E. tenax, the Drone Fly. That species has two bands of hair on each eye - can you make that out here on the eye that's not in shadow? Or am I seeing things?

The original post is almost full-size, but I've added a full-size closeup of the eyes, and tried to remove a little bit of shadow. Unfortunately, that's the best I can do for this pic. Do you see what you're looking for?

Thanks, Beatriz. How can you tell? Are those big eyes? Is there an image of a same-species female for comparison? I just noticed that, unlike this one, the other fly #21487 has a large space between the eyes. They're laid out differently. Can that be simply a gender difference? I suppose it can be in many types of insects.

Among Syrphid flies, the males have big eyes that meet in the middle (they need the big eyes to find females) and the females have smaller eyes. Visit the Syrphid pages and soon you will be telling the sexes apart like a pro.
In other species it is the antennae or a number of other features, such as the presence of pollen baskets in female bees, or ovipositor in some female wasps. This is why insects are so much fun, you are constantly learning new and fascinating things.