INTRODUCTION 31 



which also undergo a metamorphosis, such a shortage 

 arrests this stage to an almost incredible extent. 



During the varied changes that take place in the life- 

 cycle of an insect, it is extremely rare for the parent to 

 take any interest whatever in his or her progeny ; in many 

 cases, in fact, the sole duty of the adults is to increase 

 their kind, and frequently the female dies after ovipositing. 

 How then, one may ask, do the young fare so well, on the 

 whole, in the struggle for existence ? How do they know, 

 without experience or parents to guide them, what food 

 will be good for them and what harmful, for every herbi- 

 vorous insect has its appointed food plant or, at most, two 

 or three food plants and will not eat of all green things in- 

 discriminately. The mother insect lays her eggs on the 

 proper plant, one may airily reply, without advancing far, 

 for something must guide the parent in its choice, and most 

 people would call that something instinct, though another 

 word, Chemotropism, is more correct. Chemotropism has 

 been called " the guiding force perceived by an animal 

 through its olfactory sense " ; it is a very important force in 

 insects, because it instructs them where to lay their eggs ; 

 it helps them in their search for food and for mates. Most 

 people know that if the females of certain species of moths 

 be confined in a gauze-covered cage, males of the same 

 species will seek them out as swiftly and surely as a vulture 

 seeks carrion. Verschaffelt, in 1910, published some inter- 

 esting results of his experiments on the choice of food in 

 insects. He studied the larvae of the large and small 

 cabbage butterflies and discovered that they would only 

 feed on plants that contained a certain chemical known as 

 a glucoside, in this case one of the mustard oils nothing 

 would induce them to feed on any plant which did not con- 

 tain this substance ; so he made a dough of the glucoside 

 and smeared it on the leaves of a plant which the larvae had 

 previously refused, with the result that they now greedily 

 partook of the proffered food, being misled by the presence of 



