Egg Cocoon-Carrying Behavior in Female Water Scavenger Beetle, Helochares nipponicus (Coleoptera: Hydrophilidae), Functions to Increase Offspring Fitness and Survivability

Abstract

Insects of the family Hydrophilidae produce egg cocoons (a silk-like substance that surrounds the eggs). In several genera, the female attaches these egg cocoons to their abdomen and carries them until the larvae hatch. In this study, we examined whether the egg cocoon-carrying behavior of the water scavenger beetle, Helochares nipponicus (Coleoptera: Hydrophilidae), functions to protect the egg cocoon from physical, biological, and environmental stressors. The number of hatchlings from egg cocoons that were removed from female scavenger beetles and floated on the water surface did not differ from those of the that remained attached. However, few eggs hatched from egg cocoons submerged in water and from those placed in a dry environment. When first instar larvae of the conspecific species were put into small Petri dishes with egg cocoons still attached to the female, the number of hatchlings did not differ from those without larval cohabitation. In contrast, when the egg cocoons were removed from the females, the presence of conspecific larvae significantly reduced the number of hatchlings. First instar larvae fed on the eggs by inserting their heads into the egg cocoons. These results strongly suggest that the egg cocoon-carrying behavior of H. nipponicus functions to regulate air and water to the eggs and avoid cannibalism by conspecific larvae.