E. boreale has a flight period extending from the beginning of June to the end of August. Females were captured during mating and allowed to lay their eggs (endophytic oviposition) under laboratory conditions. Large quantities of eggs were placed at the following experimental temperatures: 15°, 17.5°, 20°, 22.5°, 25°, 27.5° and 30°C. The equation of the development of eggs in relation to temperature was fitted to transformed data by the method of multiple regression and finally expressed in a theoretical form, as suggested by PRADHAN (1946, Proc. natn. Inst. Sci. India 12: 385-404). A developmental index, the ’’optimal temperature of development”, the ’’developmental threshold” and a ’’developmental-hatching threshold” are defined and analysed. The ecological significance of these entities is stressed.