IfiO THE SENSE ORGANS AND REFLEX BEHAVIOUR 



the cheinotropic responses of Diptcra, especially those breeding 

 in decaying organic material of various kinds. Many species are 

 attracted to varions individual by-products produced as the result 

 of fermentation of different kinds. The early experiments of 

 Barrows dealt with the reactions of Drosophila melanogaster 

 {ainpelophila), which occurs in great numbers around cider presses, 

 packing sheds, orchards, and other situations where fermenting 

 fruit is present : within the latter it deposits its eggs and its larva? 

 develop. Barrows' experiments were made with various con- 

 stituents of fermenting fruit : ethyl alcohol, ethyl acetate and 

 acetic and lactic acids were tested separately and in mixture. The 

 insect was found to exhibit the maximum response to a mixture 

 of ethyl alcohol of 20 per cent, strength and acetic acid of 5 per 

 cent. It was further ascertained that cider vinegar, and fermented 

 cider, contain alcohol and acetic acid in percentages closely 

 approximating to those just mentioned. Subsequent investigators 

 have devoted a considerable amount of attention to the repression 

 of the house-fly by chemotropic means. Many highly complex 

 food materials have been advocated as baits for trapping this 

 insect, leaving the specific attractive constituents undetermined. 

 Among individual reagents tested, low concentrations of ethyl 

 alcohol (3 to 8 per cent.) and other alcohols are definitely attractive, 

 while the addition of sugars is stated to enhance this property. 

 Speyer (1920) found that alcohols, aldehydes and acids containing 

 the methyl group CH3, and whose molecular weight is 30 or over, 

 are positively chemotropic, especially when the methyl grouj) is 

 united to (CHg)^. He suggests that the molecular group 

 CH3(CH2)aj is the actual food stimulus concerned, but his experi- 

 ments were of too restricted a character to be regarded as other 

 than purely tentative. Imms and Husain (1920) tested a range 

 of substances by means of field traps. Ethyl alcohol alone 

 exhibited little or no chemotropic properties in various con- 

 centrations, but with the addition of small amounts of butyric, 

 valerianic or acetic acids it exercised a powerful stimulus. Since 

 dilute acids themselves did not prove attractive, the respective 

 esters were probably the agents involved. Both sexes of species 

 of Rhyphus, Sarcophaga, Calliphora and various Muscidse and 



