Based on the paper "AN UPDATED CHECKLIST OF HALIPLIDAE, GYRINIDAE, DYTISCIDAE,nNOTERIDAE & HYDROPHILIDAE FROM HONG KONG", I have thought that Rhantus sp. does not occur in Hong Kong. However, I have encountered the larvae of Rhantus recently in Pat Sin Leng and Lai Chi Wo. Thus, I am not sure species 4 is Rhantus sp./ Hydaticus rhantoides. They can be identified by Pederzan's paper (1995): Keys to the identification of the genera and subgenera of adult Dytiscidae (sensu lato) of the world (Coleoptera Dytiscidae)
These two genus is from different subfamilies. The main feature that used to distinguish between them is that: For Rhantus sp., it is from Colymbetinae and the eyes are emarginated above bases of antenna (indicated from the picture) while for H. rhantoides, the eyes are round and not emarginated
[ 本帖最後由 sykin 於 2016-8-5 15:28 編輯 ]