Chemical reactions of fruit-flies

Extract

There are certain smells remarkably attractive to male flies of the genus Dacus and by the employment of these smells the movements of the flies can to a great extent be controlled, in any given direction. The reaction is strictly confined to the male sex, and different species exhibit a variation as regards the smell which is most attractive to them. It is uncertain whether the females emit similar smells; on the whole improbable.

Three of the commoner species of the flies (D. diversus, ferrugineus and zonatus) normally breed respectively in (1) anthers of Cucurbitaceae, (2) fruits of Solanaceae and mango, and (3) peach, guava, mango and other fruits. D. diversus (1) is most strongly attracted by iso-eugenol, zonatus (3) by methyl-eugenol, and ferrugineus (2) by both methyl and iso-eugenol. The smells of these substances have not yet been identified with those of the plants which constitute the normal breeding-places, but male flies have been found attracted to mango, Papaya, a Cycad, and Colocasia. plants with a very characteristic smell similar to that of eugenol-derivatives. Females have never been seen to frequent these plants or to breed in them, but more extended observation is required on this point before it can be definitely said that they never do so. The males give the impression that they might visit these plants for food, for they suck the surface of the flowers, though they do not seem to exhibit any very particular avidity. One species (ferrugineus?) has been bred from a Eugenia at Bangalore.

Three explanations suggest themselves:.—

(a) The smell is a direct sexual guiding smell emitted by the female.

But young crushed females did not attract males.

(b) The smell is not emitted by the female, but may be termed an “indirect” sexual guide to the plants where the females are accustomed to congregate for breeding purposes.

Under these circumstances it is difficult to see why females should not also be attracted by the odoriferous chemicals.

(c) The smell is a food-smell.

It must then be a food attractive only to males.

Two other species of fruit-flies, D. caudatus (breeding in fruits of Cucurbitaceae) and an unidentified species, were captured by means of an amyl-vanillin solution and of eugenol respectively. The rarity of the flies and the small numbers caught (four and one) make it advisable, however, to postpone detailed comment on these two cases, and future work may give more definite information as to their chemical relations. No substance attractive to male cucurbitae has been found.

All the observations described were made at the Pusa Research Institute, India.