To study vampire spiders, build Frankenstein mosquitoes

In a swarm of buzzing mosquitoes, every insect probably looks the same to you. You wouldn’t notice that some have swollen abdomens, engorged with red blood, while others are hungry and empty. You wouldn’t differentiate between the antennae of the males (fluffy) and the females (straight). But there is one animal that can spot all of these traits, using eyes that have lower resolution than yours and a nervous system that’s far simpler.

It’s Evarcha culicivora – the vampire spiderEvarcha culicivora – the vampire spider.

E.culicivora is an East African jumping spider that feeds on mammal blood. Don’t worry: it’s not going to bite you. This indirect vampire only attacks mosquitoes that have recently bitten mammals, and it’s an incredibly discerning diner.

Jumping spiders are famously fussy anyway. They sit and wait for just the right victim to come along, spotting them with large eyes and pouncing upon them with well-judged leaps. Some eat other spiders, but only eat certain species. E.culicivora stalks mosquitoes, but it only female malarial mosquitoes that have recently fed. It ignores: males; individuals that aren’t full of blood; and insects of the wrong species (including other mosquitoes).

“It is the pickiest predator that we know of,” says Ximena Nelson, who studies the spider at the University of Canterbury in New Zealand.

To be that choosy, the spider must have very keen senses. Smell clearly plays a role (the spider is drawn to the odour of both bloody mosquitoes and human feet, and they themselves smell sexier once they’ve drunk some blood). But vision is important too. Even if all scents are blocked, E.culicivora can still pounce on exactly the right kind of prey. Now, Nelson, together with Robert Jackson, has worked out the visual cues that it uses.

They confronted captive spiders with lures built from body parts of dead mosquitoes, which had been glued together in different combinations like miniature Frankenstein’s monsters. The spiders saw two lures at a time, and Nelson noted which they pounced upon. “They are easy-to-handle, patient spiders,” she says. “Being so picky, it means we can ask them questions and get answers regarding their preferences that makes it seem like they answered in English.”

Nelson and Jackson found that the spiders always went for mosquitoes with blood-filled abdomens, rather than empty or sugar-filled ones, no matter which head had been stuck on top. The head matters too, though. When given a choice between two lures with bloody abdomens, the spiders picked the one with a female head rather than the one with a male head.

To check that the spiders weren’t relying on the smell of the lures, Nelson also showed them virtual mosquitoes on a screen. Again, they were more likely to pounce on virtual prey with female antennae than identical ones with male antennae. Human eyes would find it hard to tell the difference. The spiders’ eyes (and it has four pairs) have no such problem.

Having worked out the cues it uses, Nelson and Jackson are working to build the spider’s “decision tree”: the mental steps it makes in order to decide whether to pounce or hold. For now, all we know is that these preferences are innate. No learning is required. The spider appears to be born with some mental template of the ideal mosquito.

This feat is all the more impressive because the spider’s eyes and brain are so simple. The front pair is the largest and most sensitive, but even they probably only have a thousand or so receptors. The young spiders, which are just as fussy as the adults, probably just have 300 receptors per eye.

It seems hard to believe that with so few receptors these spiders can achieve that level of visual detail,” says Nelson. She says that the spider’s receptors are packed tightly in the central part of its eye, so it might be possible for it to see in extreme detail for a small part of its visual field. It probably also processes the information from its eyes in sophisticated way, but no one yet knows how it, or other jumping spiders, do this.

Reference: Nelson & Jackson. 2012. The discerning predator: decision rules underlying prey classification by a mosquito-eating jumping spider. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.069609

Images all by Robert Jackson