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

The diversity of reproductive strategies within the arachnids rivals all ot her arthropod groups. However, with the possible exception of spiders and s corpions, evolutionary biologists have overlooked these organisms. The orde r Opiliones is divided in three suborders (Cyphophthalmi, Palpatores and La niatores) 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. Amo ng 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 diffe rences when it does occur suggest that male care evolved independently in d ifferent 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 pressu re 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 be tween the parental and the offspring such as, restriction of the reproducti on effort to specific periods and places, considerable adult longevity, and aggressive mechanisms of defence. On the other hand, many of these preadap tations mentioned are absent in the Palpatores. Reduced fecundity and incre ased egg size accompanied the evolution of subsocial behaviour in laniatore an harvestmen. Palpatores, which show very simple forms of parental investm ent, have significantly higher fecundity and a smaller egg size than Laniat ores, which show subsocial behaviour.