Spiders with bizarre gift-giving male head structures: morphology, biochemistry and sexual selection

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In many species males possess traits that are involved in mating but not in sperm transfer, i.e. traits that seem to function as clasper devices or stimulatory devices. There are three main hypotheses to explain such secondary sexual traits: 1. they may function as isolating mechanisms between species or 2. have evolved under sexual selection due to female choice or 3. due to sexual conflict. Spiders of the linyphiid subfamily Erigoninae are ideal organisms to test these hypotheses as males of many species possess bizarre head structures that play a role in the mating process and are associated with secretory glands. The few observations available todate suggest that the females sink their claws (chelcicerae) into the male head structures and ingest secretion produced by the associated glands. If these bizarre head structures function as isolating mechanisms, interspecific matings should be impossible and contact with the head structures should be a prerequisite for successful copulation. If the structures and their associated glands have evolved under sexual selection due to female choice, males with more pronounced head structures or males that are able to produce more secretion may be more attractive for females if they gain material or genetic benefits for themselves or for their offspring. The head structures may also have evolved under conflict between the sexes over control of mating and fertilization. Male secretion may serve to manipulate females in that it prevents remating with additional males despite costs for female lifetime fecundity or longevity. My investigations of the possible functions involve three focal species of dwarf spiders, Diplocephatus permixtus, Oedothorax retusus and Oedothorax gibbosus. We are currently investigating the function of the secretion by manipulating its transfer to the female and plan also to manipulate its amount. The study entails a combination of behavioural, morphological, histochemical and biochemical approaches and will help to understand the function of these bizarre male secondary traits.