Living of Land: Issues of reproduction
Reproduction on land required changes since their aquatic ancestors relied on water to transport the sperm cell to the egg cell. In millipedes the reproductive cells are located close to the base of the second pair of legs. The male and female animals meet and intertwine, the male reaches forward with his seventh leg and collects his sperm and transfers it to the sexual pouch of the female. Such copulation was laborious but safe, but was not suitable for the predatory animals that evolved then but still survive today as centipedes, scorpions and spiders. These three groups of animals have all undergone a reduction in segmentation and all may indulge in cannibalism. As a consequence of this scorpions armed with large poison glands and spiders have evolved ritualized courtship patterns prior to copulation.
Do female spiders eat their mates?
It is often said that the male (usually significantly smaller than the female, down to 1% of her size for Tidarren sisyphoides), is likely to be killed by the female after the coupling, or sometimes before intercourse has occurred. This supposed propensity is what gave the Black Widow Spider, Latrodectus mactans, its name. However, the three species of North American black widows do not seem usually to kill the male (although they may) and males can sometimes live in the web of a female for a while without being harmed. The male Australian Redback Latrodectus hasselti is killed ritually by the females after it inserts its second palpus in the female genital opening; in over 60% of cases the female then eats the male. Although the male Latrodectus hasselti (known also as the Australian redback spider) certainly dies during mating without the female consuming it, this species represents a possible strategy of "male sacrifice". The male redback, while copulating, 'somersaults' and twists its abdomen directly onto the fangs of its mate. Most males get consumed at this stage (Andrade 2003 reports 65%). Males that 'sacrifice' themselves gain the benefit of increasing their paternity relative to males who do not get cannibalized (see Andrade 2003, Behavioral Ecology vol.14:531-538).
However, despite these examples, and many other similar reports, the story of the 'sacrificial male' might have become greater than the truth. Mating of spiders is not always followed by cannibalism. Rainer F. Foelix, (1982), says "The supposed aggressiveness of the female spider towards the male is largely a myth... only in some exceptional cases does the male fall victim to the female.". Michael Roberts (1995) says "It is rare for a fit male to be eaten by the female..." And yet, spider cannibalism does occur in some species more than in others, mainly species belonging to Latrodectus.
However, there has been speculation on why this sacrifice of male mates might occur at all. One theory is that once the male has mated, if he is unlikely to mate again then further extension of his life serves no evolutionary purpose, while the sacrifice of the male may help increase egg production through increased nutrition for the female. Having more offspring would give the male an advantage of having his genes passed on over other males that were not eaten. This would be consistent with the hypothesis (Roberts, 1995) that old or unfit males get eaten, whilst younger and fitter ones may survive to mate again. Another reason promoting male self-sacrifice could be low probabilities of finding another possible mate. Males of some spider species have mortality percentage of around 80% (for example L. hasselti from Andrade 2003, and other Latrodectus ssp.). Thus investing more in a mate that has already been found in order to increase paternity with that copulation, would increase the probability of transferring the males' genes to the next generation. An accessible discussion of such hypotheses can be found in the entertaining book by Judson (2002).