This species of jumping spider (Evarcha culicivora) has some interesting traits. When choosing which mosquito to attack, it prefers to take the ones laden with blood. As it gorges with blood, a mosquito's abdomen distends and swells. It may even turn a translucent red as its exoskeleton stretches to its maximum. In short, it looks different. The vampire spider notices that difference and acts accordingly.
When I first heard of the research that established this preference, I wondered if the laden mosquitoes got caught more often simply because they were slower. But scientists have ruled this out. They experimented with mosquitoes gorged on sugar water. In that state, the mosquito is just as ungainly as a blood-gorged one, but the spider still goes for the blood. It can smell the difference.
The scientists also observed a behavior more often associated with mammalian predators like cats: surplus killing. The spider will kill up to twenty blood-laden mosquitoes at a time, far more than it can eat. It may be saving them for later, or it may simply be bad at counting.
Why prefer bloody mosquitoes? Probably for the extra protein. In effect, the spider gets not just the insect to eat, but also the blood meal inside it. (It's sometimes human blood, though the Anopheles mosquitoes can prey on other mammals as well.) In courtship, the spiders prefer potential mates that have recently fed on blood-laden mosquitoes.
More recently, the scientists have demonstrated that vampire spiders are attracted to the smell of human beings. They don't parasitize us directly, but it makes sense that they'd want to be around us to prey on the mosquitoes that take our blood.
Earlier post about jumping spiders