82 Applied Entomology 
view of the fact that the X-rays have a decidedly injurious effect upon 
certain tissues and functions in the higher animals, these results are 
somewhat remarkable. The possibilities, however, of this line of 
investigation cannot yet be said to be in any way adequately tested. 
Furthermore, certain of the associated rays require isolating and their 
effects definitely ascertained. 
V. The place of Physiological Research in Applied Entomology. 
In addition to the physiological questions already discussed, there 
is a very wide field for the investigation of the tropisms of insects, 
especially chemotropism or, in other words, their orientation to olfactory 
seusations. This phenomenon is particularly in evidence in the selection 
by larvae of their food-plants, by the female when she selects particular 
plants for the purposes of oviposition, and in the pursuit of the male 
for the female. Quite recently Tragardh (1913, p. 113) in Sweden, and 
Dewitz (1912, p. 343) in Germany, have emphasised the possible 
importance of the study of chemotropism in relation to applied ento¬ 
mology. Verschaffelt (1910, p. 536) has investigated the factors which 
determine the selection of food in the case of the larvae of Pieris 
hrassicae and P. rapae. The larvae choose as their food-plants certain 
of the Cruciferae, also Tropaeolium and Reseda. In these plants there 
occurs a group of glucosides—the mustard oils. Verschaffelt took a 
solution of sinigrin, which constitutes the gluco.se 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 strong 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, Priophorus (Cladius) padi, which feed on certain of the Rosaceae, 
are attracted by the glucoside known as amygdaline. As regards the 
factors which determine the females of many insects to select certain 
plants for purposes of oviposition, definite information might be acquired 
by carrying out an analogous series of experiments. Hewlett (1912, 
p. 416) has recently experimented with certain Diptera, and in the case 
of the stable fly, Stomoxys calcitrans, he succeeded in inducing the 
female to lay her eggs on pieces of cotton wool soaked with valerianic 
acid—a substance occurring in decaying vegetable refuse. The obvious 
