116 INSECT PHYSIOLOGY 



taxis' or 'stereokinesis'). In the state of immobilization or 'akinesis' 

 which such conditions induce, these insects may require strong 

 stimuli to arouse them. Experimentally, a similar hypnotic state may 

 be induced, in the earwig for example, by many kinds of constantly 

 repeated stimulation ; and this state is doubtless related to the death- 

 feigning reflex or 'thanatosis' shown by many insects when alarmed. 

 Insects which are capable of very quick 'escape reactions', such 

 as the cockroach or the larvae of dragon-flies, possess 'giant axons'. 

 These large nerve fibres conduct impulses at a rapid rate; they carry 

 stimuli from sense organs in the abdomen and enable the insect to 

 give a very quick sterotyped response with about the same speed as 

 the eye-blink in man. 



Orientation 



A great part of behaviour consists in orientation - the direction of 

 the movements of the animal in response to external stimuli. For the 

 purposes of physiological description such reactions are regarded as 

 made up of a series of reflex responses organized in such a way that 

 the animal is moved in a predictable direction. But it is seldom pos- 

 sible to trace in detail the reflexes involved : the most that can be done 

 is to classify the general mechanisms by which certain stimuli can 

 bring about appropriate movements. 



In the first place the insect may reach its destination without being 

 truly orientated : the direction of movement is not clearly related to 

 the source of stimulus. Reactions of this type are termed 'kineses' or 

 undirected reactions. The simplest form is 'orthokinesis' : when the 

 stimulus acts the insect moves, when it ceases to act the insect comes 

 to rest. We have already considered some examples of 'akinesis' 

 brought about in this way. 



A more complex form of undirected response is termed 'klinoki- 

 nesis'. Here the insect moves in a straight line in a favourable envir- 

 onment, but as soon as it enters a mildly unfavourable environment 

 it begins to make turns, the frequency of turning increasing with the 

 strength of the stimulus. If the stimulus is very strong the insect turns 

 aside instantly, giving an 'avoiding reaction'. After a time it becomes 

 'adapted' to the adverse stimulus; it then goes straight, and continues 

 to do so as long as the stimulus remains the same or diminishes. If 



