Sexual size dimorphism and the rejection of Rensch’s rule in Diplopoda (Arthropoda)

Abstract

Rensch's rule predict the negative associations between sexual size dimorphism (SSD) and body sizes for relatively larger females. This prediction was tested for forest and savanna diplopods using a geometric morphometric approach using calculations of length and width to derive shape volume based on the mathematical formulae for estimated cylindrical worm-like millipede size (l.π.r 2) and spherical pill millipede size (4/3.π.r 3): (i) Centrobolus (were collected in February 1996, South Africa) SSD was 0.63-2.89 (1.52±0.35; 267) (ii) Sphaerotherium (extracted from literature) SSD was 1.49-5.36 (2.96±1.40; n≥7); (iii) savanna (Calostreptus, Doratogonus, Odontopyge and Spinotarsus were collected in February 1989, Zimbabwe) and forest helminthomorphs (collected in February 1996, South Africa) SSD was 0.88-1.62 (1.26±0.23; 1233). Interspecific variation regressed was SSD (0.63-5.36) on body sizes (n=1273) with no significant negative correlations rejecting Rensch's rule. Eco-morphological patterns were discussed.