Why Do Flies Have Two Wings?

Yesterday I saw this very strange insect on one of my plants.  What in the world is it?  I couldn’t tell until it moved…

…oh.  Two flies and they are mating.

I’m not sure what kind of fly they are- identifying insects can be very difficult.  As I’ve written before, when I take a close look at flies I think they actually can look quite beautiful (once you get your mind past all of the disgusting stuff they do!).

I noticed something in this photo that I don’t remember seeing before.  See the little yellow glob each of these flies have between their thorax (middle body part) and abdomen (rear body part)?  It looks like a drop of syrup or something.  What is it?

Here’s another fly I photographed a couple of minutes earlier.  It has the same glob, but it also clearly has a much larger yellowish mass right under where its wings connect to the rest of its body.

Believe it or not these are body parts, not syrup or pollen!  The large mass under the wing is called a calypter according to this Amateur Entomologists’ Society website.  Experts actually use calypters to help identify specific flies.

What about the yellowish glob that looks like a drop of syrup?  Do you think it looks a bit like a drumstick or club?  According to Field Guide to Insects and Spiders of North America by Arthur V. Evans:

Flies are distinguished from most other insects by having two, rather than four, full wings.  The second pair of wings, called halteres, are reduced in size and clublike; they function as gyroscopic balancing organs during flight.

I can practically see the folktale in my head titled “Why the Fly Has Only Two Wings” (on the bookshelf next to “How the Tiger Got Its Stripes”).  Was the fly punished by the insect gods for going somewhere it shouldn’t have?  Or was it rewarded with two clubs to protect something?  Someone needs to write this!

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