Scientists have noticed that sexual cannibalism is exhibited by certain spider species more than others, especially in those that belong to genus Lacrodetus. Many have speculated that sexual cannibalism occurs because the sacrificial male has low fecundity and is unlikely to mate again in order to produce offspring after copulating with a single female. Thus, it is eaten because the evolutionary benefit it can contribute to the population is less. However, these are merely speculations and no concrete evidences have been shown to prove it.
Dear Romelie!
Below are two "classical" studies that contradict your claim of "no evidence" for adaptive cannibalism in spiders:
Andrade, M.C.B. (1996) Sexual selection for male sacrifice in the Australian redback spider. Science, 271: 70- 72.
Andrade, M.C.B. & Banta, E.M. (2002) Value of male remating and functional sterility in redback spiders. Anim. Behav., 63: 857- 870.
There is a nice phylogenetic-comparative study showing that species with males that have little chance of remating (because their intromittent organs break inside the female's reproductive tract upon first mating) are much more likely to be cannibalized by females. I don't know of any studies actually measuring male fecundity, that would be a tricky thing to do.
Miller, J. A. (2007). Repeated evolution of male sacrifice behavior in spiders correlated with genital mutilation. Evolution, 61(6), 1301-1315.
What I can tell you is that, in accordance with what J.C.Spagna says, in Lycosa tarantula, which is a species with sexual cannibalism, the males try to avoid being caught and eaten by females at least until the end of the breeding season. The males exhibit mate guarding. They wait until the molt of the females to mate with them when their cuticule is still soft. After mating, they leave the burrow and try to find another non-molted female to mate with her. As the breeding season goes on, less non-molted females are available, and the females' willingness to catch males instead of mating with them is higher. Hence, at the end of the breeding season is when most of sexual cannibalism occurs.
I neglected to mention that there are several papers on male-cannibalism in non-orb-web building spiders (mostly Lycosidae, consistent with Jordi's answer), most of which come out of Ann Rypstra's lab at Miami U. in Ohio.
http://www.units.miamioh.edu/spiderlab/Publications.html
Thank you for answering my questions. Your answers helped me and my group mate in writing our research paper. Thank you very much! :)
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