Freeloading

When I was happily photographing tatalencho (Gymnosperma glutinosum) at Wild Basin on October 27th I came across this little drama that I had trouble understanding. Most conspicuous was the dead bee whose head-down posture on the flower stalk made it look like an airplane that had nose-dived into a crash-landing. We have to assume the bee had met its demise thanks to the spider that now too was upside down and immobile, and that I took to be dead as well. Standing on the bee’s upraised rump was by far the tiniest of the creatures in this arthropodal ménage à trois, a dark insect that appeared to be biting or sucking the tip of one of the spider’s upraised legs.

I e-mailed for help, which came quickly. From entomologist Alex Wild at the University of Texas I learned that the dark little insect is “a freeloader fly (probably Milichiidae or Chloropidae, hard to tell from the photo), taking advantage of the spider’s kill. As Joe [Lapp] said, this is a fairly common phenomenon, but since the flies are so small it is often overlooked.”

I also heard back from local expert Val Bugh: “The spider is not dead, it’s busy eating and crab spiders prefer to remain still, especially when their prey blocks them from view. The black fly, commonly called a freeloader fly (family Milichiidae) is actually just standing on the bee’s butt — it only looks like its face is touching the spider’s leg because of the angle. Really, the mouth of the fly goes down and it is probably waiting to scavenge some droplets while the spider feeds (or it is wondering whether or not it needs to leave because there is a big, scary camera pointing at it). Usually, spiders and other predators just ignore these little flies, which might walk all over both the predator and its victim.”

Joe Lapp added some more: “I think I’ve only ever seen this once on the prey of a spider other than a crab spider. I think crab spiders are preferential because they don’t masticate their prey like most do. Instead, they inject digestive fluids into holes and slurp out the yummy insides. That leaves opportunity for flies to go to town unharmed. I wonder if the venom or enzymes also assist the flies. In my mind, the wild thing is that these flies show up before the prey begins decaying.”

So there you have the explanation for this curious sight.

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I’m away for a few days. You’re welcome to leave comments, but it may take me a while to answer.

© 2015 Steven Schwartzman