Delayed egg production and a possible group effect in the blowfly Calliphora vicina

.Protein‐fed Calliphora vicina , F 1 offspring of wild flies in two cages with lower and higher fly densities showed variable delay in starting oocyte vitellogenesis at ambient semi‐natural temperatures in warm July–August weather in 1996 and 1997 at Durham in northern England (54°45′ N). The high‐density flies in 1996 showed no delay, in that the thermal sum (degree‐days) experienced was 133, comparable to 18°C constant, assuming the lower threshold for egg maturation to be 5°C. Low‐density cages and flies in a large outdoor cage (2 m 3 ) in both years showed delays in production of first eggs of 34 days (thermal sum 293 degree‐days) in 1996 and 32 days (396 degree‐days) in 1997, and longer delays for other individuals. Delays in egg production at low densities relative to high densities seem to be a group effect of unknown mechanism.