Giant spiders and the quest for bio-steel - part 1

     
There is something completely alien about the gigantic golden orb web spider that appears in Hong Kong’s woodlands each summer.  Size plays a role in forming an other-worldly impression. Made up of eight metallic legs splaying out of a chunky five-centimetre body, it would cover a good adult hand. But it’s not just the shocking dimensions. It’s the shiny robotic body parts, like a Kevlar exoskeleton created by NASA, with tribal yellow markings, painted on as toxic warnings to stay clear. A closer look reveals dangling palps that scoop liquefied prey into its mouth. But unless you’ve got too close you won’t see the real weapons she keeps tucked up until a victim is within grasp. The fangs open sideways and slam shut to pierce prey in a paralysing grip. If you’re getting that perspective you’re probably a moth or a cicada living the final moments of your brief life, you may catch a glimpse of eight beady eyes looking into the depth of your dying arthropod soul.

     Perhaps the almost-as-large huntsman spiders that often cohabit with unsuspecting Hong Kongers are just as alien. They also have eight eyes and eight legs. But they have something pet-like that makes them easier to relate to.  They’ve got visibly furry bodies like ours, almost mammalian, and they like living inside flats, like us. Not so with the golden orb webs, these have no interest in the cosy world of domesticated man, they are forest dwellers. They would thrive just as well in a post-apocalyptic world purged of humanity. The huntsmans would probably do alright too, but surely they would miss the undersides of our sofas and the shiny roundness of our bath tubs. The golden orb webs wouldn’t even notice we had gone.  

Same species, Singapore.
Credit: Joseph KH Koh

    On any hike in summer you would have to be blind to miss the golden orb web spiders, and if you were blind there would be in danger of walking into one of their webs. They build the world’s biggest, spanning up to two metres across. As their name suggests, they have a golden tint to them, almost sparkling when sunlight hits at the right angle. These webs are not hidden out of sight deep in the forest, they are cast wide across open space, exploiting all the likely insect flyways. The spider sits near the middle, dark and angular, irresistibly drawing your gaze, almost evil.  The highly visible golden orb web, also known as the large woodland spider is part of the large nephila family of arachnids, nephila pilipes in Hong Kong. They don’t hide, they are avoided. Even wild animals are scared of nephila. In Hong Kong birds and bats have been ensnared in their silk. Stuck, entangled and certainly doomed in the world’s strongest webs.  

Credit: Les Martin

    The question is whether the giant spiders eat these larger animals. Nephila have the poison to paralyse, the silk to wrap and the enzymes to digest flesh, but birds and mammals are unlikely to be their intended target. Photographs of an Australian nephila cousin wrapping a trapped bird, and apparently chewing on it, emerged in a Queensland newspaper a few years ago. Experts confirmed that a golden orb web making food parcels out of bird flesh was nothing new. But even the strongest silk in the world gets badly damaged by the death throws of a bird, and so it seems that nephila generally prefer to avoid bird strikes.  They are known to build barrier nets in front of their main webs, decorated with discarded body parts of more manageable prey, as well as leaves and twigs, to create a visible warning -- spider here, might kill, stay away.  Such acts imply intelligence, so does the casting of nets on lampposts. 
    I saw the webs on several lampposts that lined a coastal hike to a small hamlet. Along a kilometre stretch the posts were evenly placed every 100 metres or so, nephila webs fixed on each crook, formed between vertical poles and the arms that reached over the footpath. It was a woody rural walk, but the tops of the lamp posts were several metres away from the edge of the vegetation. At dusk the advantage of such strategic web placing was obvious, as insects swarmed the orange glow a few centimetres from fat spiders. But how does the spider “know” that would be good place to build? Its behaviour seems to imply calculation, including the observation of insect behaviour, and a consideration of the inverted L shape of the lamppost forming the perfect frame for a web. Do they think, like we do?  
    Before we fall too far down an anthropomorphic road to confusion, it’s as well to remind ourselves that spiders are nothing like us, at all. Like insects they are arthropods, and so any genes we share with them are buried deep in our DNA archives, hundreds of millions of years old. We branched off from their family even before the backbone was invented. Spiders offer a thoroughly alternative model of life that may as well have come from outer-space when we look at some of the basic details.  
    They have two main sections to their body, a front end that holds eyes and mouth, and a back end that they excrete from. So far so good, but as noted before, they have eight eyes. Try drawing an eight-eyed animal, and see if you can possibly avoid it looking alien. Unlikely. But the eyes are not even that good at seeing. Orb weaving spiders sense the world through vibrations and chemistry. Hairs on their legs are finely tuned to detect and interpret movement on their web, which itself becomes an external sensory organ. And the tips of their legs are equiped with probes that can detect chemical changes, such as vital information that a female nearby might just accept insemination without necessarily eating its suitor.  

