\ND ITS SIGNIFICANCE FOR ECONOMIC ENTOMOLOGY. L15 



substances likewise common to the Cruciferae, Reseda and Tropaeolum, which 

 exercised their attractive force upon the Pieris larvae, Verschaffelt took a 

 solution of sinigrin, which forms the glucose part of black mustard, and with a 

 brush spread it upon some leaves of other plants which the larvae had refused to 

 eat without previous treatment. These were then devoured with relish. From 

 these experiments it will be clearly seen that the Pieris larvae exhibit a strong 

 positive dbemotropism* towards a group of substances, mustard-oils, and that it 

 is the presence of these substances in the leaves of their food-plants which makes 

 them palatable for the larvae. 



In the same manner Verschaffelt showed, by experiments with the larvae of a 

 herbivorous Hymenopteron, Priophorus padi, which attacks the leaves of some 

 members of the natural order Kosaceae, that in these there occurs a glucoside, 

 amygdaline, which forms the attractive substance. It is hardly too daring to 

 predict that further investigations will merely go to confirm these observations, 

 and that the choice of food by herbivorous insects is determined by the occurrence 

 of certain specific organic substances in the plants. So far only a beginning has 

 been made with this problem ; but it is certain that co-operation between 

 vegetable chemists and entomologists in this (question will prove highly 

 profitable. 



The above-mentioned experiments do not, however, give an answer to the 

 question as to what factors guide the females of the herbivorous species in their 

 ovipositing, and it is a fact that in the majority of cases the larvae have no option 

 in the choice of their food. The selection is made by the females, and were 

 their instinct not reliable, the larvae would perish. I am more especially 

 thinking of the leaf-miners which, at least when quite young, would not be able 

 to quit the plant upon which they had been deposited in the egg-stage. 



It has been frequently observed that when moths are trapped with baits — in 

 opposition to what occurs when catching with lanterns — a very large percentage 

 of fertilised, egg-laden females is obtained, and this point seems very suggestive. 

 Dewitz considers that this is due to the fact that the females are in the habit of 

 feeding before ovipositing, and that they are attracted by the bait because the 

 smell of the latter is like the odour of the fluids in their food. This is quite 

 possible ; but we might also imagine that the attraction is due to the odour of 

 the bait being like that emanating from the food-plants of the larvae, and it 

 would appear to be an easy matter to solve this question on the basis of Vers- 

 chaffelt's researches by enticing the females of Pieris to oviposit upon other 

 plants than the normal food-plants by means of sinigrin, for example. 



No such experiments have, however, as yet been carried out, so far as I am 

 aware, but in some cases where the larvae and adults have nearly the same diet, 

 the attempt has been made to deceive the females into ovipositing by the aid of 

 certain organic substances extracted from their food. 



F. M. Howlett last year published the results of some experiments of this 

 kind. 3 He succeeded in enticing Sarcophar/a to oviposit in a bottle containing 

 scatol, a pungent substance found in excrement, and which is the product of 



* This term is not used by Verschaffelt, who regarded the problem merely from the stand- 

 point of vegetable chemistry. 



