UPPER THERMAL LIMITS FOR HYPODERMA-LINEATUM (DIPTERA, OESTRIDAE) EGG HATCHING AND DEVELOPMENT

During periods of Hypoderma lineatum (De Villers) oviposition activity , ambient temperatures reached 28 degrees C and dorsal bovine dermal t emperatures exceeded 45 degrees C. Egg hatch decreased linearly with i ncreased constant temperatures from 79% at 35 degrees C to 0% at 39 de grees C. Egg hatch varied from 58 to 80% after 1-, 3-, or 5-h exposure s to 40 or 45 degrees C. Exposures of 3 h or more at 50 degrees C were lethal, whereas 66% of all eggs survived 1-h exposures to 50 degrees C. The upper thermal limit for survival at constant temperatures lies between 38 and 39 degrees C, with similar to approximate to 53% surviv al at 37 degrees C. H. lineatum eggs are well adapted to typical tempe ratures occurring in their natural habitat; however, these temperature s can at times exceed the eggs' upper developmental threshold. This li mitation of the host environment may have led to the evolution of adul t oviposition preferences for the lower parts of the host animal, shie lded from incident solar radiation.