Evidence for directional selection on male abdomen size in Mecolaesthus longissimus Simon (Araneae, Pholcidae)

Bernhard A. Huber

Abstract

Abdomens of male Mecolaesthus longissimus Simon 1893 are on average more than twice as long as in females, their length is highly variable, and they show extremely steep allometric values when scaled on body size (OLS, b = 2.64). Males cohabit with females, and they likely fight to defend this position as other pholcid spiders do. Male legs, which are usually used in pholcid male-male fights, do not show the usual high allometric values but a very low value (OLS, b = 0.37). Collectively, this lends support to the idea that M. longissimus males do not use their legs in fights and that male abdomens have assumed a role in male-male fights. However, behavioral data are missing and sexual selection by female choice or inter-male display might be involved. A large sample of data from taxonomic revisions is used to document that across pholcids, males consistently have longer tibiae 1 (and probably legs in general) than females. Several possible reasons have been suggested to account for longer male than female legs in various spider groups, but the pattern in pholcids remains to be explained.

Key words: Sexual size dimorphism, sexual selection, allometry, Pholcidae