杏吧原创

WHEN black clouds come rolling across Lake Victoria towards Mbita Point, they don鈥檛 always bring rain. Sometimes the dark billowing shapes heading for the Kenyan shore are alive, a teeming mass of tiny lake flies. And when the sky rains flies, the spiders are waiting. Lurking among this army of predators is one that鈥檚 different. They call it the terminator.

Terminator doesn鈥檛 have an official name yet. It鈥檚 a species of Evarcha, a jumping spider, small, not much to look at, but with extraordinary habits never seen before in a spider. This spider drinks vertebrate blood 鈥� including that of humans. Its fangs aren鈥檛 sharp enough to puncture tough skin, however. Instead it gets its blood second-hand. 鈥淚t uses a mosquito as a straw,鈥� says Robert Jackson, a biologist at the University of Canterbury in Christchurch, New Zealand.

Female mosquitoes are little more than syringes with wings. Their hypodermic mouthparts are designed to pierce thick skin and draw blood. All Evarcha need do is find a mosquito with a swollen, blood-filled abdomen, prick a hole in it and siphon off the contents. It does this with amazing efficiency, singling out and attacking mosquitoes hidden among hordes of midges. 鈥淟ike the villain in the movie, this spider is on a mission to seek and destroy,鈥� says Jackson. And the terminator is as ruthless as its celluloid namesake. Once it has smelled blood it launches into a feeding frenzy, killing as many as 20 mosquitoes in quick succession.

For the past eight years, Jackson has been a regular visitor to Mbita Point, where the International Centre for Insect Physiology and Ecology (ICIPE) runs its Malaria Vector Programme. Jackson is an expert on jumping spiders, and when the lake flies come, this is a great place to find them. He鈥檚 spent many years studying the spiders鈥� keen eyesight and how their brains work as they plan and execute complex hunting strategies. They stalk their victims as a cat does, making detours to take advantage of nearby cover. In the final approach, they creep forward with their bodies close to the ground then leap on their prey with incredible accuracy. But in 25 years of observing jumping spiders, Jackson had never come across anything like the terminator.

He first spotted Evarcha in 1995. The spider was nothing special to look at, with a dull grey-brown body about 8 millimetres long and a red face. But every time he saw one it seemed to be sucking the life out of a blood-filled female mosquito, and that made it interesting. In a part of the world where malaria kills someone every 30 seconds, anything that seeks out and destroys the mosquitoes that transmit the disease is worth a closer look.

Although jumping spiders usually eat insects or other spiders, Jackson suspected that what the terminator was after was not the mosquito but the blood inside. To test his hunch, he offered captive spiders a choice of prey: midges, male and female mosquitoes, females that had recently fed on blood and others that hadn鈥檛. The spiders undoubtedly preferred mosquitoes. And given a choice, they picked females that had sucked blood in the past 24 hours.

Giant killers

Jackson also discovered that while adult Evarcha kill many different species of mosquito, newly hatched spiders appear to single out the Anopheles mosquitoes that transmit human malaria. On hatching, Evarcha is only 1.5 millimetres long. 鈥淢osquitoes are giants by comparison and such a small spider shouldn鈥檛 be able to bring them down,鈥� he says. But Anopheles are particularly vulnerable because they rest with their rear ends pointing skyward. A young spider will make its way behind Anopheles, move stealthily under the belly and leap upwards, firing its fangs into the abdomen.

Why do these spiders hanker after blood, and how do they pick mosquitoes out from the huge crowd of midges? Last year, with a grant from the National Geographic Society and a year鈥檚 study leave from his university, Jackson returned to Mbita Point to find out more about the mosquito terminator. With help from Simon Pollard, a spider specialist at Canterbury Museum in Christchurch, researchers at ICIPE and postgraduate students back at his lab in New Zealand, Jackson has begun to piece together the complicated life of the mosquito terminator.

