Parental investment and the evolution of subsocial behaviour in harvestmen (Arachnida Opiliones)

The diversity of reproductive strategies within the arachnids rivals all other arthropod groups. However, with the possible exception of spiders and scorpions, evolutionary biologists have overlooked these organisms. The order Opiliones is divided in three suborders (Cyphophthalmi, Palpatores and Laniatores) with considerable differences in morphology, habits and behaviour. In this review we focus on the life history, sexual behaviour and ecology of Opiliones, and discuss the possible causes of the diversity of forms of parental investment found in this order. In the Cyphophthalmi the forms of parental investment are restricted to the choice of oviposition sites. Among the Palpatores there is selection of microhabitats for oviposition, egg-hiding, and discharge of repugnant secretions on the eggs. The most common form of parental investment in Laniatores is probably egg-hiding by females but several species show subsocial behaviour, including both maternal and paternal care. The scarcity of paternal care among harvestmen and the differences when it does occur suggest that male care evolved independently in different laniatorean lineages from a plesiomorphic state of non-care rather than from female care. Both male and female care reduce egg mortality. Egg predation, rather than infection by fungi may be the most effective pressure favouring the evolution of subsocial behaviour in harvestmen. Subsocial behaviour appears to be restricted to members of the Laniatores since they show a conjunct of preadaptations that may favour prolonged associations between the parental and the offspring such as, restriction of the reproduction effort to specific periods and places, considerable adult longevity, and aggressive mechanisms of defence. On the other hand, many of these preadaptations mentioned are absent in the Palpatores. Reduced fecundity and increased egg size accompanied the evolution of subsocial behaviour in laniatorean harvestmen. Palpatores, which show very simple forms of parental investment, have significantly higher fecundity and a smaller egg size than Laniatores, which show subsocial behaviour.