Female genitalia in Pityohyphantes phrygianus, a spider with a skewed sex ratio

The functioning of female genitalia in many arthropods, especially the site and mode of sperm storage, may have a strong impact on the pattern of sper m usage, leading to differential male fertilization success and even varyin g offspring sex ratios. The female genital morphology of the linyphiid spid er Pityohyphantes phrygianus was investigated, as this species: was shown t o possess an intriguing mechanism of sex ratio determination, possibly rela ted to genital morphology. By means of SEM and serial semi-thin sectioning, two distinctly different spermathecae plus an additional sperm sac, equipp ed with several valves, were found on each side of the genital apparatus. S perm is stored under different conditions, as one of the spermathecae exhib its large gland cells of different types that discharge their products into the spermatheca through long secretory ducts, whereas the other spermathec a is surrounded by a simpler type of gland cell. Furthermore, there are fol ds instead of closed ducts leading to the spermathecae, and from the sperma thecae to the oviduct. The latter fertilization-fold runs inside the copula tory chamber and enters the oviduct from the outside. The mechanisms for se x ratio bias in spiders are discussed. The occurrence of two types of sperm atheca of which one is twisted, provides good evidence for the involvement of female genital morphology in sex ratio control of P. phrygianus offsprin g. Data are compiled on species for which information is available on sex r atio and female genital morphology. We put forward a hypothesis that in spe cies with a skewed sex ratio, there is interaction between female genital m orphology and behaviour, i.e. change of body position before, during or imm ediately after sperm transfer.