Sex determination of Japanese rhinoceros beetles, Trypoxylus dichotomus (Coleoptera: Scarabaeidae), based on their dropping shape

Abstract

The Mahalanobis-Taguchi system (MTS) is one of the multi-variate analyses which can classify by correlations among multi-variables. It has a significant advantage of applying a cause analysis which provides effective variable combinations to upgrade the accuracy of the classification. In this study, droppings, which were extruded from Japanese rhinoceros beetle larva (called Kabutomushi in Japanese) were classified into male/female based on their shape-related features using the MTS. Their droppings have a spheroid shape of approximately 10 mm in their long axis. Seven features (i.e., variables) were obtained from digital micrographic images using image analysis software, such as height/width (H/M), major/minor (Ma/Mi), perimeter/Feret diameter (P/F), projected area/Feret diameter (A/F), roundness, circularity, and solidity. Although an abnormality rate, which shows how the correlation pattern of one larval dropping is far from any standard ones, has a boundary in order to classify into male/female from 18 larval droppings, 3 larva were judged as the opposite sex. Based on the cause analysis, the A/F and solidity were the best variable combination which all the larval droppings were completely classified into male/female. Why these variables were affected by the sex most seemed to be caused by the gut components, as determined by time-domain nuclear magnetic resonance (TD-NMR). MTS has the great ability to classify from the powders’ minor difference and will be an effective tool for particle design in the near future.