32 INSECTS AND MAN 



the mustard oil. Further experiments of a similar nature 

 were made with a hymenopterous insect, Priophorus padi, 

 which will only feed on plants of the rose family, because 

 they contain a glucoside known as amygdaline. 



Another scientist, Hewlett, published confirmatory, though 

 even more striking, results in 1912. His experiments with 

 the flesh flies, Sarcophaga, were exceedingly interesting. 

 The larvae of these flies, as their popular name implies, live 

 on decomposing animal matter: the eggs therefore are 

 always laid on some substance that will provide suitable 

 food. Howlett, however, prepared a substance known as 

 sketol from decomposing albuminous matter, and, placing 

 it in a bottle, caused the flies to oviposit in its vicinity. 

 With the stable fly, Stomoxys calcitrans, which deposits its 

 eggs in horse dung and decaying vegetable matter, the same 

 observer performed a like experiment. Closeted with a 

 rag moistened with valerianic acid a product of decompos- 

 ing vegetable matter the befooled flies at once proceeded 

 to the business of egg-laying, being, evidently, ignorant of 

 the fact that there was not a morsel of food for their pro- 

 geny. Speaking of this powerful guiding force, which 

 must have an all-powerful effect on the survival of insect 

 life. Dr Tragardh says " the ovipositing of the females is 

 guided, even in those cases when the larvae has a diet 

 different from that of the adult, by Chemotropism." These 

 discoveries have a value that is almost incalculable to the 

 economic entomologist, for if, as their sequel, injurious 

 insects can be induced to oviposit in places other than on 

 the larval food, their ultimate eradication is only a question 

 of time. 



