Sexual Size and Shape Dimorphism in Three Species of Parasitoid Wasps with Burrowing Females: Spalangia endius, Spalangia nigroaenea, and Spalangia nigra (Hymenoptera: Pteromalidae)

The parasitoid wasps Spalangia endius Walker, Spalangia nigroaenea Curtis, and Spalangia nigra Latrielle (Hymenoptera: Pteromalidae) develop on filth fly pupae. Females burrow through decaying organic matter and parasitize hosts; whereas, at least in S. endius, males tend to stay above ground searching for mates. Both sexes lack obvious digging morphology such as enlarged forelegs and are not known to exhibit physical aggression. Size data were obtained from specimens from Illinois field-collected hosts for all three species and from a Florida laboratory colony for S. endius. The degree of sexual size dimorphism varied with body part and species, but the direction of bias was consistent between the field and laboratory specimens of S. endius. Females had wider abdomens in S. nigroaenea and S. nigra (not measured in S. endius). In all three species, females had longer heads than males, both in absolute size and relative to width. The latter is referred to as narrowness. Forewings were significantly narrower in females compared with in males for both S. endius and S. nigroaenea. Thorax narrowness was either greater in males (S. endius) or was not significantly different between the sexes (S. nigroaenea and S. nigra). Patterns of sexual size dimorphism seem consistent with females’ need to store eggs and burrow. For all three species, there was overlap between males and females in all body parts measured. Thus, these size measurements will be unreliable to differentiate the sexes. Size ratios also overlapped.