To become both a lover and a fighter, the male spider Nephilengys malabarensis snaps off his penis inside his partner while they have sex. He becomes better at fending off other males who try to mate with her, because his now-lightened body can fight for longer without tiring. And while he’s playing the guardian, his detached genitals can continue pumping sperm into the female. Through self-castration, he gets more stamina, and he gets more stamina.
In a swarm of buzzing mosquitoes, every insect probably looks the same to you. You wouldn’t notice that some have swollen abdomens, engorged with red blood, while others are hungry and empty. You wouldn’t differentiate between the antennae of the males (fluffy) and the females (straight). But there is one animal that can spot all of these traits, using eyes that have lower resolution than yours and a nervous system that’s far simpler.
E.culicivora is an East African jumping spider that feeds on mammal blood. Don’t worry: it’s not going to bite you. This indirect vampire only attacks mosquitoes that have recently bitten mammals, and it’s an incredibly discerning diner.
Jumping spiders are famously fussy anyway. They sit and wait for just the right victim to come along, spotting them with large eyes and pouncing upon them with well-judged leaps. Some eat other spiders, but only eat certain species. E.culicivora stalks mosquitoes, but it only female malarial mosquitoes that have recently fed. It ignores: males; individuals that aren’t full of blood; and insects of the wrong species (including other mosquitoes).
“It is the pickiest predator that we know of,” says Ximena Nelson, who studies the spider at the University of Canterbury in New Zealand.
To be that choosy, the spider must have very keen senses. Smell clearly plays a role (the spider is drawn to the odour of both bloody mosquitoes and human feet, and they themselves smell sexier once they’ve drunk some blood). But vision is important too. Even if all scents are blocked, E.culicivora can still pounce on exactly the right kind of prey. Now, Nelson, together with Robert Jackson, has worked out the visual cues that it uses.
They confronted captive spiders with lures built from body parts of dead mosquitoes, which had been glued together in different combinations like miniature Frankenstein’s monsters. The spiders saw two lures at a time, and Nelson noted which they pounced upon. “They are easy-to-handle, patient spiders,” she says. “Being so picky, it means we can ask them questions and get answers regarding their preferences that makes it seem like they answered in English.”
Nelson and Jackson found that the spiders always went for mosquitoes with blood-filled abdomens, rather than empty or sugar-filled ones, no matter which head had been stuck on top. The head matters too, though. When given a choice between two lures with bloody abdomens, the spiders picked the one with a female head rather than the one with a male head.
To check that the spiders weren’t relying on the smell of the lures, Nelson also showed them virtual mosquitoes on a screen. Again, they were more likely to pounce on virtual prey with female antennae than identical ones with male antennae. Human eyes would find it hard to tell the difference. The spiders’ eyes (and it has four pairs) have no such problem.
Having worked out the cues it uses, Nelson and Jackson are working to build the spider’s “decision tree”: the mental steps it makes in order to decide whether to pounce or hold. For now, all we know is that these preferences are innate. No learning is required. The spider appears to be born with some mental template of the ideal mosquito.
This feat is all the more impressive because the spider’s eyes and brain are so simple. The front pair is the largest and most sensitive, but even they probably only have a thousand or so receptors. The young spiders, which are just as fussy as the adults, probably just have 300 receptors per eye.
It seems hard to believe that with so few receptors these spiders can achieve that level of visual detail,” says Nelson. She says that the spider’s receptors are packed tightly in the central part of its eye, so it might be possible for it to see in extreme detail for a small part of its visual field. It probably also processes the information from its eyes in sophisticated way, but no one yet knows how it, or other jumping spiders, do this.
Reference: Nelson & Jackson. 2012. The discerning predator: decision rules underlying prey classification by a mosquito-eating jumping spider. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.069609
Images all by Robert Jackson
More on amazing spiders:
If your partner is likely to devour you after sex, snapping off your genitals inside her might seem a reasonable reproductive strategy. This game plan is used by males of the orb-web spider Nephilengys malabarensis and, it turns out, continues to work in their favour, regardless of whether they survive the encounter.
