A version of this post first appeared on June 7, 2012. Happy Halloween!
It's reasonable for a hungry predator to hesitate when its prey appears to be two halves of an animal glued together and reanimated. Jumping vampire spiders that were faced with this decision took it slowly. But they eventually chose tasty mosquitos stuffed full of fresh human blood. Before pouncing, these discriminating diners considered their victims' headwear as well as their belly size.
New Zealand biologist Ximena Nelson led the investigation of how Evarcha culicivora spiders choose which prey to attack. The East African jumping spiders are very picky eaters, dining solely on mosquitos that have recently filled up on blood. (Since only female mosquitos drink blood, this means males are mostly off the menu.)
A blood meal doesn't just satisfy a rumbly abdomen; it also makes the spiders more attractive to the opposite sex. So it's worth a spider's effort to make sure that it's hunting the right prey. Stalking and killing what it thinks is a jelly-filled, only to discover it's only a glazed, would be disappointing.
Earlier research had shown that vampire spiders can find their prey by sight alone—they don't need the smell of a blood-engorged insect abdomen to direct them. So Nelson wondered what visual cues help E. culicivora home in on its prey. The spiders have great vision and eyes that point in all directions. But will they simply attack any mosquito with a belly full of blood, or do they use other signals to find females? If they only glimpse part of a mosquito, what hints will give them enough confidence to pounce?
To find out, Nelson's team tempted vampire spiders with various mosquito "lures." They used dead mosquitos rather than living ones, since these are less prone to flying away in terror when a spider comes after them. Dead mosquitos are also more amenable to being cut up and Frankensteined together with each other, which is exactly what the researchers did.
The scientists used three kinds of mosquitos: males, females fed only on sugar water, and females recently fed on mammal blood. After being dispatched, each insect was cut in half. Then the front and back halves were shuffled and recombined. This created female heads with male abdomens, male heads with blood-filled or regular female abdomens, and control insects that were normal (except for the seam across the middle).
Those female mosquitos fed on mammal blood, by the way, had eaten well. I asked Ximena Nelson if the researchers fed their mosquitos using their own arms, an unappetizing practice I'd heard of. "Yes, unfortunately that is the case," she said. "Not a pleasant experience for the victim."
After the monstrous mosquito lures were "mounted in a lifelike posture," the vampire spider subjects went through a series of tests. In some cases, the researchers made the lures jerk up and down to seem a little more lively. Sometimes one end of the lure was hidden behind a wall, so that only the mosquito's head or only its abdomen was visible.
Whenever females with blood-engorged abdomens were on the menu, the spiders went for them. But when given the choice between a normal-looking female mosquito and one with a male head, the spiders preferred their meals with the correct heads on them. And when the mosquitos' back ends were hidden behind a wall, spiders went after the ones with female front ends. A test using virtual mosquitos on a screen (which the frustrated spiders jumped at to no avail) confirmed how the spiders were choosing their victims: In addition to blood-stuffed stomachs, they looked for female antennae.
Male mosquitos have plush, feathery antennae, while females' are simple and unadorned, like car antennas. In the photo below, a male is on the left and a female is on the right (with her head pointing toward the bottom corner).
Although E. culicivora spiders do choose victims for their blood-filled bellies, they use antenna shape as an extra clue to make sure they're choosing well. Plain female antennae tell a spider that its intended victim isn't just a male mosquito that overdid it on the sugar water.
It seems to be "a combination of cues from the head and from the abdomen that truly 'flicks the switch,'" Nelson says. Vampire spiders are cautious, deliberate hunters that look closely before they leap. Once they take their pick, that well-fed female mosquito may be left regretting her own lunch choice.
Nelson, X., & Jackson, R. (2012). The discerning predator: decision rules underlying prey classification by a mosquito-eating jumping spider Journal of Experimental Biology, 215 (13), 2255-2261 DOI: 10.1242/jeb.069609