Position of targets and period of competence for UV-induction of the malformation "Double Abdomen" in the egg ofSmittia spec. (Diptera, Chironomidae).

1. UV irradiation of anterior pole regions of eggs ofSmittia results in the formation of "double abdomens". The frequency of this malformation has been recorded as a function of developmental stage, size and position of irradiated area, and UV dose. 2. The efficiency of irradiation decreases with increasing longitudinal extent of the irradiated anterior area. 3. Within the anterior fourth of the egg, irradiation of peripheral regions is more effective than irradiation of central regions. 4. The varying efficiency of irradiation is thought to represent a corresponding distribution in the concentration of effective targets or the morphogenetic efficiency of the targets in the irradiated state. Alternative interpretations are discussed. 5. There is a multiplicity of targets distributed symmetrically to the longitudinal egg axis. 6. The effective targets have been proved to be localized in the ooplasm of eggs aged less than 1 h. There is evidence to show that the targets are confined to the ooplasm during all stages studied. 7. The targets are probably stored during oogenesis. 8. The irradiated targets affect the body segment pattern far beyond the irradiated area. 9. Possible relations between the targets for UV-induction of double abdomens and factors that are involved in the formation of the normal body segment pattern are discussed.