In East Africa lives a species of spider that drinks mammalian blood. But fear not – Evarcha culicivora is an indirect vampire – it sates its thirst by preying on female mosquitoes that have previously fed on blood themselves.
Even though its habitat is full of non-biting midges called “lake flies”, it can tell the difference between these insects and the blood-carrying mozzies it carries. Robert Jackson from the University of Canterbury discovered this behaviour a few years ago and one of his colleagues, Fiona Cross, has now found that the blood isn’t just a meal for the spiders, it’s an aphrodisiac too.
Photo of E.culicivora eating a mosquito, by R. Jackson.
Cross made spiders choose between two adults of the opposite sex, by wafting their smells down a tube on different days and seeing which drew the choosy spider’s attention for the longest time. The contenders had been fed on one of four diets: blood-fed female mosquitoes, sugar-fed female mosquitoes, male mosquitoes, or lake flies.
She found that only a menu of blood-fed mosquitoes made spiders more attractive to the opposite sex, and both males and females shared this turn-on. If spiders were switched from a diet of lake flies to one of bloody mosquitoes, their scents became more attractive. Even a single meal of blood makes the spiders smell more attractive. Likewise, fasting, or moving from mozzies to lake flies even for just a day, curtails the sex appeal of an individual’s odour.
So for E.culicivora to maintain its sensuous scent, it needs to continuously feed on blood. In this way, spiders that smell of blood are probably those that are best at catching mosquitoes, and potential partners may be using the odours as a way of sussing out the quality of their mates. Of course, that’s just a hypothesis. Next, Cross plans to see if spiders on a blood diet actually mate more often, or produce more viable eggs and sperm.
The other alternative is that spiders are using the smell of blood to lure in potential mates, by tricking them into thinking that prey is near. But Cross thinks this is unlikely – spiders were only drawn to the smell of blood if it was given off by individuals of the opposite sex.
The importance of smell might come as a surprise, especially since E.culicivora is a jumping spider, a group that’s better known for their keen eyesight. But when it comes to mating, previous studies show that smell plays an equally important role in identifying a partner. If the smell was simply making them hungry, the gender of its source wouldn’t matter.
Perhaps the actual chemical lure is produced after blood is processed in the spider’s body. Perhaps it’s a combination of blood and a sex-specific chemical that piques a partner’s interest. The only real way to find out is to work out the precise chemicals that E.culicivora finds so appealing, and that’s next on Cross’s to-do list.
In the mean time, there are probably many other examples in nature of animals to rely on the same smells in courtship rituals as in other aspects of their lives. For examples, noctuid moths use sex pheromones that mimic smelly chemicals given off by plants, the same chemicals that they track to find somewhere to lay their eggs. And the European starling adds aromatic plants into its nest to attract females.
Reference: PNAS doi:10.1073/pnas.0904125106
A gallery of incredible spiders
In the forests of South Africa lurks an arachnophobe’s nightmare – Nephila kowaci, the largest web-spinning spider in the world. The females of this newly discovered species have bodies that are 3-4 centimetres in length (1.5 inches) and legs that are each around 7.5cm long (3 inches).
This new species is the largest of an already massive family. There are 15 species of Nephila – the golden orb weavers – and at least 10 of them have bodies that are over an inch long. Many spin webs that are over a metre in diameter.
The first of these giants was discovered by Linnaeus himself in 1767 and the most recent one was described 140 years ago in 1879. Thousands of specimens have been collected and grace the displays and drawers of the world’s natural history museums. But every attempt at finding a new Nephila species since 1879 (and there are more than 150 suggested scientific names on record) has been a dead end – the “new species” are always repeats of known ones.
All of that changed in 1978, when a new Nephila spider was collected at Sodwana Bay in South Africa. The unusual spider caught the attention of Matjaz Kuntner and Jonathan Coddington from the Smithsonian Institution. The duo launched several expeditions to capture the elusive spider but all of them failed. They were beginning to think that had found a hybrid, or a species that had become extinct since its brief flirtation with discovery.
Then, their fortunes changed in 2003, when they found a second specimen in an Austrian museum, taken from Madagascar. This was no hybrid. A few years later, three more surfaces – a female and a male collected in Tembe Elephant Park in South Africa. N.komaci was far from extinct, and clearly a new golden orb-weaver species, the first to be described for over a century. The duo named it after Kuntner’s best friend, Andrej Komac, who died while these discoveries were made.
This is not the new species – it’s Nephila clavipes. Unfortunately, no photos fo N.komaci were available. Look at the size difference between the female and the male though…
Like other Nephila spiders, N.komaci‘s females are the giant ones. The male, by comparison, has a body that is less than a centimetre long and has legs that are 4cm long, no bigger than a large house spider.
