"Spiders have good chemosensory abilities, and if blood were on the menu no doubt many would be able to detect it by smell, even if not all spiders possess vision good enough to be able to detect blood-carrying prey visually," Nelson said.
Although many spiders have relatively poor eyesightthose that use webs to trap prey have no need for acute vision, Nelson saysjumping spiders are an exception.
"Salticids are predators that actively search for prey and mates and typically do not build webs," she said. "They have evolved eyes that support high-acuity vision suited to their active lifestyle."
Spiders don't have the skin-piercing mouth parts needed to feed directly on human blood, but the mosquito-munching jumping spider appears to have got around this. The strategy has other advantages as well, Nelson points out.
"Blood-feeding is a dangerous activity," she said. "Animals that are bitten have a swatting response, and often the insect is killed."
By eating mosquitoes, the spider avoids the risk of being squashed by an unwilling blood donor.
The study team suspects a blood meal is also biologically important to E. culicivora.
They say spiders expend a lot of energy breaking solid food down into liquid by injecting their prey with digestive enzymes.
"Perhaps blood is a ready-made, nutrient-rich liquid meal," Nelson said.
Another recent study suggests spiders are surprisingly picky eaters, purposely seeking a balanced diet to maintain their health.
The study, published in January, found that spiders are selective about the nutrients they take from their prey depending on their need for proteins or fats.
Even the web-building desert spider (Stegodyphus lineatus), which has little control over what it catches, was able to balance its diet, according to David Mayntz, a biology professor at Aarhus University in Denmark.
He says the spider likely does this by altering the cocktail of digestive enzymes it pumps into its prey, allowing it to extract extra protein if the last prey it ate was low in protein.
Mayntz describes the African blood-eating spider find as "a very fascinating story indeed."
While he says blood-fed mosquitoes might be easier to catch than other mosquitoes because they are heavier, their higher nutrient value may well make them more appetizing to spiders.
Mayntz adds that the East African spider must have evolved a special ability to handle large amounts of vertebrate blood.
"[The spiders] would need some special enzyme to deal with this more complex protein," he said. "It's likely they would benefit from this protein, which is more or less dissolved already and easy to extract."
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