Nut Leaf Weevils

2009 January 31

Sometimes called “Sweet Clover Weevils”, although they don’t seem to be all that picky about which clover they eat.

Every spring, and on through the first part of the summer, we get these little brown beetles all over the place. They tend to hang out on the sides of buildings, and of course get into the house a lot. When touched, they tuck in their legs and antennae and play dead, obviously depending on their very stiff wing covers and heavy exoskeleton to protect them.

The one above was photographed at the end of May of 2008, this next one was from almost a year earlier, July of 2007:

They are some of the Entiminae, the Broad-Nosed Weevils (as opposed to the weevils with the elongated snouts). Back when I first created this posting in 2009, I thought these were Clover Weevils, Sitona hispidulus. However, as of now (May 2015), it has been pointed out that I made a mistake, and that a much better match would be the Nut Leaf Weevil, Strophosoma melanogrammum.

Given that I’ve already said that they are very common, is it any surprise that the Nut Leaf Weevils are an invasive species? No, I thought not. They are from Europe and Asia originally, and the adults eat . . . well, see, I’m reading contradictory things here. The BugGuide link says that they eat the leaves of varioius broad-leaved trees. But, this paper says that they are a pest of coniferous trees, eating the needles. Maybe they eat both? Well, the ones we have around here are certainly plentiful enough, so they probably eat practically everything.

According to David V. Alford in his book “Pests of Fruit Crops” (2nd Edition), they don’t actually have functional wings, and the ones that have gotten transplanted to North America are entirely females who reproduce parthenogenically (there are males back in the Old Country, but they never made the trip over). The larvae eat plant roots, primarily of certain grasses and docks. They emerge as adults in the fall, and then hibernate until spring when they come back out to feed on the fresh greenery and lay eggs.

Looking back on this post from 2015, one thing I’ve noticed is that after their initial population boom, there was something of a crash. While they are still around, they are significantly less common than they once were. They fact that they are parthenogenic means that they are all basically genetically identical, so it is quite likely that some disease that they are susceptible to ran through the local population and knocked them back.

While we’ve got this little guy here, I just wanted to mention something about beetle anatomy. When you read about the fundamental traits of insects, one of the things that is mentioned is that they have three distinct body segments – head, thorax, and abdomen. The head is obvious, the thorax is the segment that has the wings and legs on it, and the abdomen is the segment where most of the digestive tract and other vital organs are located. In most insects, there are clear constrictions between each of these segments, particularly in things like wasps and ants.

Beetles are a bit different, though: if you look at the underside of this fellow, we can see that the legs do not look like the are all together on one segment. We have the head, then a segment with two legs on it, then after the constriction there is a much larger segment that has the other four legs on it that runs all the way to the end of the body.

That last segment is not all abdomen. The part of it that has the legs on it is the last 2/3 of the thorax, which blends smoothly into the abdomen, and the whole thing is tightly packaged under the wing covers. What has happened with beetles is this: like most other insects, beetles had four wings. The front pair of insect wings are on the middle third of the thorax, and the hind pair of wings are on the last third of the thorax. When beetles developed their hard, armored forewings, they then started compacting their abdomen together with their thorax so that everything would fit under the armor. Ultimately, the abdomen and thorax practically merged together into one segment. But, since the front third of the thorax was ahead of the first pair of wings, it was never under the armor. So, that front third of the thorax developed its own armor plate (the pronotum), and basically turned into a separate segment so that the beetle could retain a little bit of flexibility. As a result, we ended up with beetles looking like they have a thorax with just two legs.

Why do I bring this up? Well, some years ago I was goofing around trying to draw beetles from memory[1], and they kept coming out all wrong, because I was assuming that all the legs went onto that middle segment. It wasn’t until I looked at a beetle closely to figure out why it was all coming out wrong that I realized that they’d seriously messed with the whole “head, thorax, abdomen” body plan. So, I thought I’d point it out just in case somebody else was having the same problem.

[1] See, there is this short-short science fiction story that I currently don’t remember the title of, or the name of the author,[2] “The Figure”, by Edward Grendon, and the plot has stuck in my mind. In the story, this small team of researchers has developed a time machine that can reach into the future, and pull back objects. So they operate it, and bring back something that causes them to lose all hope for the future. It is a statue, a piece of fine art, reaching upward in an aspiring manner, and obviously intended to be inspirational. The only problem is, it is a statue of a beetle! Anyway, I thought it would be amusing to actually sketch out what this statue might look like, possibly with the aim of using my metal-casting kit to cast it in pewter. Unfortunately, all my sketches came out all wrong because I didn’t understand about beetle segmentation and how the legs were really attached. Ultimately, I ended up making pewter castings of the asteroid Eros instead.

[2] Found it! It’s in Fifty Short Science Fiction Tales, edited by Groff Conklin and Isaac Asimov. And yes, I remembered correctly. The statue is, in fact, of a beetle (not a cockroach, although roaches are mentioned in the story).

6 Responses
  1. January 31, 2009

    I remember that story, though the name escapes me too. I thought it was fantastic. Wasn’t it specifically a cockroach statue though? Maybe I’m remembering that bit wrong.

  2. February 2, 2009

    Consistency in beetle design is the hobgoblin of little beetle minds.

  3. February 2, 2009

    It’s ok to have a comment.

  4. April 14, 2011

    Great post!

  5. Jason permalink
    May 6, 2015

    Although you pick-out the notable features of Sitona hispidulus correctly, the species photographically-depicted here featuring similar equivalents is more likely of the genus Strophosoma.

    The divisions of head-thorax-abdomen; the thorax is the ‘small’ central portion, delimited by the head and elytra. You anotate the thorax to include the mid-coxae, which in reality only possesses the fore coxae (‘leg bases’).

    Nice article, an useful-quality images.

  6. May 8, 2015

    Thanks, Jason. I’ve corrected the identification.

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