118 INSECT PHYSIOLOGY 



This same 'fixation reflex' doubtless provides the basis of the res- 

 ponse to moving objects that is shown by many insects. Some forms, 

 like the larva of the dragon-fly, will follow any object moving near 

 them; others, like the adult dragon-fly, after turning momentarily to 

 such an object, instantly leave it if it is not suitable for food, the 

 initial reflex being inhibited by the higher centres. In the bug Rhod- 

 nius, the response to moving objects is normally elicited with dif- 

 ficulty, but it becomes very marked if the antennae are removed. 



Then there are insects which will move at a constant angle to a 

 source of light; a method of orientation known as 'menotaxis'. In 

 some cases this response seems to result from the balanced turning 

 effect of two lights - that from the source itself and that reflected 

 from the background. Sometimes included in this mechanism is the 

 'light-compass' orientation of ants and bees, which make use of the 

 direction of the sun's rays in finding their way back to the nest. Cer- 

 tain aquatic insects (e.g. Notonecta) maintain their position in run- 

 ning water by a comparable mechanism - by keeping constant the 

 image of objects on the bank. If these objects are moved, the insect 

 can be caused to move with them. Even such insects as Gyrinidae, 

 which swim in all directions in the water surface, seem, surprisingly 

 enough, to maintain their position by the same mechanism. 



Sense organs : mechanical senses 



We must now consider briefly the sense organs of insects from the 

 point of view of their relation to these forms of orientation. Most of 

 these sense organs take the form of 'sensilla', small organs in the 

 integument each of which is furnished with a sense cell (or group of 

 sense cells) with a sensory axon. The primary type of sensillum is the 

 tactile hair. Indeed, nearly all the articulated hairs or spines of insects 

 are provided with a sense cell which is stimulated when the hair is 

 moved. Such organs are often most abundant on the antennae, 

 which are thus important tactile organs, but they occur all over the 

 body and the feet and are responsible for the responses of 'akinesis' 

 or 'stereokinesis' already described. 



In some parts of the body sensilla are developed without any spine 

 or bristle: there is just an innervated dome set in the cuticle. These 

 structures are termed 'campaniform sensilla'. They are stimulated by 



