Thread-legged Bug

2007 October 20

I’ve never seen one of these before, but I think that’s because they’re hard to spot, not because they’re particularly rare:

This is a “thread-legged bug” that I found in the bottom of the wheelbarrow after moving some old lumber. At first, I thought it was a tiny splinter of wood, before I realized that it seemed to be levitating slightly[1]. The legs are long, thin, and practically invisible, hence the name.

Looking at just the body, it looks rather strikingly like a praying mantis nymph. This is even more striking when we look at it from the side in silhouette:

If you look at a picture of the forelegs of a praying mantis, it looks almost exactly the same as the forelegs on this specimen, right down to the spines on the middle segment of the leg that get progressively longer as they approach the elbow. The body looks a lot like a mantis too, with the four long walking legs projecting from the center of the body, and the elongated abdomen. And yet, when we look at the head it is obviously not a mantis, because the thread-legged bug has a small head and piercing/sucking mouthparts that firmly identify it as a “true bug”, order Hemiptera, and not as a member of the order that mantises are in (Dictyoptera). It also holds its body horizontal, rather than having the head and thorax held upright the way mantises do. Still, this is one of the most startling examples of convergent evolution I’ve seen. This is just a very good body plan for an ambush predator, and two almost completely unrelated groups of insects happened to converge on it.

So, anyway, thread-legged bugs are a subfamily of the Reduviidae, or assassin bugs[2], and this particular one looks like it is in the genus Barce. There doesn’t seem to be a lot of information about this genus specifically on BugGuide, probably because they are, as I mentioned, kind of hard to spot.

An interesting thing with this one is that, while I was photographing it, it didn’t stay still. It slowly swayed from side to side, back and forth, back and forth . . . I think it does that so that it can triangulate on its prey. The eyes are pretty close together, so its depth perception probably isn’t that great if it stays still. But, by swaying, it can watch the parallax shift and be able to tell when something is in striking range.

If you look up thread-legged bugs with Google, most of the sites talk about the large species Emesaya brevipennis, which can be a couple of inches long and often gets mistaken for a walking-stick insect. One thing that I saw mentioned is that, because thread-legged bugs are so light and their legs have such a delicate touch, they can walk onto spider webs and steal food from the spiders without being noticed. They evidently do this most of the time to funnel-web-weaver spiders, whose webs are flat, quite dense and not all that sticky. When the funnel-web-weaver spiders come out again next summer, I should go out and look for this.

[1] I was actually checking over the other small arthropods that had fallen out of the wood at the time to see if there was anything interesting. The others were all woodlice, and I already had pictures of them.

[2] The typical assassin bug is a chunkier, broader-bodied insect, and the larger ones are reported to be able to give a nasty stabbing bite if they are abused. Some of the assassin bugs even intentionally go after human blood (see kissing bugs). The thing is, some of the assassin bugs look quite a lot like other, completely harmless bugs that live on plant juices, so you might not know which kind it is until much too late. So far, I don’t think I’ve found any of the bigger assassin bugs, although I’ve got some pictures coming up[3] later of some of the plant-sucking bugs that could be mistaken for assassin bugs. At least, I think they are plant-sucking bugs. They didn’t bite me, in any case.

[3] Update on the arthropod count so far:83 distinct species have been photographed, and at least 80% are things that either got into the house briefly, or that live in the house full-time. And I’m still at the point where, when I find an insect, more often than not it is one that I haven’t photographed yet.

3 Responses
  1. nadja permalink
    July 8, 2008

    i found this creature is some tall grass he or she is very small less then an inch….
    he has long antenna and long slender legs….its dull 2

  2. October 19, 2008

    These guys are actually supremely effective spider predators. Their herky-jerky progress apparently sends sufficiently random vibrations to the spider whose web has been invaded that no alarm is raised. Some of the smaller forms (perhaps all) have a hook-like notch on the rear tibia which allows them to “play” or “pluck” the web to determine where the spider sits. A careful approach, a back-and-forth feeling out of the quarry aided by oh-so-delicate antennal taps, and- – bang! The immature instars can be found feeding, sometimes together, from spider egg sacs.

  3. kookimebux permalink
    February 1, 2009

    Hello. And Bye. 🙂

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