Relative sexual size dimorphism in Centrobolus digrammus (Pocock) compared to 18 congenerics

Abstract

The present research was aimed to study relative sexual size dimorphism of Centrobolus digrammus (Pocock) compared to 18 congenerics. Millipedes illustrate reversed sexual size dimorphism (SSD) as females are larger than males; and corroborate Rensch's rule as this dimorphism increases with body size. SSD was calculated in 18 species of the genus Centrobolus and illustrated as a regression. The approximate relative position of C. digrammus was shown from measurements taken at Simon's Town Waterfall, South Africa (March 1997). The size of C. digrammus was 49.9 X 4 mm: 54.5 X 4.8 mm (males: females; n=6) and logged (females/x = 2.993; males/y = 2.790). The mean volume ratio for C. digrammus was 1.599. The evidence suggests the proximate cause for SSD in C. ruber is sexual bimaturism while the ultimate cause in Centrobolus is intersexual competition.

Centrobolus dubius (Schubart, 1966) Monomorphism

“…Previous studies on SSD in invertebrates and these results consistently give a positive correlation and break the rule [5,[21][22][23][24][25]26] . Figure 1 shows the finding for Centrobolus dubius where the regression of log male volume on log female volume was highly significant with a positive slope of 1.…”

Centrobolus anulatus (Attems, 1934) reversed sexual size dimorphism

“…Previous studies on SSD in invertebrates and these results consistently give a positive correlation and break the rule [5,10,[12][13][14][15][16][17][18][19]36] . Figure 1 shows the finding for Centrobolus anulatus where the regression of log male volume on log female volume was highly significant with a positive slope of 0.86207; showing females get larger than males with an increase in body size [2][3][7][8] .…”

“…Size differences correlate with factors such as color, sexes, species, urbanisation and water relations [25-26, 31, 34] . Diplopoda resemble the majority of invertebrates where SSD is reversed [12][13][14][20][21][22] . SSD has consequences for outcomes of sexual encounters in diplopod mating [1, 4-9, 11, 15, 25-27, 29, 34] .…”

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

“…C. digrammus was unlike C. inscriptus with ordinarily small males and larger females which are similar compared to 18 Centrobolus species for which data was available [34] . C. fulgidus was similar to C. inscriptus with small males and larger females compared to 18 Centrobolus species for which data was available [35] . C. ruber males and females followed the trend for SSD and (break) Rensch's rule in Centrobolus [36] .…”