Do Mecolaesthus longissimus males fight with their abdomens?
(Published in Journal of Arachnology, 2005, see Abstract, PDF)

Mecolaesthus longissimus is very unusual among spiders in that the males (lower left) are much larger than females (lower right). Interestingly, it is the abdomen that is on average twice as long in males than in females. Revealingly, the male abdomen is extremely variable in length (compare the three male abdomens in the upper right), and show extremely steep allometric values when scaled on body size. Morphometric data suggest that male abdomens are under sexual selection, probably by inter-male competition.
In addition, this paper summarizes information on leg-length dimorphism in pholcid spiders. Over 2600 measurements from 100 species show that longer male than female legs are a universal trend in pholcids. The complete data matrix is available here:
as an Excel file
as an SPSS (.sav) file