REACTIONS TO CHEMICAL STIMULI 163 



of fruit-fly, viz., Ceratitis capitata. They showed that certain 

 mineral oils, including kerosene, naphtha and benzene, similarly 

 exercise a powerful chemotropic attraction for the males of this 

 species. The significance of the phenomenon, however, remains 

 unexplained. 



Practical measures designed to exploit the reactions of insects 

 to chemical stimuli afford abundant promise of extended 

 api^lication. Much of the data at present available, however, 

 is of a purely tentative character. It is too heterogeneous and 

 scanty to allow of any fundamental conclusions being drawn. 

 We know as yet, for example, very little with respect to the 

 relations between chemical constitution and the attractive 

 properties of different compounds. An indication of possibilities 

 in this direction, however, has been given by Cook (1926), but 

 his results are marred by the fact that, although about 50,000 

 insects were involved in his experiments, their specific identities 

 are unstated. The discovery of chemical attractants is only one 

 aspect of the problem. Due cognisance needs to be taken of the 

 fact that reactions to such substances are only the end-results of 

 a series of interacting stimuli. On the one side are external 

 factors, including temperature, humidity and light, and on the 

 other are the internal physiological states of the insects themselves. 



The preliminary study of the reactions of insects in the 

 laboratory under properly controlled conditions is obviously an 

 important aspect of any work designed to exploit the subject 

 from an economic standpoint. A variety of instruments or 

 " olfactometers " has been used. One devised by Mclndoo 

 (1926) has already been referred to. When investigating blow- 

 flies this worker used an instrument of different pattern (Mclndoo, 

 1938) and partly made of wood. Other kinds have been described 

 by Barrows (1907) in connection with experiments with Drosophila : 

 by Ripley and Hepburn (1929) in their studies with the South 

 African fruit-fly Pterandus rosa : by Hoskins and Craig (1934) 

 with reference to Lucilia : and by Marshall (1935a) for the hive 

 bee. Some of these instruments show obvious disadvantages : 

 either they are too slow in operation or give inconclusive results, 

 or are not easily washed and rendered inodorous. Also, the 



