Factors affecting size, longevity and fecundity in the reindeer oestrid flies Hypoderma tarandi (L.) and Cephenemyia trompe (Modeer)

1. Laboratory reared reindeer oestrid flies Hypoderma tarandi and Cephenemyia trompe (Diptera: Oestridae) were weighed to determine progressive weight loss and death weights at treatments with various temperature and humidity conditions. 2. Four individual measurements of size were taken: larval weight, wet weight of newly eclosed flies, wing length, and weight of flies after dehydration and fat extraction. In H. tarandi, males were bigger than females (except for wing length), whereas the reverse was true for C. trompe. 3. Size variation was not significantly related to conditions (temperature, humidity, duration) during the pupal stage, but individual reindeer produced flies (both species) of different mean sizes. These size differences were not correlated with larval burden (= number of larvae per individual host), but are hypothesized to be connected to unknown host quality factors. 4. Longevity of flies kept in vials and subjected to various temperature and humidity conditions revealed that C. trompe lived significantly longer than H. tarandi (range: 4-44 and 1.2-27 days, respectively) at 5-33 degrees C. Male H. tarandi survived longer than females; female C. trompe survived longer than males. Longevity was not significantly correlated to any of the size measures. 5. Most flies had a large portion of their fat reserves left at death. 6. In H. tarandi, mean number of eggs was 609 +/- SD 73 (range 354-772, n = 119). Egg number was slightly dependent on larval size, but not on wet weight of newly eclosed flies or wing length. In C. trompe, mean number of eggs was 960 +/- SD 208 (range 493-1349, n = 31). 7. The possible adaptive value of large size in oestrids is questioned. Benefits of flexibility in size in oestrids are hypothesized.