On the habitats of three species of the genus Hygrotus Stephens (Coleoptera: Dytiscidae)

SUMMARY. 1. Adult Hygrotus beetles were collected from about 400 localities out of 621 sampled in the Netherlands. Distribution of the three Dutch species of this genus has been related to different environmental variables, principally by use of the Index of Representation (I.R.).
2. Hygrotus decoratus is an acidophilous and haloxenous species. It inhabits small water bodies such as peat-ditches, reed-lands and overgrown ditches where emergent plants are abundant. Hygrotus inaequalis occurs in all of the habitats studied and has no clear preferences or aversions with respect to the environmental variables examined. Hygrotus versicolor inhabits permanent water bodies such as clay- and sand-canals which have a rich growth of submerged plants and a weak current. The species has a significant preference for waters with a pH between 7.1 and 8.0 and a chlorinity between 60 and 200 mg 1-⁻¹.
3. H. decoratus and H. versicolor rarely co-exist because of their different environmental requirements, but both species often co-exist with H. inaequalis. Structural environmental variables such as water depth and abundance and growth-form of the aquatic plants play a greater role than chemical factors in habitat selection by Hygrotus spp.