"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

August 3, 2015

Hymenoepimecis argyraphaga

Those who have been reading this blog for a while realise that August is the month when I featured some guest posts written by students from my Evolutionary Parasitology  (ZOOL329/529) class.  One of the assessment I set for the students is for them to summarise a paper that they have read, and write it in the manner of a blog post. The best blog posts from the class are selected for re-posting (with their permission) here on the Parasite of the Day blog. I am pleased to be presenting these posts from the ZOOL329/529 class of 2015. To kick things off, here's a post by Alison Cash on a paper published in 2001 about a parasitoid that uses its spider host to weave a tangled web.

Left: The usual web constructed by a Plesiometa argyra. Right: A web constructed under Hypmenoepimecis' influence
Photo from this paper.
The parasitoid wasp Hymenoepimecis argyraphaga can be considered to be pretty unremarkable at first glance. However, the life history of this killer insect contains more drama and intrigue than an episode of Game of Thrones - maybe with just a little less incest. This wasp is found in the tropical forests of Costa Rica. Here, when an expectant mother wasp is prepared to lay her solitary egg, she seeks out one particular species of orb-weaver spider - Plesiometa argyra.

This spider is known for its elaborate web-spinning abilities, with which it uses to capture its prey. Each day, it meticulously recreates its skilled masterpieces and for this talent H. argyraphaga targets it with the burden of raising its life-sucking young. The larva of this wasp not only makes a meal of the spider, it also turns the unfortunate arachnid into its personal slave via mind control - using it to create a perfect haven to pupate.

When the female wasp locates a P. argyra, it temporarily paralyses its victim with a sting before it glues an egg on the spider and leaving. After 10-15 minutes, the spider wakes out of its stupor, and resume life as normal, apparently unaware of its new and sinister backpack. The egg soon hatches and the larva anchors itself to its spider host, riding it triumphantly for the next two weeks, all while feeding on the spider's blood (call hemolymph) from small holes it has punctured in the host's abdomen.

Once the larva has matured and is ready to begin its transformation into an adult wasp, the relationship becomes more menacing. The larva injects the spider with a cocktail of chemicals that alters its web-weaving behaviour. Under this influence, the spider custom-build a unique reinforced web, fit to encase the wasp larva in its a cocoon while it metamorphoses. Once the spider had completed this highly altered web, the spider moves to the center of the web where it remains somewhat dazed. The wasp larva then dismount from its naive eight-legged steed, then kills it and suck the corpse dry for its last supper as a larva. It then weaves a cocoon which nestles securely in the middle of the web, suspended away from potential threats. After ten days, the adult emerges to begin the grisly cycle once again.

What sets this wasp apart from many other parasitoids is that it modify the host's behaviour, via an injected chemical cocktail, in such a specific and detail manner. Instead of weaving the usual intricate five-step web, P. argyra is reduced to repeated the first two step of web construction. The scientist who conducted this study observed that by blocking the ability to construct the multi-step web, the result was a "custom-built" structure which is more durable and less likely to be damaged by falling debris. Even when the larva is removed from the spider before it is able to kill its host, the webs made by the previously parasitised spiders are still malformed for the following few days, but eventually return to normal, which suggest that the behavioural change is induced by a chemical rather than just physical interference by the parasitoid larva.

By chemically inducing this altered host behaviour, H. argyraphaga ensures that it will successfully raise another generation of spider-enslaving wasps.

William G. Eberhard. (2001). Under the influence: Webs and building behavior of Plesiometa argyra (araneae, tetragnathidae) when parasitized by Hymenoepimecis argyraphaga (hymenoptera, ichneumonidae). Journal of Arachnology, 29(3), 354-366.

This post was written by Alison Cash

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