In Kenya, a vampire spider is hunting down its prey by tracking the smell of human feet. If that sounds nightmarish, don’t panic – the spider Evarcha culicivora is only an indirect vampire. It’s not interested in attacking humans; it’s after mosquitoes that have fed on mammal blood. If anything, the spider is our ally – it kills female Anopheles mosquitoes that spread malaria.
Robert Jackson from the University of Canterbury discovered the spider in 2003. He quickly showed that it likes to target malarial mosquitoes, especially those that had just fed on blood, and it can recognise them based on either appearance or smell. And in 2009, Jackson, together with his colleague Fiona Cross, showed that the blood is both an aphrodisiac and a meal for the spiders. When they’ve drunk their fill, they become more irresistible to the opposite sex.
Now, Cross and Jackson are back with a new study, which shows that Evarcha likes to hang around humans. After all, what better place to find a blood-filled mosquito than the source of the blood?
The duo wafted the scent of human socks into test tubes occupied by captive spiders, which could leave at any time. They were more likely to stay if the smell came from a sock that a human volunteer had worn for 12 hours beforehand. The scent of unworn sock was less attractive. They spent anywhere from 15 to 30 minutes longer sampling the fragrance of feet, and spiders both male and female, young and old, behaved in the same way.
Evarcha’s keen sense of smell is unusual for a jumping spider, a group that’s better known for their exceptional eyesight. It’s possible that its sense has evolved to mirror that of its prey. Both the mosquitoes and the spiders are drawn to the smell of humans, in search of mouthfuls of blood. The only difference is that the mosquito takes it from our bodies and the spider takes it from the mosquito.
Reference: Cross & Jackson. 2011. Olfaction-based anthropophily in a mosquito-specialist predator. Biology Letters http://dx.doi.org/10.1098/rsbl.2010.1233