Animals do the most amazing things. Read about them here in this series by Janaki Lenin.
Many wasps are parasites. They paralyse caterpillars or spiders, lay their eggs in them, and seal them alive inside their nests as ready-to-eat meals for their larvae once they hatch. If that isn’t diabolical enough, some wasps take victimisation to a whole new level by becoming zombie masters.
Reclinervellus nielseni, a wasp known only by its scientific name, is found throughout North Africa, Europe, and across Asia to the far east. Its larvae commandeer orb-weaver spiders to spin a special web for their own cocoons. In Japan, the unfortunate victim is a small spider called Cyclosa argenteoalba.
A normal spider without a wasp overlord produces two types of web. One is the classic spider web with the circular, sticky, spiral threads used for trapping insects. The other is a minimalist web that the spider weaves when it moults its skin. Entomologists call it the resting web. It has the frame and radiating spokes of the regular web, but not the closely spaced spiral. A moulting spider doesn’t want to trap prey. While it waits for its soft body to emerge from the hard crusty old shell, it is vulnerable and can’t do much. The resting web is no more than a spider hammock as it enjoys its time-out from regular life.
A female R. nielseni wasp climbs up the non-sticky anchor lines of the spider’s web and pretends to be helplessly caught. Once the arachnid unsuspectingly approaches close enough to its “meal,” the wasp wrestles the spider, often knocking it to the ground. It lays an egg, glues it to the arachnid’s body, and flies way. The spider’s fate is sealed even as it scampers back to its web.
Once the worm-like larva hatches, it drills a hole into the spider’s abdomen and feeds off its body fluids. The spider continues its normal life even though it is saddled with a worm. When the wasp larva is ready to pupate, it manipulates the spider to weave a web to hold its cocoon.
Since cocoon-holding webs are similar to resting webs, do wasp larvae exploit spiders’ innate skill for their own ends? Or was the resemblance mere coincidence, wondered Keizo Takasuka and his colleagues from Kobe University, Kyoto Women’s University, and Osaka Museum of Natural History, Japan.
The researchers visited shrines in the cities of Tamba and Sasayama between mid-April and mid-May and collected 36 spiders and their webs. They taped each web’s anchor lines to a round wooden hoop, holding it in place, and removed it from the field. In the lab, they struggled to keep the spiders alive.
Takasuka told The Wire, “The most onerous obstacles were the rarity of finding wasp larvae on spiders and rearing spiders. Many spiders wouldn’t weave indoors. They starved and eventually died. The larvae also died.”
“So I found another method. I kept many unparasitised spiders of same species in the campus, and then replaced the spiders with precious parasitised ones. They settled down in the already constructed webs. This method provided me with several wasp larvae that reached pupal stage and manipulated the spiders.”
The larvae bid their hosts to strengthen the structural threads of the web by repeatedly laying more and more silk. The spiders work an average 10 hours at night to build the web hammocks for their masters.
Once their jobs are done, the spiders return to the centre of the web. The ill-fated arachnids serve their masters well, feeding them and constructing safe houses. The larvae have no further use for their victim-hosts. They suck their spiders dry and chuck the carcasses away. Almost immediately, the larvae build themselves a cocoon, by producing silk of their own.
A new web
The researchers say the cocoon webs are no more than slightly modified and reinforced resting webs, with structural strands but no spiral. When spiders go into a moult, they produce a specific hormone that guides them through the change. Takasuka speculates the larvae must inject a similar substance into the spiders. But it’s not a simple instruction: build a resting web. The wasp larvae somehow commandeer the spiders to go back and forth, laying more silk filaments to reinforce the web.
Although the silk filaments were no different, cocoon webs were much tougher than regular webs. A spider takes only two days to moult, but a pupating wasp requires 10 days. So the frame of cocoon webs are 2.7 times and the spokes 40 times stronger than average resting webs.
William Eberhard of the Smithsonian Tropical Research Institute, Costa Rica, told The Wire, “The measurements of the strengths of the lines are particularly useful and new. They have documented in this species types of changes that have also been seen in the cocoon webs of other species.”
In addition, the spiders also add finishing touches: little fluffs of silk that wave in the wind along the spokes of the web. This is a standard ornament in resting webs too. Entomologists cannot agree whether the role of these appendages is to deter predators, attract prey, or advertise the web so birds and large insects don’t unwittingly crash into it.
The researchers say the silk fluffs were clearly visible under ultraviolet light. So they may function as ‘Do Not Disturb’ signs. The last thing moulting spiders and pupating wasps want to deal with are web crashers. However, the classic trapping web with a spiral has no such decoration and is invisible under ultraviolet light. After all, spiders don’t want to alert their insect prey.
Eberhard says, “One possible new question for this species, and in fact for all the others that have been studied, is whether the wasp larva may induce the spider to modify the chemistry of the silk lines themselves, to produce stronger fibres. This kind of change, in response to environmental factors, is beginning to be documented in other species of spiders, but has never been looked for in the cocoon webs of parasitized spiders.”
Takasuka says, “Wonderful adaptations shown by minor organisms are seemingly useless for human welfare, but they may have invisible potential. Don’t take little account of them.” One thing prevents most spiders from suffering this awful fate – these wasps are relatively rare.
The paper was published by the Journal of Experimental Biology on August 5, 2015.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.