The Case Of The Web-Jacking Wasp

The fear of spiders and insects are among the most common phobias in America, and being mind controlled by larvae that live inside you isn’t exactly a national pastime, so the wasp genus Polysphincta probably isn’t anyone’s idea of a good time. Though there is individual variation between species, each one has a similar parenting strategy – handing off the kids to a trustworthy spider for a while. The process starts when a female wasp finds a good unwilling surrogate to lay her eggs in, usually a female spider of the genus Cyclosa. She attacks the spider, usually by ambush, and puts her ovipositor -the egg-laying organ that has been modified into a stinger in bees, wasps, and ants – in its mouth. A sting is delivered, most likely to the subesophageal ganglion (a group of nerve cells that controls the mouthparts and neck muscles), that paralyzes the host for long enough that the wasp is able to lay a single egg on top of the spider’s abdomen. Around two days later, the egg hatches and the larva starts to grow and feed on the spider’s hemolymph (the invertebrate version of blood). At around 22 to 24 days old, it begins modifying the spider’s behavior [1]. At this point, the larva has outgrown its place on the host’s back, and needs a safe cocoon in which it can develop into an adult wasp. The spider is able to provide – it spins an unusual type of web that differs pretty significantly from that of a normal spider. This new web is simpler, with fewer spirals and spokes than the webs of healthy spiders. At the same time, the larva develops tubercles that it will use to cling on to the finished web. Once the cocoon is built, the spider dies and the larva eats its corpse, giving it the nutrients it needs to grow into a fully developed wasp.

Left to right: Web of an unparasitized female, early-stage parasitized female, modified cocoon web, and unparasitized moulting web. (Kloss et al, 2017)

How does this little wasp control an entire adult spider? The neural mechanisms are actually pretty simple. Arthropods have a steroid called 20-Hydroxyecdysone (or ecdysterone, or 20E) that controls their moulting and metamorphosis. In spiders, an increase in this hormone causes them to create a moulting web – a web with a structure somewhat similar (though still more complex) to the one seen in parasitized individuals. Spiders that have just built a cocoon nest have a higher level of this hormone than spiders that haven’t been parasitized, or those with larva that are in an earlier phase and not yet ready to metamorphize [2]. It is hypothesized that the larva create either 20E or a chemical precursor to the steroid and inject it into the spider’s hemolymph as they’re feeding, causing this highly beneficial change in behavior through just a single hormone. This biological simplicity is a breath of fresh air in the tangled web of host manipulation.

[1] Kloss, T. G., Gonzaga, M. O., Roxinol, J. A. M., & Sperber, C. F. (2016). Attack Behavior of Two Wasp Species of the Polysphincta Genus Group (Hymenoptera, Ichneumonidae) on their Orb-Weaver Spider Hosts (Araneae, Araneidae). Journal of Insect Behavior, 29(3), 315–324.
[2] Kloss, T. G., Gonzaga, M. O., de Oliveira, L. L., & Sperber, C. F. (2017). Proximate mechanism of behavioral manipulation of an orb-weaver spider host by a parasitoid wasp. PLoS ONE, 12(2), 1–11.

One thought on “The Case Of The Web-Jacking Wasp

  1. What a web of deception! I was not familiar with this particular case study, but it is really interesting. I was thinking about similarities to another system, the jewel wasp / cockroach system, that also targets stings to the subesophageal ganglion (SEG) to alter the cockroach’s behavior to become less intrinsically motivated to walk or escape. It seems like a similar shift in “motivation” towards a directed behavior is going on here.

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