The larvae of the parasitic wasp Reclinervellus nielseni turn their host spiders Cyclosa argenteoalba into drugged navvys, to modify the web structure into a more persistent ‘cocoon’ web so that the larvae can pupate safely on this web after the spider’s death, according to a team of scientists led by Dr Keizo Takasuka of Kobe University in Japan.
Types of web constructed by Cyclosa argenteoalba, from left, clockwise: normal orb web; resting web with a molting spider (inset) somewhat away from the hub; normal orb web constructed by a spider parasitized by Reclinervellus nielseni on the eve of being manipulated by the wasp larva; cocoon web constructed by the same spider as in the previous image, where the inset shows close-up of the parasitized spider. Scale bars – 5 cm. Image credit: Keizo Takasuka et al.
Orbweaving spiders Cyclosa argenteoalba produce several different styles of web over the course of their lives – ‘orb’ webs when hunting and ‘resting’ webs for protection when molting – each produced by a specific set of behaviors.
However, it wasn’t clear which of the spider’s behavioral patterns and web-types the parasitic wasp Reclinervellus nielseni was adapting to its own ends until Dr Takasuka and co-authors investigated how the wasp larva manipulated its host.
In their study, the scientists found that the wasp larva forces its host spider to build a modified and reinforced resting web before steering them back to the center of the web to construct a cocoon.
First, Dr Takasuka’s team collected spiders complete with their webs and parasitic larvae. They then analyzed the cocoon webs of Cyclosa argenteoalba and were struck by their similarity to the resting webs, complete with fluffy decorative structures.
And when the team analyzed the spiders’ behavior as they constructed the cocoon webs over a 10 hour period, they saw that the manipulated spiders always constructed the new web on the site of the old orb web, painstakingly removing the sticky spiral first, then reinforcing the radial and frame threads and then adding the fibrous web decorations.
And, when the web was complete and the wasp larva done with its spider slave, the larva directed the spider to return to the hub of the web before murdering it.
“So, the cocoon webs looked like resting webs, but were they true adaptations of the more conventional web? We photographed the webs in UV light and were impressed to see the fibrous decorations shining brightly, to deter other insects from inadvertently blundering into the pupa’s nursery,” the scientists said.
When the researchers investigated the strength of the different types of silk that contribute to the web’s structure, they were amazed to see that the cocoon web was significantly tougher than the orb and resting webs: the breaking force of the cocoon radius and frame silks was 2.7 to 40 times greater than for the orb and resting webs.
However, the breaking stress of the silks was not increased significantly, leading the scientists to suspect that instead of forcing the spiders to alter silk production, the wasps were directing the spiders to lay down more fibers to strengthen the cocoon web, which is essentially a reinforced resting web.
“Resting web construction is triggered by the same hormones that control molting and I suspect that the wasp larva may inject a substance similar to a molting hormone into the spider during the later stages of its stay, ready to trigger cocoon web building when the larva is ready to pupate,” said Dr Takasuka, lead author of a paper published in the Journal of Experimental Biology.
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Keizo Takasuka et al. 2015. Host manipulation by an ichneumonid spider ectoparasitoid that takes advantage of preprogrammed web-building behaviour for its cocoon protection. J Exp Biol 218, 2326-2332; doi: 10.1242/jeb.122739
