I walked face-first into the large web of the giant wood spider, also called golden orb-weaver, on a jungle walk. I could barely see the laboriously made, mathematically precise gossamer trap, which the female had slung right across the path even though it was about a metre in diameter. If I were a small insect, I’d get more and more entangled as I struggled. Only after I wiped the sticky strings from my face did I notice the silhouette of the spider high above my head scampering away, seemingly treading the air. I kicked myself for being blind, especially since the weaver had a leg spread of 10 centimeters and was strikingly coloured with bright turmeric yellow stripes down her long black body.
Insects get snagged in the world’s original glue traps because they cannot see the webs either. Small birds such as warblers, tailorbirds, and swiftlets are no better, flying right into the well-laid durable nets. I’ve even seen a photograph of a young bronzeback tree snake helplessly tangled in one. Knowing that wild jungle-sharp creatures are just as blind makes me feel better.
To produce these invisible sturdy lairs, the Indian orb-weaver (Nephila pilipes) has five silk-producing glands in its rear end, but some of its relatives have as many as seven. The spider squirts strands from the appropriate gland: thick fibres for the frame and radials of the web, tough threads to truss up prey, and special silk to envelope the egg sac. Embedded in the fine sticky filaments are tiny droplets of glue, which stick fast to anything that encounters them. These globules contain much more than adhesives.
Mario Sergio Palma, a professor of biosciences at São Paulo State University, Brazil, studies plant toxins and insect venoms. While living in a rural area a few years ago, he watched little bugs caught in the silken nets of the banana spider (Trichonephila clavipes), a close relative of the Indian orb-weaver. The spiders hadn’t bitten them, yet the insects trembled and their little tongues, called glossa, stuck out of their mouths. When he rescued them from the webs’ clutches and placed them on a flat surface, they didn’t take off after cleaning themselves as one would expect. They staggered like drunks, crawling instead of walking. For an expert well-versed in poisons and venoms, this behaviour suggested symptoms of neurotoxicity.
On analysing the sticky globules, he found more than 200 toxin-like proteins, peptides, and fatty acids, typical of spider venoms but also similar to those produced by other venomous creatures such as snakes and marine life forms. The banana spider secretes venom from both ends. Some toxins in the web droplets resembled neurotoxins that could interfere with insect nervous systems.
To understand how topical application of toxins in the web would act on prey, Palma and his colleagues smeared a fatty acid called palmitic acid, found in abundance in the glue droplets, on the backs of honeybees. A layer of wax usually coats the hard shell of insect bodies to reduce the loss of water by evaporation. This wrapping could prevent poisoning through contact. The scientists discovered that the palmitic acid corroded the honeybees’ waxy protection, opening the way for the toxins to work their way in. Then the researchers tested the efficacy of the toxins by injecting them into bees which became paralysed.
In the marine realm, the Portuguese man-of-war follows a similar template, dangling an invisible curtain of tentacles with venom-loaded barbs under the water surface to snare unsuspecting fish that blunder into it.
When invisible glue traps strung through the air catch insects flying into them, poisoning them seems to be overkill. Exhausted from struggling to free themselves, insects can die quickly. Then why do the spiders go through all that effort to produce these complex chemical substances? That’s because spiders want their meals served live. The neurotoxins in the microscopic droplets are just strong enough to knock out their victims and hold them quiescent until the eight-legged predators feel the pangs of hunger.
Some substances in the droplets are antimicrobial proteins that preserve the web and prey from becoming infected by microbes. Other elements start pre-digesting the insects while they are still entangled, so the spiders have a half-prepared repast.
Isn’t embedding neurotoxins throughout the web more expensive and wasteful than attacking the enmeshed prey? On the contrary, more proteins are needed to produce fibre for enveloping insects than for spreading toxins all over the web, says Palma. The banana spider does inject venom and silk-wrap large meals such as dragonflies, cicadas, and butterflies, since they provide more nourishment for the amount of effort it spends. For smaller insects like bees and flies, the predator relies on the web to paralyse, safeguard, and liquefy because the nutrients it gains from them does not compensate for the use of venom and silk fibre.
Although this research was conducted on an American species, the researchers suspect many others, possibly including our Indian orb-weaver, construct similarly toxic webs. As disquieting a thought as that is, I take comfort in knowing that these venoms probably have little or no effect on a klutz like me.