The male dimorphism in the dwarf spider Oedothorax gibbosus (Blackwall, 1841) (Erigoninae, Linyphiidae, Araneae): Results of laboratory rearing experiments

Oedothorax gibbosus is a dwarf spider bound to oligo- and mesotrophic alder carrs. Two male morphs occur: gibbosus, characterised by a hunch and a hairy groove on its carapace, and tuberosus without these features. The hairy groove is supposedly of importance during gustatorial courtship behaviour. Earlier studies indicated that this dimorphism was presumably determined by a di-allelic gene only expressed in the male sex. Allele G for gibbosus is dominant over allele g for tuberosus. An elaborate laboratory rearing experiment was set up to test this hypothesis, which was based on only a small number of observations. Analysis of a large number of family trees was in agreement with this model of inheritance. A small fraction of the reared spiders (5\%) needed a fifth moult to reach the adult instar. This is the first time that an exception to the normal number of four moults in dwarf spiders of the subfamily Erigoninae has been observed. The dwarf spiders that moulted five times also hatched significantly earlier, and this probably made an additional moult necessary. The juvenile phase of the gibbosus males was significantly longer than that of the tuberosus males. The juvenile phase is the period between the emergence of the spiders and the last moult. Two possible mechanisms compensating for the advantage of gibbosus in sexual selection were observed: sex ratio distortion in favour of the female sex in tuberosus-genotypes, and shorter juvenile development.