THE POLYMORPHIC FORMS OF MELITTOBIA CHALYBII ASHMEAD AND THE DETERMINING FACTORS INVOLVED IN THEIR PRODUCTION. (HYMENOPTERA: CHALCIDOIDEA, EULOPHIDÆ)

¹ ZOÖLOGICAL LABORATORY, UNIVERSITY OF PENNSYLVANIA

1. The parasitic chalcid Melittobia chalybii Ashm. occurs in two forms, the type-form and the short-lived or second-form. The production of one or the other of these two forms is determined by the trophic conditions obtaining during the growth of the larva. A third form of female was obtainable experimentally and very rarely in the normal cultures.

2. The males of the second-form differ from the type-form in their darker color and in the more aborted condition of their optic organs. The females of the second-form differ from typical females in their lighter color, in the failure of their wings to spread upon emerging from the pupa, in the large swollen condition of the abdomen which is full of ripe eggs at the time of emergence, and in the shortness of their imaginal life. Both sexes of the second-form emerge as adults in a minimum of fourteen days after oviposition compared with the ninety days required for the development of individuals of the type-form.

3. The life history is as follows: A female of the type-form enters the nest of a host wasp (e.g., Trypoxylon politus Say), and remains upon the host larva within the cocoon, feeding and ovipositing for about 70 days, until it dies. The first few (12 to 20) eggs laid develop in about 14 days into a generation of males and females of the second-form. These do not leave the host cocoon and the females deposit their eggs upon the same host upon which their mother continues to oviposit. All eggs, those of the mother and those of her offspring of the second-form, now develop in about 90 days into chalcids of the type-form. After mating, the females alone gnaw their way to the exterior.

4. Experiments involving the transfer of eggs from one host to another and the transfer of ovipositing females to different individual host larvæ demonstrated that all eggs of both forms of females are potentially alike and capable of developing into imagoes of either form. In every experiment, the first few larvæ which feed upon a given host invariably give rise to adults of the second-form, all the remaining larvæ to adults of the type-form. The conclusion is that the first larvæ ingest mainly the blood of the host while the later ones must feed to a larger extent upon the remaining tissues; and that this trophic difference determines the production of one or the other form of adult from a single type of egg.

5. Experiments involving the changing of the trophic conditions during the life of an individual larva give some insight into the nature of the determination and the time at which it occurs.

6. When larvæ were transferred from a partly consumed host to a fresh host, a third form of female individual, termed intermediate-form, was obtained.

7. In crossing females of one form to males of the other form, including males derived from virgin females, the results obtained differed in no way from those obtained from the usual matings which occur in nature.

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