There are plenty of good reasons for the spider鈥檚 bloodlust. Blood is rich, nutritious and the spider equivalent of fast food. Spiders feed on fluids and must turn their prey鈥檚 tissues into soup before eating. First they immobilise prey with venom, then they pump in digestive fluids. 鈥淭he prey becomes an extension of the spider鈥檚 digestive system, an external stomach,鈥� says Pollard. Feeding is a slow process. If the prey is midge-sized it might take 40 minutes to turn it to a nutrient slush. For big prey it can take hours. 鈥淏ut in this case what鈥檚 inside the prey is already liquid,鈥� he says.

What鈥檚 more, the mosquitoes remove the lumps: blood cells are crushed and torn to pieces during their passage through its gut. And a spider may not even have to suck to get its food. 鈥淎 freshly filled mosquito with a distended belly is under pressure, and when it鈥檚 punctured the blood probably flows straight into the spider with no effort at all,鈥� says Pollard. 鈥淭hey may not even have to inject any venom because once the mosquito鈥檚 belly is punctured it dies. These guys may be getting a free lunch.鈥�

But terminator wouldn鈥檛 be able to enjoy such rich food without its extraordinary ability to pinpoint blood-filled mosquitoes in a crowd of small insects. 鈥淚t鈥檚 fantastically good at doing this,鈥� says Jackson. Experiments show that Evarcha can identify a mosquito by sight alone. Jumping spiders have unique eyes that give them acute vision, six times keener than the best-sighted insects and not far short of a human鈥檚.

Although most spiders have four pairs of eyes, they see only poorly and rely instead on touch, smell and the ability to detect vibrations. However, the frontmost pair of a jumping spider鈥檚 eyes are large and can see shape, size and colour well enough to identify mates, rivals and different types of prey from a distance of at least 30 body lengths. The other six eyes function mainly as movement detectors.

The jumping spider鈥檚 principal eyes are camera eyes, rather like ours, with a single lens at the front and a retina behind. Pioneering work by Mike Land at Sussex University and later research by David Blest at the Australian National University in Canberra showed that despite the similarities, jumping spiders鈥� principal eyes work very differently to ours (see Graphic). Each one consists of a fixed lens at the front of a long tube with a small retina at the rear. A second lens near the back of the tube magnifies the image from the front lens, so not only do these pairs of eyes look like binoculars, they have a similar system of telephoto optics.

The spider鈥檚 retina is uniquely complex, with four layers of photoreceptors that intercept light of different wavelengths and give the spider colour vision. But it is the closely packed receptors of the rearmost layer that enable the spider to see detailed shape and form. Unlike us, the spider can鈥檛 alter the shape of its front lens to focus the image on the retina. Nor can it change the length of the eye tube. Instead, the retina is curved and the photoreceptors lie at different distances from the lens. This means that however far an object is from the spider, it will cast an in-focus image on some part of the retina. And just to make sure, the spider can move the eye tubes about inside its head.

Distinguishing features

Being able to move the eye tubes has another function. The image focused on the tiny retina is only a fraction of what is captured by the lens. But Land believes that by moving the eye tube from side to side, the spider sweeps its retina across the image, 鈥渟canning鈥� more of the picture. This is a slow process, however. So to speed up decisions 鈥� whether to attack what it sees or maybe mate with it 鈥� jumping spiders may simply search for specific details in the image, says Duane Harland, who is also at Sussex. They might look for the shape of a leg or a palp, perhaps, or the size of an eye or the outline of an antenna.

To find out exactly what terminator recognises, Jackson has had to resort to deception. Working with Godfrey Sune at Mbita Point, and with student Ximena Nelson at the University of Canterbury, he presented spiders with various lures 鈥� dead mosquitoes mounted in lifelike poses on corks. The team systematically altered parts of the body, such as the number and outlines of the appendages, the shape and size of the abdomen, and watched how the spiders responded. These tests revealed that the key features were the antennae and the shape of the abdomen.

Male mosquitoes have long hairs on their antennae which give them a feathery look. Females don鈥檛 have such prominent hairs. In the tests, Evarcha was less likely to attack female lures if they bore male antennae, but pounced on males with female antennae. 鈥淭heir eyesight is good enough to discern something the size of an antenna, and they also see that it鈥檚 attached to something,鈥� says Pollard. 鈥淚t鈥檚 like looking at the Mona Lisa with binoculars. You can just see the mouth and you know it鈥檚 her.鈥�

The shape of the abdomen provides another clue: a distended, stretched belly is more likely to prompt attack than a slim one. There鈥檚 even a suggestion that the spiders might be able to distinguish the red colour of blood showing through the overstretched abdomen of a well-fed mosquito.

