Guarding your sperm with your life

Like all spiders, the male dark fishing spider (Dolomedes tenebrosus) mates by giving his mate a pedipalp job. Pedipalps are short stubby “arms” that spiders carry on both sides of their heads. In the males, these essentially have become their penises: the males have a regular sex opening in their nether regions, but when mating time is there, they squirt a droplet of sperm in a specially-fashioned “sperm web”, suck up this sperm droplet in their fountain-pen-filler pedipalps, and go seek out a female. When they find a willing female, they use their pedipalps to transfer the sperm into her vagina (of sorts).

But whereas most spiders try to get away after having donated their sperm, not be eaten by the female, the dark fishing spider does not make a quick get-away. Instead, as Stephen Schwarz and colleagues report in Biology Letters, it locks its pedipalp to his female’s holiest of holies, and then simply expires in pure bliss. The female walks around with the dead male dangling from her pudenda, and eventually still ends up eating it. A nice video-report of the New York Times sums it up for those of you who can’t walk through the paywall of Biology Letters.

Why would these spiders do this? It’s a bit of a mystery. There is one other spider that euthanises itself after mating, Tidarren sisyphoides (it’s in my book), and there the point seems to be that the dead male locks the female against potential subsequent (and posthumous) suitors. The male as its own chastity belt. The same sort of thing could be going on in the dark fishing spider. But there is a twist: Schwarz and colleagues found that the dead male’s second, unused pedipalp, is also full of sperm. So why fill up both sperm stores if you’re going to commit suicide anyway? And why not reserve your chastity-belt suicide for your second mate, rather than your first one? Spiders are weird.

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