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

Protein-fed Calliphora vicina, F1 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 degrees 45' N). The high-density flies in 1996 showed no delay, in that the thermal sum (degree-days) experienced was 133, comparable to 18 degrees C constant, assuming the lower threshold for egg maturation to be 5 degrees C. Low-density cages and flies in a large outdoor cage (2 m3) 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.