The Olfactory Sense of Lepidopterous Larvæ

Abstract

A thorough investigation of the above subject will undoubtedly throw some light upon how plants attract insects, because as yet no one has shown experimentally that lepidopterous larvæ can smell and the olfactory organs have never been described. Whenever it is proved that insect larvæ respond to chemical stimuli, and whenever organs suitable for the reception of these stimuli have been found, then we can intelligently ask why do the cotton caterpillar (Alabama argillacea Hübr.) and cotton boll weevil (Anthonomus grandis Bah.) feed exclusively on the cotton plant?; or why does the silkworm (Bombyx mori L.) feed almost exclusively on mulberry-tree leaves? Many more similar examples could be given by any entomologist, but why is it that some larvæ are very selective in regard to their food while others show little or no preference between members of a large list of plants. Also, how does a female lepidopteran distinguish the best or only suitable host plant for her progeny so that she can deposit her eggs on this particular plant? The only plausible answer for all of these questions is to suppose that plants, as well as animals, emit odors, and that insects in searching for food, either for themselves or for their young progeny, are guided by the odors emitted by the plants. In order that the odors of plants may differ, it may also be assumed that the various chemical constituents of plants emit particular odors and that the odor emitted from a plant may be a combination of all the odors from the various constituents, or possibly one odor might be so strong that it masks all the others. Upon this hypothesis, the odors from plants would vary according 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.