Nelson, a scientist at Macquarie University in Australia, and her team conducted a food preference test with
Evarcha culicivora serving as the critic. They first put the spider in a glass vial so that it could not smell prey choices, which were a mixture of male mosquitoes that do not consume blood, female mosquitoes fed a sugar concoction and female mosquitoes that had just feasted on blood.
Using sight alone, the spider always chose the blood-engorged females, who looked fat and somewhat red.
The researchers next pumped odors of the mosquitoes into a test chamber that the spider entered. The spider, using only its weak sense of smell, went for the females that had just dined on blood.
Nelson told Animal Planet News that feeding on blood is a dangerous activity, so this spider appears to minimize its risk.
"Animals that are bitten have a reflective 'swatting' response — humans use their arms, tailed animals use their tails, etc. — and often the insect is killed," Nelson explained. "It may be safer for Evarcha to obtain blood by killing a mosquito, then risk being swatted, even if they did have the mouth parts required to pierce skin and locate a blood vessel."
The spider also uses a rather clever technique for catching its fat female mosquitoes. The spider stalks the mosquito like a cat, and then pounces either on top or underneath the mosquito before taking a bite.
With such a hunger for blood, evolution would seem to favor this spider gaining the ability to directly suck blood from victims, but Nelson thinks the way Evarcha holds its fangs might prevent this from happening.
"They hold them close to their face, not forward projecting as mosquitoes do," she said. "Perhaps they might stab themselves, and this would kill them as spiders rely on a high hydrostatic pressure inside their bodies to 'hold them up.'"
The spider, then, would sort of burst like a holiday parade balloon that has hit a pointy light post.
Steve Heydon, senior scientist and collection manager for the Bohart Museum of Entomology at the University of California at Davis, was surprised to hear of the discovery.
"I know of parasitic wasps that find their caterpillar prey based on the smell of the caterpillar's feces, but I'd never heard of a spider like this before," Heydon told Animal Planet News. "Spiders don't have much sense of smell, so that part of the study is especially surprising."
Heydon agrees with Nelson that spiders now probably lack the right body parts and structure to evolve into direct bloodsuckers, but he does not completely rule this out for the distant future.
"Maybe spiders will end up like bed bugs," he said. "They could have that bed-bug lifestyle of laying around and coming out at night when a big, huge, monstrous food item comes tantalizingly near them and simply goes to sleep."