Thus begins my new piece for Nature News. Honestly, I can’t believe they let me keep the lede. Here’s more:
Daiqin Li at the National University of Singapore and his colleagues studied the species and found that after the male breaks away his severed organ continues to pump sperm into the female. This allows him to fertilize her remotely, while denying entry to other males. Even though the male cannot regrow his genitals and so renders himself sterile, he increases the odds that he will father the offspring of his one and only mate.
Male spiders deliver their sperm through a pair of structures known as palps, which are found on the sides of their heads. By serving sexual encounters between 25 pairs of virgin N. malabarensis, Li’s group found that every coupling ended with damage to the male’s palp. In 12% of cases it was partially severed; in the rest it snapped off completely.
Li thinks that this bizarre strategy, found in only two spider families so far, evolved to counter the female’s penchant for cannibalism. “The females are very aggressive and 75% of them kill the males during sex,” he explains. “The duration of copulation is also very short, and the females initiate the break-off.”
By dissecting the mated spiders, Li and his co-workers found that the palp has dispensed only about one-third of its sperm by the time the female pushes the male off. But it continues to transfer sperm after it breaks off, and does so at a faster rate.
And head over there for the rest of the story.
(In the picture at the top, the bigger female devours the smaller male while his palp clings on to her underside (in the red box).)
We don’t like blurry vision, and we go out of our way to correct it with glasses and contact lenses. But some animals aren’t so fussy. The jumping spider not only tolerates blurry images, it deliberately produces them.
Jumping spiders, as their name suggests, leap onto their prey from afar. They judge their jumps using the two huge (and rather beautiful) eyes on the front of their faces. And to gauge how far away their targets are, they use special retinas that produce sharp images and out-of-focus ones at the same time.
Other animals have many different ways of judging depth, but none of them apply to jumping spiders. Humans mostly rely on our two eyes. Each gets a slightly different view of the world and our brain uses these differences to triangulate the distance to objects in front of us. But this ‘binocular vision’ only works if the two eyes see overlapping parts of the world. Those of jumping spiders do not.
Chameleons can judge distance by sensing how much they have to focus their eyes to bring an object into sharp relief. But jumping spiders have no way of actively focusing their eyes. Finally, some insects judge distance by shaking their heads from side to side, which makes nearby objects move further across their field of view than far ones. But jumping spiders can accurately pounce onto their prey without moving their heads.
Without any of these three methods, how could they possibly gauge their precise killing pounces with any sort of accuracy? Takashi Nagata from Osaka City University has the answer.
Each of the front eyes has a unique staircase-shaped retina, with four layers of light-sensitive cells lying one over the other. By contast, our retinas only have one such layer. Scientists have known about the staircase retinas since the 1980s, but Nagata has finally shown exactly what they do. He found that the top two layers are most sensitive to ultraviolet light. The two on the bottom have a penchant for green.
And that’s a bit odd. The way the layers are stacked means that green light only ever focuses sharply on the bottom one (layer 1). Blue light focuses on the one above it (layer 2), but those cells aren’t sensitive to blue. Instead, they see the world in fuzzy out-of-focus green.
Nagata thinks that this fuzzy vision isn’t a bug; it’s a feature. The amount of blur depends on an object’s distance from the spider’s eye. The closer it is, the more out of focus it is on the second retina. Meanwhile the first retina always gets a sharp image. By comparing the images on both layers, the spider can gauge depth with a single unmoving eye.
To test this idea, Nagata placed Adanson’s house jumpers in a special arena where they had to leap at prey. If the arena was flooded with green light, the spiders made accurate jumps. If Nagata used red light of equal brightness, they fell short of the mark. Nagata even created a mathematical model for the spider’s eye to predict how far it would miss its jump under different wavelengths of light. The model’s predictions matched the animal’s actual behaviour.
