Tuesday, August 28, 2012

I just want to bee ... Mistaken for a wasp by wasps

Spilomyia sayi (Goot, 1964), not a wasp, just a good mimic
Every summer from late July through August an unusual flower fly (Syrphidae) shows up in the Home Bug Garden. Many, if not most, flower flies are brightly and contrastingly coloured more or less like wasps or clothed in furry hairs like bees. But some look more like the stingers than others. That is true of this fly, Spilomyia sayi (Goot, 1964) (known in earlier works by the preoccupied name Spilomyia or Paragus quadrifasciatus Say,1824). 
Two syrphids of varying waspishness - both in looks and behaviour
As well as being coloured like many a vespine wasp, its antennae are longer and more wasp-like than most syrphids, its wings waspishly dark, and its demeanour as it struts across the heads of yarrow and goldenrod as recklessly fearless as any hornet. Most flower flies dodge quickly away when approached, but not Spilomyia sayi. In fact, this fly is easy to catch by hand - in which case it emits a loud and very bee-like buzz.
Flies have died from time to time, and birds have eaten them, but how about wasps?
But who is Spilomyia trying to fool? Well, it is pretty good at fooling people, but the usual argument for this kind of Batesian Mimicry is bird brains. I suppose birds do eat flower flies, from time to time, but how often does a bird swoop down on a goldenrod or yarrow in a field? I can't say that I have ever seen this happen, but I have seen many a fly scarfed up from such flowers by patrolling hornets. Could it be that these flies just want to be left alone by their fellow insects as they gather some nourishment on flowers that are often covered in wasps (many of which are stingless males)? I suspect that predatory wasps and hornets have exerted more evolutionary pressure on these flies than any bird. Alas, behavioural ecologists seem obsessed with birds, so I am not sanguine about seeing any studies that test this idea.
Blackjacket (Vespula consobrina) - a likely model for and predator on our local Spilomyia


  1. There was a study done back in the 1980s that looked at the phenology of wasps and bees, the mimics, and the fledged birds. As I recall, the recently fledged birds were more likely to encounter the models than the mimics, so that later in the season, the mimics would end up being confused with the Hymenoptera. I that the author was Waldbauer.

    1. Could be, but here the wasps and mimics aren't common until the birds are fledging (I don't think any passerines raise broods after July here), so there's not much time to learn or hide. Also, at least when they are hanging out on composites in open areas, birds do not seem to be much of a threat. In the studies I've read (mostly with caged birds) the birds pounce from a perch. I suppose swallows may be doing some selecting in fields, but the other birds are hunting lower down. Wasps, spiders, ambush bugs, robber flies, etc seem to be the main predators of flies visiting composites in open areas.

  2. I photographed what appears to be the same syrphid sp. (http://bugguide.net/node/view/695966)down in the river valley last Sunday. Its behavior was interesting too, as it was constantly flicking its wings as it roamed over the flowers. Could this behaviour perhaps be another way to deter patrolling hornets?

    1. Hi Adrian,

      The wing-flicking behaviour - a left-right waving of the wings - is interesting, but I'm not sure why they do it. I can't remember seeing any wasps doing it, so it doesn't seem to be mimicry. Maybe it's a "I'm bad, leave me alone' swagger.

      The larvae of Spilomyia sayi occur in tree holes and rotting wood. I'm wondering if they like the birch bolts I spread around the garden after the drought? (Remember the drought - seems a long, wet time ago.)



  3. Wow x 2.

    1st = that is a VERY impressive wasp mimic. Jeepers.
    2nd = your catching it by hand = a VERY good time to have strong invert. ID skills. =)

    I like your theory. I suspect trolling out the old " 'cause then birds won't want to eat it" chestnut is a reflex for many, and I suspect few of those who posit that theory (looked it up and "positer" is not a word, darn it!) know what you know about who actually eats those flies. I LOVE evolutionary/natural selection theory and the places it can take you.

    1. A lot of 'mimics' seem very pro forma and one tends to wonder if their mimicry isn't more in the eyes of the (human) beholder than potential predator. But this is one of those multidimensional mimics that are beyond chance or cultural bias.

      A little training, though, and the fly jumps out - only one pair of wings, too large eyes, wrong antennae. Telling a male from a female wasp is more difficult. Grabbing a male wasp to impress your audience is a better test of id skills (but I'm usually too chicken to try it when sober).

      But I think the most rigorous test of taxonomic skills is eating wild mushrooms. If you are right, you have a good meal (at least with enough butter and garlic). If you are wrong, you may or may not have a good meal (allegedly deadly amanitas taste good), but you may live just long enough to regret your error.

  4. I'm never sure what is a true wasp or not, and it looks like there is a good reason reading this post. Any guesses if this is a wasp or a look-alike of some kind?


  5. Hi Middle Earth - looks like a look-alike to me.

    I think you have a pair of mating Clearwing Moths (Family Sesiidae) - most of which mimic one wasp or another.

    A good guess would be the American Hornet Moth aka Poplar Clearwing Borer, Poplar Crown Borer: Sesia tibialis (Harris, 1839). The larvae burrow through the cambium of aspen, poplar, green ash, lilac and other hardwoods.

    Pretty cool mimic, because yellowjackets even mate that way - with the male dangling from the female (although male yellowjackets are more similar in size to females).

    See: http://bugguide.net/node/view/160324

    Also: http://www.entomology.museums.ualberta.ca/searching_species_details.php?fsn=Sesia&sb=1&r=2&o=1&c=2&s=6200&sn=Sesia+tibiale

    NB - species name seems to be misspelled - tibialis in my books

    1. Re the species name - 'tibiale' was Harris' original spelling (Trochilium tibiale Harris, 1839), but apparently his Latin should have been 'tibialis'. Some people follow the original orthography and some the ICZN suggested correction. An interesting piece of nomenclatorial anarchy!