Determination of Paronychiurus kimi (Collembola: Onychiuridae) age structures by head width measurements with reference to cadmium toxicity

The age structure of Paronychiurus kimi (Lee) was determined by analysis of the head capsule width distribution using a HCAP program. The HCAP program successfully separated five distinct peaks, which represented a single age group, with a low misclassification probability of 0.1. The mean head capsule widths for each age group were 0.123, 0.165, 0.258, 0.304 and 0.350 mm, respectively. The age groups of P. kimi could be subdivided into two postembryonic developmental stages, the juvenile stage (groups 1 and 2) and adult stage (groups 3–5), on the basis of stage at first reproduction of P. kimi. Based on this information, we examined the chronic effect of cadmium on the age structure of P. kimi. After 28-day chronic exposure to various cadmium concentrations, the total number of P. kimi individuals decreased in a concentration dependent manner. The proportions of the juvenile and adult stages to the total number of individuals in the cadmium treated soils were significantly different from those in the control. In addition, the overall proportions of age structures were affected by cadmium treatment. These results indicate that head capsule measurements can be used to determine the age group of P. kimi and chronic exposure of cadmium to P. kimi at the individual level may be translated and manifested at the population level, by changing the age structures of P. kimi populations.