158 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



numbers of such insects that were caught in the traps, he found 

 that 30 per cent, was the maximum number caught over a period 

 of nine days after release. Assuming that this percentage is 

 approximately the percentage taken of the entire local population 

 of the species, the particular chemotropic method adopted is 

 ineffective, especially since many of the moths caught have 

 opportunity for oviposition before capture. 



Lloyd (1920) recommends the use of chemotropic measures 

 against the moth Hadena (Polia) oleracea, whose larvse are 

 destructive to tomatoes grown under glass in England. He 

 found that a mixture of cane molasses, ale and 1 per cent, sodium 

 fluoride is an effective means of control, particularly when following 

 periods during which spraying against the larvae is impracticable. 

 The relatively constant environmental conditions afforded by 

 glasshouse cultivation greatly favours the use of this measure, 

 which is now regularly practised by growers. 



In 1910 Verschaffelt conducted experiments in order to ascertain 

 the factors which determine the adoption of specific food-plants by 

 phytophagous larvae. He utilised caterpillars of Pieris which feed 

 upon certain of the Cruciferae along with Tropwolium and Reseda. 

 In these plants there occurs a group of glucosides — the mustard 

 oils — and Verschaffelt took a solution of sinigrin, which is a 

 constituent of black nuistard, and spread it uniformly over leaves 

 of plants the Pieris larvae had previously refused to eat. Leaves 

 thus treated were readily devoured, and he concluded that Pieris 

 larvae exhibit a strong positive reaction to mustard oils, and it is 

 their presence in the leaves of certain plants that determines the 

 selection of the latter by the larvae for their food. By similar 

 methods this same investigator showed that larvae of the saw-fly 

 Priophorus {Cladius) pyri, which feed upon certain Rosaceae, are 

 apparently attracted by the glucoside amygdalin. 



Mclndoo (1926) has described a laboratory instrument which 

 he terms an " olfactometer " designed for the purpose of testing 

 the chemotropic responses of insects. His main object was to 

 prove that plants attract insects by the odours which they emit. 

 A small potted potato plant was placed in a special chamber of 

 the instrument and a gentle current of air was drawn over it into 



