92 THE BIOLOGY OF INSECTS 



questions before bringing tliis section of our study to a 

 close. 



Manv insects, like the proverbial moth, fly directly 

 towards a lamp, and the obser^'ations of J. Loeb (1905) and 

 others show that a definite relation of the creature's body 

 axis to the source of light is always brought about. The 

 hght seems, one may say, to exert a pull on the insect, 

 inciting it first to turn head on towards the light, and then 

 to move its body as a whole along the path of the luminous 

 rays in the direction opposite to their course, so that at 

 length it flies against the lamp. Such a response is usually 

 called a tropism, because the insect is ine\'itably turned in a 

 certain direction and along a certain path under the impact 

 of the stimulus. As this response draws the insect towards 

 the source of light it is defined as " positively phototropic." 

 The same reaction is shown by many lars'ae as well as by 

 winged insects. Caterpillars commonly move towards the 

 light. " If they are obKquely placed," writes E. L. Bouvier 

 (1922), " on a plane surface opposite to the source of light 

 they quickly turn their heads, follo\^'ing the axis of the light 

 ray and then the rest of the body moves in this direction." 

 On the other hand, there are insects on which the influence 

 of light is the exact reverse of this ; they turn away from it, 

 as may be observed by W. B. Herns (191 1) in the maggots of 

 flies or bluebottles exposed to bright daylight or strong lamp- 

 light. These lan-ae first direct their heads from the source 

 of light and then move away towards the gloom. So 

 experimenters call them " negatively phototropic." It is 

 important to realise that such terms as these, while con- 

 venient for analytical definitions of behaviour, offer no 

 explanation of such beha\*iour ; they simply express in 

 tw^o or three long words that some insects appear to seek 

 and others to shun the light. But the use of the word 

 tropism to define such responses of insects to stimulus is 

 generally taken to imply that the response is automatic and 

 not necessarily accompanied by any distinct sensation or 

 experience ; the term was long ago applied to describe the 

 behaviour of plants in response to luminous and other 



