Sunday, December 25, 2011

Spider Sunday: Spider Enemies

Spiders may seem invincible, especially to those who fear them. Not true. Spiders are under constant threat from predators, parasites, and other mortality factors. It is impossible to even list all the agents that kill spiders, but some creatures that prey exclusively on spiders, or nearly so, are worth investigating.

Even tarantulas, the largest spiders, are not immune to attack. Enormous wasps in the genus Pepsis aggressively seek these gargantuan arachnids. Only the female wasp is armed with a stinger, a formidable weapon to match the spider’s fangs.

In the southwest U.S., the wasp fuels her superhero lifestyle on nectar, especially from milkweed flowers. Once juiced-up, she scours the ground for signs of her prey. Encountering an inhabited burrow, she lures the spider out of it and the battle begins. Tarantulas can move fast when they have to, but the wasp is usually quicker. She stings it in a nerve center on the underside of its cephalothorax, rendering the arachnid almost instantly paralyzed.

Once she has immobilized the spider, the wasp drags her victim to a hole where she caches it. She sometimes uses the spider’s own burrow. She lays a single egg on the spider, then leaves. She will repeat the process until she dies. The larva that hatches from the egg consumes the still-living spider, eventually pupating and emerging as an adult wasp the following year.

All wasps in the family Pompilidae, including Pepsis, are dedicated spider-slayers. So are mud daubers in the families Sphecidae and Crabronidae. The Black and Yellow Mud Dauber, Sceliphron caementarium, builds clod-like mud nests under the eaves of houses and other buildings, stocking each cell with numerous spider victims. The wasps are generalist hunters and almost any kind of spider will do. The Blue Mud Dauber, Chalybion californicum, is famous for killing black widow spiders, but it is also a generalist. It uses the old nests of the Black and Yellow Mud Dauber instead of building a nest from scratch.

Still other wasps don’t bother making a nest at all. The ichneumon Acrotaphus wiltii (above) simply locates a host (the orb weaver Neoscona arabesca in this case), stings it into brief paralysis, and lays a single egg on it. The wasp leaves, but her larval offspring will feed as an external parasite on the spider.

Flies are the quintessential victims of spiders, but some turn the table. Small-headed flies of the family Acroceridae, are parasites of spiders, especially trapdoor spiders.

The female fly lays hundreds or thousands of eggs, scattering them throughout the landscape. The larvae that hatch are highly mobile, host-seeking missles. Upon contacting a spider, the larva climbs up its host’s leg and burrows into the spider’s body wall. It takes up residence around the book lungs, feeding internally on its host.

A similar scenario is played out by members of the family Mantispidae, order Neuroptera. Females lay large numbers of eggs and the larvae emerge to go in search of spiders. Instead of eating the spider, they simply climb aboard and wait. What are they waiting for, you ask? A larva waits for a female spider to spin her egg sac, at which point it climbs inside and becomes wrapped up with the eggs. Then it eats the eggs, pupates, and eventually emerges as an adult mantispid. What if a larva climbs onto a male spider? It will transfer to a female during mating.

Among the most amazing spider predators are “helicopter damselflies” of the family Pseudostigmatidae.

These tropical giants hover in front of a spider web and simply pluck the owner off its silken platform. They are aquatic as nymphs, living in pools of water inside treeholes and preying on other small animals.

It should come as no surprise that perhaps the most lethal spider predators are…other spiders. Spiders may be able to negotiate their own webs, but can easily become entangled in another spider’s snare. Further, mating is risky business, even if you are not a black widow. Female spiders are usually larger and more powerful than their mates, and may eat potential suitors instead of courting them.

No spider hunter, however, matches the skill and stealth of the jumping spiders of the genus Portia, found in Africa, Asia, and Australia. They may build their own webs, unusual for jumping spiders, or actively stalk other kinds of spiders in their webs. Even other jumping spiders are a potential meal.

Have you ever seen a spider as the victim of another animal? Did it surprise you? Please feel free to share your stories here. I may also revisit this topic to address other spider-eating creatures.

Aknowledgements: Acrotaphus wiltii image by Tom Murray in Massachusetts and borrowed from Bugguide.net; Acrocerid fly image is of Eulonchus tristis, taken in Washington state by Stephen Hart and borrowed from Bugguide.net; Portia spider image taken from sgmacro, by Nicky Bay of Singapore; all other images by Eric R. Eaton unless otherwise noted.

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