Way back in March of 2001, my wife and I were out hiking in Phoenix’s South Mountains. I had just gotten my first digital camera, a Nikon Coolpix, and was eager to take some photos.
I was trying to take pictures of Messor pergandei, a type of Harvester Ant that is common around Phoenix. My wife noticed something strange in the vegetation near the nest – there was a cluster of dead ants hanging from the stem of a plant.
We’d both recently read Carl Zimmer’s excellent book Parasite Rex, so we first thought of the fluke that “takes control” of ants at a certain stage in the fluke’s life and makes them climb to a prominent place. This makes it more likely that that the ant will get ingested by the fluke’s next host. The ants we were looking at, though, were hanging by silken threads.
As we looked around the nest some more, we found more ants hanging from the vegetation.
All of these hanging ants were within a foot or two of the nest, in or near the nest’s midden of discarded seeds.
As we looked at the strange sight of the dead ants, I noticed what appeared to be a dead ant moving across the ground. The black coloration of the ant made her very easy to see, and it looked as though the ant was moving as she lay on her side, without moving her legs.
But the dead (or paralyzed!?) ant was being dragged along by a spider, one much smaller than the ant.
The ant was dragging the ant with strands of spider silk attached to the spider’s spinnerets. In the photo below, you can see that there is a gap between the spider and the ant – that gap was spanned by spider silk.
We looked at another nearby Veromessor nest, and found more dead ants hanging around the nest. It was a creepy sight, like a scene out of the movie Apocalypse Now.
We saw another Euryopis spider dragging an ant. This one was a male spider – we could tell by his enlarged pedipalps. But the spider didn’t drag the ant up into the vegetagion – rather, he dragged the ant under a potato-sized rock that was near the entrance of the Messor pergandei nest.
As we kept watching, we saw another spider. And then another. And another…male and female spiders.
This was really quite extraordinary.
I turned over the rock that seemed to be the center of the spiders’ activity. I could see both male and female spiders on the underside of the rock.
As I looked at the rock, I saw more and more spiders. There are at least four spiders in the photo below, along with two or three of their Euryopis prey. At one point I was able to count eight(!) of the spiders in my field of view at the same time.
The ants seemed to be passive throughout all of this. There were few ants walking about. Rather, the workers were clustered in and near the nest entrance. In contrast, other nearby Veromessor nests that did not have obvious spiders had streams of foragers coming and going.
I wonder if one of the reasons that Messor pergandei nests often have multiple entrances is to mitigate the effects of these spider infestations.
Amanda Hale etal (2018) also saw groups of Euryopis hunting around the entrance of Veromessor pergandei nest. She says that these large numbers of spiders at the same ant nest isn’t just happenstance – she suggests that the spiders are actually aggregating. If the spiders are aggregating,I wonder what the mechanism for that aggregation would be. How would the spiders find each other, how would they choose which ant nest to hit?
Alison Bockoven has a truly neat blog post about an encounter she had with Euryopis spiders. She saw a group of these spiders at night around a Pogonomyrmex (barbatus?) nest. Some of these spiders…actually went into the nest and dragged Harvester Ants out.
Can you imagine how horrifying that would be?
A lot of ants block off the entrance of their nests at night, sealing themselves in with pebbles. I’ve always wondered why they go to this effort, since they will only need to dig themselves out in the morning. Maybe these spiders are part of the reason for the nest-sealing.
Amanda Hale, Tierney Bougie, Elisa Henderson, MadisonSankovitz, Mari West, et. al. 2018. Notes on hunting behavior of the spider Euryopis californica Banks, 1904 (Aranaeae: Theridiiddae), a novel predator of Veromessor pergandei (Mayr, 1886) harvester ants (Hymenoptera: Formicidae). The Pan-Pacific Entomologist, 94(3): 141-145.