Not Exactly Rocket Science

In East Africa lives a species of spider that drinks mammalian blood. But fear not - Evarcha culicivora is an indirect vampire - it sates its thirst by preying on female mosquitoes that have previously fed on blood themselves.

Even though its habitat is full of non-biting midges called "lake flies", it can tell the difference between these insects and the blood-carrying mozzies it carries. Robert Jackson from the University of Canterbury discovered this behaviour a few years ago and one of his colleagues, Fiona Cross, has now found that the blood isn't just a meal for the spiders, it's an aphrodisiac too.

Photo of E.culicivora eating a mosquito, by R. Jackson.

Cross made spiders choose between two adults of the opposite sex, by wafting their smells down a tube on different days and seeing which drew the choosy spider's attention for the longest time. The contenders had been fed on one of four diets: blood-fed female mosquitoes, sugar-fed female mosquitoes, male mosquitoes, or lake flies. 

She found that only a menu of blood-fed mosquitoes made spiders more attractive to the opposite sex, and both males and females shared this turn-on. If spiders were switched from a diet of lake flies to one of bloody mosquitoes, their scents became more attractive. Even a single meal of blood makes the spiders smell more attractive. Likewise, fasting, or moving from mozzies to lake flies even for just a day, curtails the sex appeal of an individual's odour.

So for E.culicivora to maintain its sensuous scent, it needs to continuously feed on blood. In this way, spiders that smell of blood are probably those that are best at catching mosquitoes, and potential partners may be using the odours as a way of sussing out the quality of their mates. Of course, that's just a hypothesis. Next, Cross plans to see if spiders on a blood diet actually mate more often, or produce more viable eggs and sperm.

The other alternative is that spiders are using the smell of blood to lure in potential mates, by tricking them into thinking that prey is near. But Cross thinks this is unlikely - spiders were only drawn to the smell of blood if it was given off by individuals of the opposite sex.

The importance of smell might come as a surprise, especially since E.culicivora is a jumping spider, a group that's better known for their keen eyesight. But when it comes to mating, previous studies show that smell plays an equally important role in identifying a partner. If the smell was simply making them hungry, the gender of its source wouldn't matter.

Perhaps the actual chemical lure is produced after blood is processed in the spider's body. Perhaps it's a combination of blood and a sex-specific chemical that piques a partner's interest. The only real way to find out is to work out the precise chemicals that E.culicivora finds so appealing, and that's next on Cross's to-do list.

In the mean time, there are probably many other examples in nature of animals to rely on the same smells in courtship rituals as in other aspects of their lives. For examples, noctuid moths use sex pheromones that mimic smelly chemicals given off by plants, the same chemicals that they track to find somewhere to lay their eggs. And the European starling adds aromatic plants into its nest to attract females.

Reference: PNAS doi:10.1073/pnas.0904125106

More on spiders:

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• Spider mimics ant to eat spiders and avoid being eaten by spiders
• Spiders gather in groups to impersonate ants
• Traumatic insemination - male spider pierces female's underside with needle-sharp penis
• Singaporean spiders spit venomous glue, work together, eat each other