 

 sequence of actions is interfered with. For example, if the clay cage 



