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ON THE CHEMOTROPISM OF INSECTS AND ITS SIGNIFI- 

 CANCE FOR ECONOMIC ENTOMOLOGY. 



By Ivar Tragardh, D.Sc, F.E.S., 



Hxperimentalfii Itet, Sweden. 



In a highly interesting' article in a previous number of this journal (vol. iii, p. 

 343) J. Dewitz discussed " The bearing of physiology on economic entomology ," 

 and gave in this connection, amongst other things, an account of the various 

 kinds of tropisms and their use, besides which he also pointed out the necessity 

 for studying these for the purpose of discovering weapons for fighting noxious 

 insects. It strikes me, however, as if the importance of chemotropism had not 

 been emphasised in this survey as in my opinion it ought to have been, and the 

 purpose of these modest lines is partly to draw attention to two important 

 researches in this branch of science, recently carried out independently of each 

 other, partly to point out that with the aid of chemotropism we shall in all 

 probability be able to discover new weapons for the fight against noxious insects. 



By the term " chemotropism " is meant, as it is well known, the automatic 

 orientation of the animals to any olfactory sensation in such a manner that both 

 sides of the body are struck by the lines of diffusion at the same angle. 

 Theoretically, when a substance diffuses an odour, fine particles are ejected in 

 straight lines, but in reality the air currents cause the lines to deviate from their 

 straight track, and for that reason we cannot expect insects in the case of 

 chemotropic movements to follow such straight lines as in the case of 

 phototropic movements. 



Chemotropism is a very important factor in the life of insects, and its positive 

 action is especially evident in the following circumstances : in the search of the 

 sexes for each other ; in their search for food ; and in oviposition. On the other 

 hand, the occurrence of negative chemotropism is demonstrated by the fact that 

 many insects protect themselves against their enemies by discharging evil- 

 smelling fluids, and so forth. 



Entomologists have been aware for a long time that the males of certain 

 species of moths are attracted from a great distance by the females, and the 

 phenomenon has been confirmed by many experiments. It is perfectly well 

 known also that the antennae of many males are far better developed than those 

 of the females, and that olfactory organs of different kinds are found both in 

 male and female Lepidoptera. All these arrangements go to prove that chemo- 

 tropism plays an important part amongst the Lepidoptera from a sexual point 

 of view. It is, however, quite obvious that sexual chemotropism cannot be 

 utilised in practice, unless the females were attracted by the males. But this 

 appears to occur only in extremely rare cases {Hepialus liecta, according to 

 Degener). 



The position is quite different in the case of that form of chemotropism which 

 guides the insects in their search for food, or the females when ovipositing ; and 

 the former action has actually been turned to practical use for along time by 



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