Sexual size dimorphism and corroboration of Rensch’s rule in Chersastus millipedes (Diplopoda: Pachybolidae)

Abstract

Sexual Size Dimorphism (SSD) was investigated in the millipede genus Chersastus. Mass, length and width were used to compare intraspecific variation in four species. Interspecific variation in volume was calculated in 18 species and an allometric coefficient of 0.85 found, which corroborates Rensch's rule in this genus.

Fire millipedes obey the female sooner norm in cross mating Centrobolus (Myriapoda)

“…Copulation may be female mediated in C. inscriptus. The explanation for the reduced copulation duration in the hetero-specific crosses cannot be based on species sexual size preference [18] , sex ratio [17] or sperm load [25] as these factors were controlled in the cross mating experiment. It is possible that variation in genital morphologies and loss of genital contact are responsible for the reduced copulation durations [26] .…”

“…The inscribed red millipede Centrobolus inscriptus and the red ringed millipede C. annulatus exemplify this sympatry [17] . They have equal sexual size dimorphism [18] , but differ in copulation duration [19] and genital structure [20] . A one-way cross experiment between C. inscriptus and C. annulatus was performed to find changes in mediation of copulation duration.…”

Heavier-shorter-wider females in the millipede Centrobolus inscriptus Attems (Spirobolida: Trigoniulidae)

“…The arcsine of length: width ratios places the sexes as equally apart as was the distribution for width (Figure 1d). The combination of these two parameters may be perceived as sexual size dimorphism as represented by the distribution of body volumes (Cooper, In Press) [2] .…”

“…Centrobolus millipedes obey Rensch's Rule as females are larger and there is a steady rise in sexual size dimorphism with overall body size with the allometric coefficient of 0.85 [2] . This allometric finding was also made by earlier myriapodologists [3] .…”

Sperm dumping in Centrobolus inscriptus Attems (Spirobolida: Trigoniulidae)

“…The active release of ejaculate post-insemination and copulation duration was studied in Centrobolus inscriptus where sperm or ejaculate precedence correlates with prolonged second copulation duration and re-analysis of sperm volumes from single and double mating experiments with 24 hour (h) delays post-mating [14][15][16][17][18][19][20][21][22][23][24][25][26] . In order to provide supporting evidence for female control of ejaculate and sperm dumping after mating, ejaculate volumes released from females 24h after mating was studied and compared alongside double matings.…”