Relationship between size and mating success in the blow fly Phormia regina (Diptera : Calliphoridae)

Size variation occurs in natural populations of the black blow fly, Phormia regina (Meigen); head width varied from 2.85 and 1.89 mm to 3.68 and 3.72 mm for males and females, respectively. Using these values as a guide, smal l flies were produced in the laboratory by early removal of larvae from the rearing medium. Mating experiments, pairing flies of various size categori es, showed that when small males mated with large females the percentage of successfully inseminated females was significantly reduced compared with t he other combinations (42% versus 73-89%). Previous worked us to suspect th at small males might have small aedeagi and therefore have a problem in suc cessful sperm transfer when they mate with large females. Measurement of ae deagus size and head width showed that in wild flies the size of the aedeag us is positively correlated with both size.