This blog also appears at zombieants.ca a science blog dedicated to parasite induced altered behaviour, host mind control.
The numbers of small insects and spiders on the planet is staggering and these two groups are both immensely fascinating and terrifying all at once. They posses amazing abilities like completely transforming their bodies during metamorphosis, producing venom and silk or the evolution of the eusocial lifestyle. With all these incredible traits comes too the cold and horrifying ones that this tiny world demands as well. The swarming hive mentality of the eusocial ants that will consume and attack everything in their path, overwhelming larger insects and tearing them apart alive and taking the pieces home to feed the nest. The beauty of the spider web too gives way to a mode of feeding that is truly unimaginable in larger animals, wrapping up prey in silk, piercing it with sharp straw like appendages and slowly sucking up all the liquid insides. It is a harsh world for these tiny animals and a take no prisoners, show no sympathy way of life is required to survive here. One of the most horrifying ways of life is the parasitoid lifestyle and its adaptations that create living (for a time) nurseries for metamorphosing young.
Parasitoids are essentially parasites except that they usually only require a host for part of their life cycle and will more often than not kill and consume that host when they are done. Many species of wasps have adopted this strategy and some have evolved some impressive/heinous ways to get the job done. As fully mature adults these wasps are loners, unlike hive building eusocial wasps. They live and hunt on their own, mate and then parasitize other arthropods like cockroaches, spiders, caterpillars and all varieties of bugs. They will lay their eggs on or in these hosts which will serve as a food source for the developing young to consume alive until they are ready to become adults.
It isn’t all as easy as it sounds for these wasps though as they have to locate a host, often immobilize it long enough to plant the eggs and once present on the host the young wasps can be vulnerable to other parasitoids or predators. In order to overcome some of these difficulties some parasitoids appear to have evolved the ability to subvert the minds of their hosts for their own purposes.
A stunning example of this involves orb weaving spiders and the parasitoid wasp Hymenoepimecis argyraphaga found in Central American rain forests.The wasp will attack, paralyze the spider with venom and lay an egg on the abdomen. The spider will resume normal activity after the initial venom wears off but the wasp larva remains and feeds on the insides of its host by slowly sucking them out from its position on the abdomen.
As the wasp gets closer to maturity it induces the spider to forgo its normal web building process for the construction of a special cocoon web. Inside this structure the wasp molts, finishes consuming the spider, after which it spins its own pupal cocoon anchored to the specialized spider web. Here it finishes growing and emerges as an adult. By inducing the change in web building the spider creates a protective structure for the wasp to finish feeding and then to anchor its final cocoon too. Without this the young wasp would be exposed to the environment and scenarios like being washed away and killed during heavy rains.
The truly terrifying aspect of this relationship, even worse than being eaten alive, is the fact that the wasp can not just prevent a host from resisting being consumed but will actually drive the host to aid the creator of its own gruesome demise. It is believed that is done with some sort of venom because the weird spider behaviour will increase over time as the larva grows, and releases the chemical. This mechanism also seems likely as the wasp is not in direct contact with hosts brain or nervous system. The effects of altered behaviour also gradually diminish if the larval wasp is removed. All signs point to a chemical means of manipulation and this is hardly the only wasp for which mind controlling venoms have evolved. Many of the parasitoid wasps induce their hosts to protect the very things that are eating them alive. A quick YouTube search will provide ample video of these horrifying relationships many more of which will also be covered here at ZombieAnts.ca.
Original work on Hymenoepimecis argyraphaga :
Eberhard, W.G., 2000. Spider manipulation by a wasp larva. Nature 406, 255-256.
Eberhard, W.G., 2001. Under the influence: webs and building behavior of Plesiometa argyra (Araneae, Tetragnathidae) when parasitized by Hymenoepimecis argyraphaga (Hymenoptera, Ichneumonidae). Journal of Arachnology 29, 354-366.
Eberhard, W.G., 2010. Recovery of spiders from the effects of parasitic wasps: implications for fine-tuned mechanisms of manipulation. Animal behaviour 79, 375-383.