Sexual dimorphism is a ubiquitous phenomenon, but its ecological role and evolutionary significance are still poorly understood in many hyperdiverse lineages. We used geometric morphometrics to characterize and quantify sexual dimorphism in the head and mesosoma of 24 species in the wasp subfamily Ichneumoninae (Hymenoptera: Ichneumonidae). Our results show that sexual dimorphism varies substantially among species but is generally more pronounced on the head, with males occupying a noticeably smaller region of the morphospace than females. Phenotypic trajectory analyses showed a trend towards conserved trajectories in the difference between males and females, indicating that dimorphism seems to manifest as predictable shape changes across species. Species parasitizing pupae, particularly in concealed substrates, were found to have higher sexual dimorphism for the head than species parasitizing more exposed hosts such as larvae. For the mesosoma, this difference was not significant, with the degree of sexual dimorphism more influenced by phylogenetic history. We hypothesize that female head shape has adapted to the demands related to finding hosts in concealed habitats, whereas species that attack exposed or weakly concealed hosts have kept a generalized head shape in both sexes. These results highlight the importance of investigating intersex ecological differences as drivers of sexual dimorphism.

This dataset consists of photos of the mesosoma and lateral view and head in front view for 24 species of Ichneumoninae (Hymenoptera, Ichneumonidae), with 10 males and 10 females of each species photographed. Standardized photographs of the head in front view and the mesosoma in lateral view taxa were produced using the extended-focus system EntoVision (GTVision, Hagerstown, Maryland), consisting of a Leica MZ16 zoom lens attached to a JVC KY-75U 3-CCD digital video camera that feeds image data to a desktop computer. The program Archimed 5.3.1 is then used to merge an image series (representing typically 15–30 focal planes) into a single in-focus image. Lighting was provided by an EntoVision dome light.

Any image-related software can open the files; landmarks for geometric morphometrics can be generated through the freely available software ImageJ: https://imagej.nih.gov/ij/download.html