272 Cheinotro^yic Responses of Insects 



situations where fermenting fruit is present; within the latter it deposits 

 its eggs and its larvae develop. Barrows investigated the reactions of 

 this fly to the various chemical constituents of fermenting fruit. Ethyl 

 alcohol, acetic ether and acetic and lactic acids were experimented with 

 separately, and in mixture. The insect was found to be attracted in the 

 most marked degree by a mixture of ethyl alcohol of 20 per cent, strength 

 and acetic acid of 5 per cent. It was further discovered that cider vinegar 

 and fermented cider contain alcohol and acetic acid in percentages very 

 close to those just quoted. A series of experiments were conducted 

 proving that the fly discovers its food by means of the olfactory sense, 

 the latter being located in the terminal joint of the antennae. In 1908, 

 Forbes experimented with various reagents with reference to the Corn 

 Root Aphis, with the object of discovering substances towards which 

 this species reacts negatively. The seeds were treated with various 

 chemicals of which carbolic acid, formalin, kerosene and oil of lemon were 

 found to be of value as repellants, aphis attacks being noticeably 

 reduced after the seeds had been thus dealt with. In 1910 Verschaffelt 

 published an important and highly suggestive paper, embodying his 

 investigations of the factors which determine the selection of food in 

 the case of the larvae of Picris brassicae and P. rapae. The larvae select 

 as their food plants certain Cruciferae, also Tropaeoliu7n and Reseda. In 

 these plants there occurs a group of glucosides — the mustard oils. Ver- 

 schaffelt took a solution of sinigrin, which constitutes the glucose agent 

 in black mustard, and uniformly distributed it over the leaves of plants 

 which the Pieris larvae had previously refused to eat. Leaves so treated 

 were devoured readily. From such experiments it appears that the Pieris 

 larvae exhibit a marked chemotropism towards mustard oils, and it is 

 due to their presence in the leaves of certain plants that determines the 

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

 of research, Verschaffelt has shown that the larvae of the saw-fly, Prio- 

 phorus (Cladius) padi, which feed on certain of the Rosaceae, are 

 attracted by the glucoside known as amygdaline. As regards those 

 factors which determine the females to select certain species of plants 

 for purposes of oviposition, definite information might be acquired by 

 carrying out an analogous series of experiments. In 1911 Patterson 

 observed that flies (Sarchophagidae) would not oviposit on freshly killed 

 material (caterpillars) in the cages even though the females had been 

 ovipositing previously on older decomposing caterpillars. This would 

 tend to show that the material must leach a ceitain stage of docomj)osi- 

 tion before the female would oviposit. In 1912 llowlett published some 



