120 A BOOK OF INSECTS 



Although these reactions may be, and often are, quite 

 independent of a nervous system, there can be no question 

 that such a mechanism confers a distinct advantage upon 

 its possessor by rendering the responses more speedy and 

 effective. A plant, by turning towards the window, 

 evinces its ability to distinguish light from darkness ; but 

 its response is far less rapid than that of an insect, whose 

 eyes enable it to detect on the instant the slightest shadow 

 passing across its field of vision. Nevertheless, the re- 

 sponse in each case is a simple reflex act in which con- 

 sciousness plays no part. 



These reflex acts are often spoken of as tropisms — a 

 convenient word, culled from the Greek, which we may 

 translate " turnings." They form the basis of all insect 

 behaviour. Thus, an insect may be phototropic as it 

 turns to or from the source of light ; thigmotropic as it 

 courts or is repelled by the touch of a foreign body; 

 anemotropic as it faces or turns from the prevailing wind. 

 Many other tropisms are recognisable. But the essential 

 point to bear in mind is that these " turnings " are inevit- 

 able on the part of the being concerned, no matter what 

 the result. A moth rushes headlong into the flame of a 

 candle and sears its wings. It does this again and again 

 until its power of flight is destroyed, and it falls maimed 

 and dying upon the table. The moth is attuned, so to 

 speak, to a low intensity of light. In common with other 

 nocturnal animals it is repelled by strong sunlight, but 

 responds by active movement to the softened radiance of 

 approaching night ; these adjustments being in exact 

 accordance with its needs. That it meets its death in the 

 flame is a mere contretemps, due to the impetus of its 

 rapid flight and to the fact that artificial luminaries are 

 abnormal to its environment. Other animals that 

 move slowly react to the counter stimulus of increasing 





