Landmark-free morphometrics reveals sexual dimorphism in shape and integration of tagmata in the forensically important blowfly Calliphora vicina

Phenotypic integration has been shown to influence the degree and direction of organismal evolution in vertebrates. Comparatively fewer multivariate analyses of shape have been conducted in invertebrates, partly due to difficulties in identifying unambiguously homologous landmarks. We present the first landmark-free deterministic atlas analysis (DAA) of shape and integration between insect tagmata, characterizing intraspecific variation in the blowfly Calliphora vicina (Diptera: Calliphoridae). We recover sexual dimorphism in the shape of all three tagmata, even where differences are gradational or indistinguishable from visual inspection. Partial least square analysis reveals high integration between tagmata within females, but not males. In males, integration is low between the thorax and abdomen, and absent between the head and abdomen. This reflects independence in the mechanisms underlying shape variation, including the development of highly sexually dimorphic eye morphologies and of structures that house the genital apparatus. Furthermore, we find a relationship between within-segment integration and morphological variance suggesting within-tagma integration may be facilitating shape variation in C. vicina. These intraspecific insights highlight the potential for landmark-free studies investigating the influence of development on shape and integration between species and across clades. Moreover, these techniques may aid forensic entomology, where sex determination can assist with postmortem interval estimates.