A spider wasp drags a paralyzed spider across the deck in Troy. Photo by Dana Wilde
Sitting on the deck on a summer afternoon, talking with friends. Nice in the shade. Pugnacious hummingbirds buzz past to make sure we know where we belong. A mosquito or two. Or three.
One of my eyes always keeps track of the sub-ecosystem on the deck. The creatures down there make their ways around with particular gaits, that seem for all the world to signal their intentions. Ants scurry in starts, stops and hairpin reverses. Beetles shuffle past in straight lines. Jumping spiders hop, dart, pause to look around, repeat. Tiny mites motor around on cruise control. I once watched a running crab spider loop around and around the rim of a coffee cup as if she were trapped in a never-ending, two-dimensional cage.
This afternoon, I spot somebody dragging something. My first thought is that it’s an ant with a huge prize. I once watched two ants pull, push, nudge and knock the carcass of a wolf spider across a road. But on second look, this is not an ant. It has wings. It’s some kind of small wasp. Whatever it’s dragging has more than six legs — a spider.
I have a pretty good idea what’s going on here. It’s not in the spider’s favor.
I think it’s a spider wasp, of the family Pompilidae. The spider is motionless, and I can’t tell what kind it is. How did the wasp come to be dragging the spider? Some pompilids can knock a large orb-weaving spider out of its web onto the ground, where they grapple the spider with forelegs and mandibles and then sting it, paralyzing it. It looks like the spider wasp on the deck has already accomplished this. Now, it’s dragging the immobile spider to a pre-excavated hole, where it will store the spider to eat later. It could be weeks before the meal ensues.
Some species of spider wasps lay an egg on the body of the warehoused spider. When the egg hatches after a few days, the wasp larva eats the paralyzed spider alive. Some ichneumid wasps, species of which we also have in Maine, lay eggs not only on the spider’s body, but sometimes in the egg sacs of wolf spiders, where the larva feeds after hatching out.
An ichneumid wasp in Costa Rica attacks the long-jawed orbweaver, or tetragnathid, spider right in its web, paralyzes it with a sting and then glues an egg to the spider’s abdomen. For the next week or two, the spider goes about its business undisturbed, building its usual webs using its web-grade silk.
Meanwhile, the egg hatches and the larva, still attached to the spider’s body, drills tiny holes to suck the spider’s hemolymph (roughly speaking, the spider’s blood).
On the night it intends finally to kill the spider, the wasp larva injects a psychotropic drug that induces the spider to use its dragline-grade silk to build a whole different kind of web. The lines of this “cocoon web” are attached to the surroundings differently from the regular orb web, and are more solid for holding a cocoon than the spirals of the orb web. The larva then molts to its next instar, and that night kills and eats the spider.
The next night, the larva spins a cocoon that it hangs by a line from the drug-induced web. It completes its cycles and reaches adult wasphood in a little more than a week. Studies have found that long-jawed orbweavers saved from being killed by the wasp larvae gradually rediscover how to build a normal orb web.
Researchers have observed that most spiders appear to be terrified of spider wasps and try to run away from them. Some wolf spiders have been seen trying to push a wasp away or hide behind a plug of sand. Hackledmesh weavers of the Amaurobius genus, which we have abundantly here in Maine, seem unfazed by pompilids and sometimes overpower and eat an attacking wasp when it gets entangled in a hackledmesh web.
The spider being dragged across the deck is almost certainly still alive. I wonder if it’s still awake.
At that point, I get sidetracked in one of my grandson Silas’ adventures, and by the time I’m back on scene, the wasp and spider are gone. Off to some pompilid underground chamber.
Life moves in mysterious gaits.
Dana Wilde lives in Troy. You can contact him at naturalist1@dwildepress.net. His book “A Backyard Book of Spiders in Maine” is available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.
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