With measurements of this new species, Kuntner and Coddington reconstructed the evolutionary history of this family of eight-legged giants. By building a family tree of the golden orb-weavers and related families of spiders, the duo showed that the females became increasingly large as the group diverged and evolved. N.komaci is the epitome of that evolutionary enlargement and is 7 times larger than the group’s ancestor probably was.
While the giant females all cluster around one part of the family tree, the males show no such patterns in terms of their size. Species with large males aren’t any more closely related than they are to those with small males. These patterns strongly suggest that male and female Nephila have massive size differences between them because the females grew big rather than because the males shrank.
This family has much to teach us about the evolution of size differences between the two sexes – a trend that is commonplace throughout the animal world. And yet, its discovery comes with a familiar warning. It may well be already endangered. With only five individuals ever seen, it’s hard to say, but the spider has only been found in two areas – part of South Africa and Madagascar – that are hotspots of endangered wildlife.
Reference: PLoS ONE 10.1371/journal.pone.0007516
A gallery of incredible spiders
In Latin America, there lives a unique spider called Bagheera kiplingi. It’s a jumping spider and it shares the group’s large, acute eyes and prodigious leaping ability. But it also has a trait that singles it out among all 40,000 species of spider – it’s mostly vegetarian.
Virtually all spiders are predators. They may hunt using different methods but they all end up sucking the liquidised innards of their prey. If they consume plants, they do so rarely, even accidentally. Some take the odd sip of nectar to supplement their diet of flesh. Others accidentally swallow pollen while recycling the silk of their webs.
But B.kiplingi is an exception. Christopher Meehan from Villanova University has found that this spider exploits a partnership forged between ants and acacia trees. The trees employ ants as bodyguards and it pays them with shelter inside hollow thorns, and nutritious nodules called “Beltian bodies” that grow from its leaves. B.kiplingi has learned to steal these delicacies from the ants, and in doing so, it has become the world’s only (mostly) vegetarian spider.
Meehan spent seven years observing the spider and filming its foraging trips. He showed that the spiders are almost always found on acacia trees that are occupied by ants, for the trees only grow the tasty Beltian bodies when ants are around. In Mexico, Beltian bodies make up 91% of the spider’s diet, while in Costa Rica, they make up 60% of it. More rarely, they will also drink nectar and evern more rarely, they will have a meat treat by taking ant larvae, flies and even others of their own kind.
Meehan confirmed his results by analysing the chemical make-up of the spiders’ bodies. He looked at the ratio of two types of nitrogen: N-15 and N-14. Plant-eaters tend to have relatively less N-15 than meat-eaters do, and sure enough, B.kiplingi‘s body had 5% less of this isotope than other species of jumping spiders. Meehan also considered the ratio of two carbon isotopes, C-13 and C-12. Meehan found that the vegetarian spider and the Beltian bodies had virtually identical ratios, as is usually the case between an animal and its food.
Feeding on Beltian bodies is worthwhile but far from straightforward. First, there’s the problem of the bodyguarding ants. B.kiplingi‘s strategy is stealth and evasion. It builds its nests at the tips of the oldest leaves, where ants rarely patrol. They will actively avoid ant guards if they see them approaching. If cornered, they will use their powerful legs to leap away. Sometimes, they even drop to safety using a line of silk, hanging in midair until the danger passes. Meehan documented several different strategies, all evidence of the impressive mental skills that jumping spiders are known for.
Even if it avoids the sentries, B.kiplingi has another problem. Beltian bodies are extremely high in fibre and spiders really shouldn’t be able to handle that. Spiders can’t chew their food; they rely on digesting their prey outside their own bodies using venom and digestive juices, and ‘drinking’ the liquefied remains. Plant fibre is a much tougher mouthful and we still don’t know how B.kiplingi copes with it.
Even so, it’s clear that the rewards are worth it. Beltian bodies are a ready-made source of food that’s available all year round. By exploiting this feast that’s produced for others, B.kiplingi has become very successful. Today, it’s found throughout Latin American, wherever ants form partnerships with acacias.
Reference: Current Biology in press
A gallery of incredible spiders
It’s been just three weeks since I last wrote about the dark-footed ant-spider Myrmarachne melanotarsa, but this is one species that just keeps getting more and more interesting. To quickly recap, M.melanotarsa is a jumping spider that protects itself from predators (like other jumping spiders) by resembling an ant. Earlier this month, Ximena Nelson and Robert Jackson showed that they bolster this illusion by living in silken apartment complexes and travelling in groups, mimicking not just the bodies of ants but their social lives too.
