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.

A review of studies on the fire millipede genus centrobolus (diplopoda: trigoniulidae)

“…Intersexual competition was believed to drive SSD in diplopods with forest taxa containing the diversity of species and sizes [40][41] . Sexual size dimorphism and the rejection of Rensch's rule in Diplopoda (Arthropoda) was based on Trigoniulids, Sphaerotheriids and Spirostreptids [41][42][43] .…”

Centrobolus dubius (Schubart, 1966) Monomorphism

“…The cross-mating experiments in Centrobolus suggest a combination of size assortative mating without a size based preference operates [2] . C. dubius does not resemble the majority of Diplopoda where SSD is reversed [28][29][30] . Studies of diplopod sexual dimorphism may include more taxa and make use of the length and width measurements to calculate volumes using the geometric morphometric approach shown here for finding causal relationships of dimorphism.…”

Does sexual size dimorphism vary with female size in forest millipedes Centrobolus Cook, 1897?

“…The finding of converse or inverse Rensch rule implies SSD increases with body size when females are larger [1, 2, 4, 13, 15, 18-19, 22-30, 34, 35, 37-40] . This has implications in the class Diplopoda because females are larger than males and SSD increases with body size [6][7][8][9][10][11] . The forest genus Centrobolus of the diplopod Order Spirobolida diplopod found along the east coast of southern Africa were the subject of this study.…”