Humans actually do something similar. We can use the blurry nature of background images to get a sense of distance, even if all other cues are removed. Indeed, photographers often use blurry backgrounds to create a greater sense of depth. But this is just one of the tricks we use to judge depth, and perhaps a minor one. For the jumping spider, it seems to be the only trick in the playbook.
Reference: Nagata, Koyanagi, Tsukamoto, Saeki, Isono, Shichida, Tokunaga, Kinoshita, Arikawa & Terakita. 2011. Depth Perception from Image Defocus in a Jumping Spider. Science http://dx.doi.org/10.1126/science.1211667
Photo by Alex Wild
The eyes have it – a tour through the stunning world of animal eyes
In a lab at the University of Wyoming, some silkworms are spinning cocoons of silk, just as every silkworm has done for millions of years. But these insects are special. They have been genetically engineered to spin a hybrid material that’s partly their own silk, and partly that of a spider. With spider DNA at their disposal, they can weave fibres that are unusually strong and tough. It’s the latest step in a decades-long quest to produce artificial spider silk.
Spider silk is a remarkable material, wonderfully adapted for trapping, crushing, climbing and more. It is extraordinarily strong and tough, while still being elastic enough to stretch several times its original length. Indeed, the toughest biological material ever found is the record-breaking silk of the Darwin’s bark spider. It’s 10 times tougher than Kevlar, and the basis of webs that can span rivers.
Spider webs are great at catching flying insects, but they’re an inviting target for walking ones. The spider sits pretty in the middle of its home, surrounded by the pre-packaged morsels of the insects it has caught. It’s an all-you-can-eat buffet, and ants should easily be able to raid it. Ants are excellent predators, they hunt in large numbers, and they can negotiate their way along the non-stick parts of the web. And yet, there are very few reports of ants successfully pillaging spider webs. Why?
Shichang Zhang has found one possible answer: some spiders lace their silk with an ant-repelling chemical. Their sticky webs, which so effectively trap some insects, can also deter others.
Spiders can tackle all manner of prey, from insects to fish to birds. But some of them specialise in killing their own kind. Palpimanus gibbulus and Palpimanus orientalis are two such spider-slayers, and they use special adaptations to tackle their dangerous prey, including ninja-like stealth, blinding-fast strikes, unbreakable grips, and heavy body armour.
Stano Pekár from Masaryk University in the Czech Republic confirmed that these two species (hereafter known as Palpimanus) are indeed specialist spider-hunters. They pounce upon a wide variety of other species and attack spiders more often than insects like flies or crickets. Using high-speed video cameras and staged battles, Pekár revealed their killer technique.
In the days before scuba tanks, people used to explore the underwater world with the aid of diving bells. These large open-bottomed chambers were dunked into the water, and divers used the air trapped inside them to breathe. The bells have been around since at least the time of Aristotle, but in the rivers and lakes of Europe, one animal has been using similar structures for far longer – the diving bell spider.
The diving bell spider is the only member of its group to spend its entire life underwater. But it still needs to breathe air, and it does so by building its own diving bell. First, it spins a dome-shaped web between underwater plants. Next, it rises to the surface and traps bubbles using the fine hairs on its legs and belly. It carries them down to its web and releases them, gradually filling the dome with air. After a few trips, the spider has amassed a bubble so large that it can fit inside.
If Spider-Man really could do “whatever a spider can”, he ought to shoot webs from somewhere less salubrious than his hands. All spiders spin silk from their rear ends, using special organs called spinnerets. But one group – the tarantulas – can also shoot silk from their feet, and they use this ability to climb up sheer vertical surfaces.
Tarantulas have been kept as pets for decades, but their silk-spinning feet were only discovered in 2006 by Stanislav Gorb from the Max Planck Institute. Gorb watched Costa Rican zebra tarantulas climbing up glass plates, and saw that they left behind silken footprints – dozens of fibres, just a thousandth of a millimetre wide.
As the spider climbs, four of its legs leave the glass plate at any one time. As the legs land, they begin to slip but small nozzles secrete a viscous silken fluid that rapidly hardens and adheres to the surface. The silk acts as a tether, firmly holding the spider to the pane.