Bulbal muscles in spiders: an overview
(published in J. Morphol., 2004; PDF)
 

This is maybe the largest comparative histological study so far in spiders. It is based on some 40.000 sections produced from 107 spider species representing 73 families. The focus is on the muscles that move the genital bulb (in this photo on the left, with the sperm duct inside). The intriguing question that I try to answer is why have spiders (i.e. most of them) given up on muscles and evolved hydraulic movement of the bulb instead?