ARTÍCULO: BIMODALITY IN THE BODY SIZE DISTRIBUTION OF MEDITERRANEAN TARANTULA JUVENILES: HUMPHREYS ’ RUSSIAN ROULETTE REVISITED

Bimodality in the distribution of body sizes within a cohort of organisms can be originated by different mechanisms. In the Mediterranean tarantula, Lycosa tarantula (Linnaeus, 1758), spiderlings disperse in two bouts, with approximately half of the spiderlings in the population dispersing before winter and the other half remaining in the burrow with their mothers until the onset of spring. Humphreys (1983) hypothesized that one advantage of early dispersal relative to late dispersal is that, after the winter, early dispersers will be several instars larger than late dispersers. If this is the case, we expect to find a bimodal distribution of spider sizes after the winter. We provide evidence for bimodality in a natural population of L. tarantula, hypothesize proximate mechanisms of biphasic dispersal and expand Humphreys ’ hypotheses on the costs and benefits of diphasic dispersal. Key words: Araneae, Lycosidae, Lycosa tarantula, size bimodality, diphasic dispersal