280 Chemotropic Responses of Insects 



recording the results. So far as possible we have worked with simpler 

 substances, leaving the more complex organic compounds for later 

 work. It is hoped that by this method it will be possible to discover 

 whether any general relationship exists between chemical constitution 

 and attractive properties. 



The question of a suitable trap for exposing the agents used is of 

 very great importance. It is essential that the substances to be tested 

 should be exposed in traps so constructed as to admit any insects that 

 are attracted thereto and, at the same time, preclude their escape. A free 

 outlet for the uniform dispersal of the odour of each reagent used is 

 essential and, furthermore, the constructional materials must be in- 

 odorous, and must not be readily acted upon by chemicals. Convenience 

 of size and handling should not be overlooked, and the traps should be 

 readily washable whenever necessary. After trials with various types of 

 traps it was found that one based upon the Minnesota model (Wash- 

 burn, 1912) was more suitable than any other. It possesses, however, 

 the defect that insects can only enter around the base of the trap, and 

 it is extremely likely that a certain number of insects which may be 

 attracted do not succeed in entering. We have mainly used a rectangular 

 trap of this type measuring 11" x 7" x 9" covered with ordinary fine 

 white muslin, which can be readily renewed when necessary. The colour 

 or condition of the trap appears to exercise some influence in the attrac- 

 tion of insects. Thus Dewitz (1912) states that white saucers are more 

 attractive than coloured ones. Morrill (1914) also observed that insects 

 came more readily to a new balloon trap than to a rusty trap. During 

 trials conducted with clean empty traps, however, we found that the 

 number of insects which entered therein was negligible. The greatest 

 number was five Diptera and one Lepidopteron, after 36 hours exposure. 

 In each experiment 50 c.c. of a chemical substance of a known strength 

 was exposed for a given time, and full details recorded of any species 

 attracted. It proved difficult during these preliminary trials to keep 

 full meteorological data but, as far as possible, a record of maximum 

 and minimum temperatures, both by means of ordinary and wet and dry 

 bulb thermometers, was kept and the general conditions of the weather 

 noted. 



During one trial an empty trap was exposed on one occasion and 

 we were surprised to find a considerable number of Diptera entrapped. 

 On the previous day this same trap contained a markedly attractive 

 agent and many insects were caught. It appears, therefore, evident that 

 either the odour of the previous reagent lingers, and still exercises chemo- 



