Tuesday, February 19, 2013

Zombie-Slave Spiders

I love spiders.  I'm also a big fan of zombie-lore, and of course of parasites.  This week, I decided to bring those three things together to tell you about a species of parasitic wasp that infects spiders.  This parasitic wasp (belonging to the genus Zatypota) hijacks the spider's body making it not only a zombie, but also a slave.  Infected spiders spin slightly abnormal webs to make better areas for the parasites' cocoons.

Zatypota specimen
mounted on a pin.

There are probably hundreds of species of parasitic wasps.  Most of theses belong in the family Ichneumonidae.  (In fact, they might all be in this family...but I can't remember, and I don't want to lie to you!) Within this family is a subgrouping that includes wasps that specifically parasitize spiders.  Members of this group are known as polysphinctine wasps. The female parasitic wasps use their ovipositors to insert eggs into the bodies of many different types of hosts...caterpillars, cockroaches, and as you will soon learn, spiders.  These wasps' eggs usually hatch within the host, using it as a food source, shelter, and in some cases as a vehicle to a more suitable emergence area.  Today, we will look specifically at a type of parasitic wasp that infects a spider that lives in the rainforests of Costa Rica.

Anelosimus octavius is a type of spider belonging in the trash-web family, Theridiidae.  This family is full of spiders with large abdomens that are carried in a distinctive way by members of this family.  I can't describe this very well...it's really something you just have to see to understand.  This family is also known as the family of  "comb-footed" spiders due to these neat little modifications they have on their back set of legs. Spiders in this family are also sometimes referred to as "cobweb" spiders because their webs take on a haphazard cotton-y appearance rather than the intricate patterns created by orbweavers or the distinctive sheets made by funnel-web spiders and their relatives.  Some of the more infamous members of this family include members of the genus Laterodectus...also known as black widows.

A typical web built by Anelosimus octavius.
Getting back to our little Zatypota, this wasp, like other polysphinctine wasps, modifies the behavior of their spiders hosts.  More specifically, these parasites modify the spiders' web-building behaviors. After a female wasp inserts its eggs into the spider hosts, the wasp larvae grow for about a week, feeding on the spiders' hemolymph for sustenance. Then, the larvae commands the spider to construct a modified "cocoon web".  Then the larvae emerge from their hosts, eat the hosts, then build a cocoon next to the cocoon web in which to pupate.  The idea is that these cocoon webs and other parts of modified web structures increases the likelihood of survival for the wasps by serving as additional protection during the pupal stage.

Central platform of web
showing top of wasp cocoon.

According to reports of observations of infected spiders, the hosts spend a great deal of time and effort preparing the cocoon webs for their parasites.  After tireless efforts, the exhausted spiders crawl back into the central part of the site only to become immobile  before being consumed by its parasite.  The following day, the parasite would began constructing its own cocoon right on top of the area where it ate its host.

It is interesting to think about how this could have evolved among these two species! However it happened, this is yet another awesome example of behavioral modification due to parasitism.  Hooray for making your hosts into zombies!

This is from a paper describing the odd behavioral changes induced by these parasitic wasps.
"Figure 2: Cocoon web of A. nr. studiosus (a) in which the wasp larva holds onto the densely-meshed central area just after having discarded the corpse of the spider. Lateral (b) and dorsal (c) views of wasp cocoons in cocoon webs of A. octavius, showing the radial pattern of lines around the upper end of the cocoon (c), and that the lines intersecting the cocoon (indicated by small “pimples”) are in the upper portion of the cocoon, with an open space below in which the cocoon hangs free (bar in (b)). The cocoon web spun from scratch in captivity (d) incorporated flat leaves (covered with white dust in the photo) as parts of the sheet."

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