V \ 



66 Annals Entomological Society of America [Vol. XII, 



Upon this hypothesis, the odors from plants would vary accord- 

 ing to the number, combination and quantitative percentages of 

 the various constituents. Reasoning along this line of thought, 

 we may be able to explain why a few insects have only one host 

 plant; why many have a preferred host plant, but will eat 

 other allied plants; and why others eat a large number of 

 plants. If we could positively answer the above questions, we 

 might be able to devise practical methods for the control of 

 certain insects, as by trap baits, etc. 



The present investigation will show that lepidopterous 

 larvae respond to chemical stimuli and that they have organs 

 suitable for receiving these stimuli; and the following review of 

 the literature will indicate how insects are attracted by certain 

 host plants. 



Verschaffelt (1910), experimenting with the cabbage-butter- 

 fly larvae, Pieris hrassicce and P. rapce, found that they are very 

 fond of the cultivated species of the Cruciferae and that in cap- 

 tivity they ate leaves from 15 indigenous species, representing 

 14 genera, of the same family. They did not eat all of these 

 species, however, equally readily, and refused to eat species 

 belonging to 17 other families, but did attack Tropceolum 

 and Reseda which belong to two other families. Chemists have 

 determined that all of the plants eaten by the above larvae 

 contain mustard oils. To determine whether these larvae 

 could be induced to eat leaves not ordinarily attacked, leaves 

 of Apios tnherosa were smeared with the juice from the leaves 

 of a crucifer {Bimias orientalis) ; these leaves were at once 

 attacked and in a short time devoured. Wheat flour and corn 

 starch, which when dry or moistened with water are rejected 

 by both Pieris-larvcE, are eaten with avidity when .soaked with 

 Bunias-]mce. The larvae behave in a similar manner toward 

 filter paper saturated with Bunias-]\x\ce. 



To ascertain the exact constituent in the plants and juices 

 which attracted the larvae, Verschaffelt wet leaves of Apios 

 tuberosa and Rosa (two species not containing mustard oil) with 

 a fairly strong solution of pure sinigrin (potassium myronate), 

 the glucoside of black mustard ; these leaves were eagerly eaten 

 by the larvae. He says: " It is clear that P^'em-caterpillars seek 

 out various mustard oils, just like the various glucosides derived 

 from them. They are clearly attracted by the whole group of 

 substances." 



