On finding a black swan

Today’s post is a short adventure in photo identification.

Earlier this week, I had the chance to survey about sixty acres of orchard in the Wenatchee Valley for native bees.  (I’ve spent a fair amount of time on the valley slopes, but this was my first substantial survey on the valley floor.)  I haven’t really processed all that I learned: I’ll probably write a longer post when I do.

The short version is that I didn’t find anything.  If the orchardist didn’t have rented honeybees (which he did), there would be no crop at all this year.  That was not what I expected, and so I was motivated to photograph anything that looked even slightly out of the ordinary.

One of the things that caught my eye was a layer of dead bees surrounding an irrigation drip.  I don’t know what killed them: there were half a dozen similar drips, all of which were clearly attractive to the bees, but only the one was dangerous.  I noted it, recorded it so I could refer it to the orchardist for further investigation, and moved on.

image

When I got home, I took a closer look at the photographs, and saw that one of them had captured an (unfortunately dead) bee that looked very different from the rest.  That caught my attention: I had after all done the survey in the hope of finding native bees, and the possibility that I might have missed one was better than the (likely) alternative, which is that I didn’t.

Looking at the photo, a few things immediately stand out:

  • Its entire body is black, with sparse, pale hair.  That is very much unlike the honeybees that surrounded it: there is such a thing as a “black honeybee”, but the name generally means “… when compared to an Italian honeybee.”  Most “black” honeybees actually have quite a bit of yellow and brown.
  • The abdomen appears to be very short and rounded, which is also unusual for a honeybee.
  • The body is shorter than the honeybees around it, but otherwise not much smaller.  (That rules out most of the most likely candidates: it’s much too big to be an orchard mason bee, for instance.)
  • The wing shape has a pronounced point on the trailing edge.  Honeybee wings are rounded.
  • The hind legs are quite broad, which suggests the presence of a corbiculum: a wide spot on the leg, adapted for carrying pollen.  (Among other things, that would mean that the bee is a female.)

At this point, everything except the corbiculum suggested a megachilid bee, that is, a relative of the mason bee.  (The Megachilidae store pollen below the abdomen, rather than on the legs.)  The next most likely candidate was Andrena – there are some large Andrenidae in the area – but the shape of the head is not quite right.  Overall, it didn’t add up.

Then I took a closer look at the wing, and saw that I had missed something important.  Like all of the Hymenoptera, bees actually have two pairs of wings: a pair of large wings in front, and a smaller pair behind.  When the bee is alive, the edges of the wings are fastened together into a single surface on each side, but when they die the wings can come apart.  That happened here: the posterior wing has come loose, and is visible near the leading edge of the anterior wing.  When I traced it back, the result looked a lot more like a honeybee.  In particular, the point on the trailing edge of the wing went away.

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Finally, I got the bright idea to copy the wing of the bee and put it next to one of the honeybees nearby, so I could make a direct comparison.  As you can see, they are identical.  (The effect is even stronger if you superimpose one over the other and play with the transparency slider, but that is beyond my current animation skills to show.)  This is the anterior wing of a honeybee.

After that, everything else fell into place.  The leg looks like it has a corbiculum because it does: all honeybee workers do.  The abdomen looks as short as it does because of perspective: the bee is a bit curled, and the abdomen is pointing towards the ground.  Honeybees also extend their abdomen as they drink water or nectar, which means that this one probably died with a relatively empty crop.  (That makes me wonder even more about what killed her, but that’s a worry for another day.)

The black body is genuinely unusual.  There aren’t many truly black honeybees in this part of the world.  Until recently, though, there was at least one.

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