Back in the lab in Christchurch, Jackson and his team have begun to 鈥渋nterrogate鈥� Evarcha further with the help of digital images and a miniature TV screen. The spider responds to an image on TV as if it were the real thing. Show it a spider of the opposite sex and it throws up its legs and begins its courtship display. If it鈥檚 hungry and sees a mosquito, it will pounce onto the image. The advantage of these virtual lures is that they can be manipulated more precisely than a dead mosquito.

The TV tests should reveal a lot more about what the spider homes in on: for instance, how distended must a belly be or how feminine an antenna, before the spider responds. These experiments, coupled with an analysis of how the spider moves its eye tubes, may go a long way towards revealing precisely how Evarcha pieces together perception of its prey.

Although visual clues are enough for the terminator to find its prey and attack, the spider also recognises the smell of a blood-fed mosquito. Jackson experimented with a Y-shaped 鈥渙lfactometer鈥�, a simple maze with the spider at the end of one arm and a choice of two meals out of sight at the tips of the others. These tests showed that terminator is equally attracted to the smell of a female that鈥檚 eaten blood in the past 48 hours, and to the contents squashed from its abdomen. But the spiders don鈥檛 like fresh blood, at least not from human volunteers or domestic animals. 鈥淏lood alone doesn鈥檛 work. It must have been inside a mosquito,鈥� says Pollard.

Evarcha may have a strong preference for blood-filled mosquitoes, but it eats other foods too, including midges, nectar and pollen. To find out what the spiders gain by eating blood, Jackson and the staff at Mbita Point reared large numbers of them on different diets and monitored how fast they grew, how long they lived and how many eggs they produced in a lifetime. 鈥淏lood alone wasn鈥檛 the best diet, but a mixed diet that included blood was better than any other,鈥� says Jackson.

There was, however, one aspect of the spider鈥檚 life where blood made a big difference: sex. 鈥淭he really dramatic effect of a blood diet is on their mating behaviour. It was quite shocking.鈥� Eating blood makes spiders more attractive and more promiscuous. Both males and females attract more mates if they鈥檝e been feeding on blood. This is partly because spiders that have grown up eating blood are bigger. But they also acquire an odour that other spiders find irresistible. 鈥淏lood is like an aphrodisiac,鈥� says Jackson. Males respond faster to females if there鈥檚 a smell of blood around. They鈥檒l even try to mate with a fruit fly if it has the right odour. 鈥淭he smell lowers the threshold for courtship and mating,鈥� says Jackson.

The smell of blood also seems to trigger a lust for more blood. Terminator launches into a feeding frenzy only when the prey are blood-filled mosquitoes. But if stimulated by the blood-odour it will kill almost anything in sight 鈥� male mosquitoes, even midges. 鈥淭he smell makes them behave as if there were female mosquitoes there,鈥� says Pollard. The odour of predigested blood has some sort of mind-bending effect on Evarcha, says Jackson. 鈥淚t alters the spider鈥檚 visual perception so it reacts differently to what it sees.鈥�

For the staff at ICIPE, a spider that goes on mosquito killing sprees is an exciting discovery, opening up the possibility of a form of biological control for malarial mosquitoes. 鈥淣o one has found a species that targets mosquitoes before,鈥� says Jackson. It鈥檚 too early to say how realistic an idea this is, so last month Jackson returned to Mbita Point to investigate further.

鈥淭here are lots of things about it that are encouraging,鈥� he says. The spider is very common in the Lake Victoria region of Kenya and Uganda. This equatorial region of Africa is plagued by Anopheles gambiae, the most effective vector for human malaria. And the most dangerous of the malarial parasites, Plasmodium falciparum, is common. 鈥淓varcha lives in houses, the very places where it鈥檚 needed,鈥� says Jackson. 鈥淚t ought to be possible to encourage the terminator by growing the plants they need close to houses. At the very least it鈥檚 worth a try.鈥