The function of the secretory cephalic hump in males of the dwarf spider Oedothorax retusus (Linyphiidae: Erigoninae)

Abstract

In some dwarf spiders (Linyphiidae, Erigoninae), males have cephalic modifications that produce chemical secretions that females touch during courtship and/or copulation. We investigated the influence of these cephalic hump secretions on courtship, copulation probability, copulation behaviour, remating, oviposition and paternity in the dwarf spider Oedothorax retusus. We compared these parameters between males that offered secretions and males whose cephalic hump was experimentally covered. Females mated twice and we limited each male to a single 1 min insertion of his palp into one of two spermathecae. Thereby, the sperm masses of rival males were transferred into separate spermathecae in potentially similar amounts. This set-up prevented direct sperm competition within a spermatheca and allowed us to test selective sperm usage by the female. Virgin females were more likely to mate with males that were able to release secretions. When copulation occurred, males with a covered hump needed significantly more insertion attempts and required more time from first hump contact to the onset of sperm transfer in both virgin and mated females. The number of eggs in the subsequent eggsac was significantly increased for females that only mated with males with an open hump, indicating that substances transferred with the male secretions promote egg production. Paternity of the two male types was not significantly different suggesting that the gland products do not trigger female choice of sperm.