Vol. L. LONDON, NOVEMBER, 1918 No. 11 



POPULAR AND PRACTICAL ENTOMOLOGY. 



Insect Tropisms.* 



by professor a. willey, mcgill university, montreal. 



The behaviour of insects, Uke that of other animals, is the 

 result of the interaction between the organism and the medium 

 or environment in which it lives. When properly understood it 

 has the value but not always the precision of a chemical reaction, 

 being the summation of a long series of physico-chemical changes. 



The fundamental relation between organism and medium is 



determined by the necessity of the former to extract its nutriment 



from the latter. The medium not only furnishes food for the 



organism but also for its enemies and, in addition, it is the source 



of catastrophic danger. In order to procure its food the insect 



has to circumvent the enemies which persecute it and evade the 



dangers which threaten it. There are thousands of ways of 



avoiding or minimizing risk and thousands of species to adopt 



one or other of these ways. They all come under the head of 



avoiding reactions," whether they operate as visible movements 



or attitudes, or as structural features; in the former case they may 



be called "global" reactions, in the latter they are cell or tissue 



reactions, i. e., reactions of the idioplasm resulting in adaptive 



growth. 



' After the emergence from the egg the life-history of the holo- 



metabolous insects is broken up into two sharply marked periods 



of active struggle for existence. The larval activities are directed 



towards solving the problem of food and growth; those of the 



imago are concerned directly or indirectly with the problem of 



reproduction to which all the sense-organs are subordinated. 



A larva has only one question to answer : What can be eaten with 



impunity? For the imago the burning question of the day is: 



Where can the eggs be laid with safety? It is one of the tasks of 



the entomologist to ascertain how these elementary realities are 



*Abstract of paper read at 54th Annual Meeting of the Entomological 

 Society of Ontario, Nov. 9th, 1917. 



