84 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 10 



hydrotropism, phototropism, and anemotropism. If time permitted, 

 a consideration of that extensive reahn of insect behaviour included 

 within the category of instinctive behaviour would be desirable, but we 

 must content ourselves with the knowledge that this type of behaviour 

 is the result of reflex responses to the various types of tropisms. 



Chemotropism is the reaction to stimuli of a chemical nature per- 

 ceived through the olfactory sense. Inasmuch as odour is undoubtedly 

 the most important factor in the environment of insects the significance 

 of this tropism is evident. The chief objects of animal or plant life, 

 are feeding and reproduction, and in the search for food or for the 

 sexes, or in oviposition, chemotropism plays a predominant part. 

 The sexual chemotropism of insects, particularly among the Lepidop- 

 tera, has long been a familiar phenomenon to entomologists. But it 

 is in their response to chemical stimuli as affecting the search for food 

 and oviposition that we find a tropic reaction that has untold possi- 

 bilities in its practical application. 



The vital functions of search for food and oviposition are closely 

 associated. The female insect deposits its eggs on substances best 

 suited for the nourishment of the larvae. The females of Pieris rapce 

 and P. hrassicce select the leaves of cruciferous plants, attracted thereto 

 by the mustard oils, a group of glucosides present in these plants, as 

 shown by the experiments of Verschaffelt. The same investigator 

 showed that the larvae of the sawfly Priophorus padi (L.) Thomas, 

 which feeds on the foliage of certain rosaceous plants, are probably 

 attracted by a glucoside, amygdaline. The chemotropic reactions 

 on the part of carrion beetles, and to excrement on the part of coproph- 

 agus Coleoptera and Diptera, are well known. Howlett induced 

 Sarcophaga to oviposit in a bottle containing scatol, a decomposition 

 product of albuminous substances; and he stimulated the oviposition 

 response in Sto7noxys calcitrans by means of valerianic acid. Rich- 

 ardson's recent work on the oviposition response of the house-fly, in 

 which flies were induced to oviposit in response, apparently, to an 

 attraction of ammonia in conjunction with butyric and valerianic 

 acids, opens up suggestive lines of investigation. Barrows finds that 

 the positive reaction of Drosophila to fermenting fruit is due in a large 

 measure to amyl, especially ethyl alcohol, acetic and lactic acid and 

 acetic ether. 



While the aforementioned cases, which might be multiplied, illus- 

 trate the chemotropic responses of insects in so far as they affect the 

 oviposition response, that is, the search for food as affecting the future 

 larvae on the part of the ovipositing female, there is the large class of 

 chemotropic reactions which affect only the adult without reference 

 to the progeny. An illustration of this class is afforded by the investi- 