Now Nelson and Robert are back with another side to the ant-spider’s tale – it also uses its impersonation for attack as well as defence. It also feasts on the eggs and youngsters of the very same spiders that its ant-like form protects it from. It is, essentially, a spider that looks like an ant to avoid being eaten by spiders so that it itself can eat spiders.
Its actively raids the silken nests of other spiders and snatches the eggs and hatchlings within. These youngsters would be safe from any normal ant but being a spider in ant’s clothing, M.melanotarsa has no problem with moving through silk. But they still have to get past the parents.
The animal world is full of harmless liars, who mimic species more dangerous than themselves in order to avoid the attention of predators. But none do it quite like the dark-footed ant-spider Myrmarachne melanotarsa.
As its name suggests, this small species of jumping spider, discovered just nine years ago, impersonates ants. In itself, that’s nothing special – ants are so aggressive that many predators give them a wide berth and lots of species do well by imitating them. The list includes over 100 spiders but among them, M.melanotarsa‘s impression is unusually strong. It doesn’t just mimic the bodies of ants, but their large groups too.
Unlike all of its relatives, the spider lives in silken apartment complexes, consisting of many individual nests connected by silk. These blocks can house hundreds of individuals and while moving about them, the spiders usually travel in groups. Now, Ximena Nelson and Robert Jackson from the University of Canterbury have found evidence that this social streak is all part of the spiders’ deception.
The courtship rituals of the spider Harpactea sadistica start innocently enough, with a dance and a hug. The male spider taps the female gently with his front legs and embraces her. But from that point onwards, things for the female go rapidly downhill. The male bites her and she becomes passive, allowing him to manoeuvre her into position. Like all spiders, his genitals are found next to his head, on a pair of appendages called the pedipalps. But unusually, his penis ends in a needle-sharp tip called an embolus.
The embolus sits at the end of a loop called the conductor. The male hooks one of these loops around the opposite embolus to steady it. Then, by rotating the anchored needle, he drives the point straight through the female’s underside and ejaculates directly into her body cavity. On average, he does this six times, moving slowly downwards and alternating between his two penises. The entire cringeworthy sequence lasts about 15 minutes and throughout it, the male spider never penetrates the female’s actual genital opening.
The species was discovered in Israel last year by Milan Rezac from the Crop Rsearch Institute in the Czech Republic. He named it well. H.sadistica practices a style of sex that’s understandably known as “traumatic insemination“. It’s disturbingly common among insects and other invertebrates, and is most famously practiced by bedbugs. But this is the first time that the behaviour’s been seen among the chelicerates – the group of animals that includes spiders, scorpions and mites.
You can see it happening in the videos below. In the first, the male spider bites and incapacitates the female. In the second, he hooks the conductor of one pedipalp around the embolus of another and, with rotating motions, drives it into the female. These videos aren’t pretty – you’ve been warned.
The wiping of unwanted memories is a common staple of science-fiction and if you believe this weekend’s headlines, you might think that the prospect has just become a reality. The Press Association said that a “drug helps erase fearful memories“, while the ever-hyperbolic Daily Mail talked about a “pill to erase bad memories“. The comparisons to The Eternal Sunshine of the Spotless Mind were inevitable, but the actual study, while fascinating and important, isn’t quite the mind-wiper these headlines might have you believe.
The drug in question is propranolol, commonly used to treat high blood pressure and prevent migraines in children. But Merel Kindt and colleagues from the University of Amsterdam have found that it can do much more. By giving it to people before they recalled a scary memory about a spider, they could erase the fearful response it triggered.
The critical thing about the study is that the entire memory hadn’t been erased in a typical sci-fi way. Kindt had trained the volunteers to be fearful of spidery images by pairing them with electric shocks. Even after they’d been given propranolol, they still expected to receive a shock when they saw a picture of a spider – they just weren’t afraid of the prospect. The drug hadn’t so much erased their memories, as dulled their emotional sting. It’s more like removing all the formatting from a Word document than deleting the entire file. Congatulations to Forbes and Science News who actually got it right.
Kindt’s work hinges on the fact that memories of past fears aren’t as fixed as previously thought. When they are brought back to mind, proteins at the synapses – the junctions between two nerve cells – are broken down and have to be created from scratch. This process is called “reconsolidation” and scientists believe that it helps to incorporate new information into existing memories. The upshot is that when we recall old memories, they have to be rebuilt on some level, which creates an opportunity for changing them.
A few years ago, two American scientists managed to use propranolol to banish fearful responses in rats. They injected the animals in their amygdalae, a part of their brains involved in processing emotional memories. The drug didn’t stop a fearful memory from forming in the first place, but it did impair the memory when the rats tried to retrieve it. Now, Kindt has shown that the chemical has the same effect in humans.