Credit: visual dictionary online

    The front end of the body, considered a fused head and thorax, is where the legs radiate from. That would be like your arms and legs sticking out from your neck, or dangling below your chin, all four pairs.  The back end, called the abdomen, contains the heart, as well as primitive lungs, made from evolved gills, indicating a "recent" previous aquatic existance. Air also enters the body through a kind of straw lodged near the anus. There are no veins or arteries, blood floods the body, swamping the organs, all contained in the waxy exoskeleton the spider sheds as it grows. 
    Compared to spiders, even civet cats with their feral ways are our close kin.  But if the basic anatomy of a spider confounds the expectations of a normal person, such as the average hiker on Hong Kong’s hills, their feeding habits are likely to upset the squeamish. 
    All spiders are predators and most of them are venomous, though only a small number are harmful to humans. Depending on the species, the venom falls into two large groups, the neurotoxins which mess with the nervous system, and the necrotics which cause cells around the bite area to die off.  Commercial interest in extracting or synthesising spider poisons focus on pesticides and drugs for cardiac patients.
    Nephilia are equipped with a neurotoxin that befuddles brain signals in the victim. The spider pumps the poison into the body of its prey, usually an insect, through hypodermic fangs, paralysing the victim though not necessarily killing. It then tightly wraps the insect up in silk to thwart any chance of escape when the poison wears off. Once secured, the spider liquifies the insect with digestive enzymes, sucking it dry until the husk of an exoskeleton is all that's left. 
    As devastating as this may be to a small animal, to humans the effect of a bite has been compared to a wasp or bee sting. In any case in Hong Kong it's not a common occurrence, as despite the menacing look of a golden orb web, the spider is a reluctant biter of people, unlikely to sink its fangs into you unless provoked. 
    The work of hunting and securing food is done by large female Nephila, but theives are never far away, including mite-sized kleptoparasites called argyrodes, and Nephila males lurking in the corner of the web trying to survive long enough to mate. Argyrode spiders wait until a meal has been wrapped, taking advantage of nephila's tendency to store food for some time before tucking in. The thieving spider ingeniously fools the nephila's sensitive trip-wires by securing the food with external drag lines, and methodically cutting the web around it without giving away any movement. With the final snip the contraband is transfered to the argyrode's silk line and pulled clear from the unsuspected giant at the centre of  the web. 
    The males of the species can also be mistaken for parasites. They are tiny compared to the females. Most casual viewers do not notice the reddish 5 to 10 mm bodied boy-spiders lurking in the corners of a massive web. Like argyrodes, they also steal from the female, but who can blame them, they have to stay alive long enough to fulfill the task of mating.  Female spiders are known for eating anything that gets within grabbing distance, so life is precarious for the spindly males. Males of some spider species use elaborate dance moves in the lead up to mating, to persuade the female not to eat them. The ritual is like a password to prove he is of the same species.  One wrong step and the female will take him for a pretender, which she will have to kill and consume. 
    Nephila males take a different approach that helps to explain the sexual dimorphism of the species. They go for stealth, by sneaking up on the female while she’s feeding, climbing onto her gigantic abdomen, and inseminating her before she notices. Spider sex is a strange methodical act where the male uses his dangling mouth palps to carefully place little packages of sperm in her genital openings. Having done this the male will soon shrivel and die. The female will have about a month left to feed, climb down to the ground, make a silk lined egg chamber, lay her fertilised eggs and then die. 
    It’s usually around March that the first nephila webs of the year appear in Hong Kong forests. They start off a modest 50 or 60 centimetres across, policed near the centre by an unassuming 2 centimetre female. It is hard to believe that she’ll turn into the giant arachnid of nightmares within months.