Bergmann's Rule: Size Correlates with Longitude and Temperature in Forest Millipedes Centrobolus Cook, 1897

Eco-geographical rules maintained within a taxonomic clade, populations and species of larger size were found in colder environments, and species of smaller size were found in warmer regions. The objective was to test Bergmann's rule. It was tested in the millipede genus Centrobolus with reversed sexual size dimorphism (SSD). Two factors were measured from 22 Centrobolus species - body lengths (mm) and widths (mm). Longitude correlated with female length (r=0.46, Z score=2.15, r2=0.21, n=22, p=0.02), female width (r=0.41, Z score=1.91, n=22, p=0.03), male length (r=0.43, Z score= 2.01, n=22, p=0.02) and male width (r=0.28, Z score=1.27, n=22, p=0.10). Female volume correlated to longitude (r=0.29, Z score=1.31, n=22, p<0.10). The smaller species (C. digrammus) occurred westerly (18.4°E) while larger species (C. inscriptus and C. anulatus) were easterly (31.7°E). In between these two longitudes all the medium sized species ranged. Mean annual temperature from type localities correlated with body size. There was a correlation between species size and temperature (r=0.55, Z score=2.69, n=22, p<0.01). There was a positive correlation between female size (volume) and temperature (r=0.49, Z score=2.31, n=22, p=0.01) and male size and temperature (r=0.48, Z score=2.26, n=22, p=0.01). There were positive correlations between male length (r=0.57, Z score=2.79, n=22, p<0.01), male width (r=0.42, Z score=1.94, n=22, p=0.03), female length (r=0.57, Z score= 2.79, n=22, p<0.01) and female width and temperature (r=0.49, Z score=2.31, n=22, p=0.01). As terrestrial arthropods, size increases an order-of-magnitude with temperature in these univoltine arthropods. This was a converse Bergmann clade in agreement with latitude-size trends in terrestrial arthropods.