A good view of all 5 eyes this fly shares with typical insects. The two compound eyes on the sides of the head are obvious.They each contain very many lenses, each with its own detectors. The 3 simple eyes ("ocelli") are in a triangle at the top of the head. They look like spider's eyes. Each has a lens and a retina. The lens cannot focus on the retina -- they are used to detect light, not images. They are more light-sensitive than the compound eyes, and have faster nerves to the brain. One faces forward, and one to each side. It is thought these give rapid feedback to aid in flight. (I think they may also serve to rapidly warn of an approaching flyswatter (?)) Head of Female Syrphid Fly (Taxomerus marginatus) Pierre Martineau writes: "The three spots are called ocelli (plural of ocellus). You are right to say that they look like spider eyes, as they are indeed spider-like in structure: they are simple. Insects have typically 5 eyes:
- the 2 familiar compound eyes
- 3 simple eyes (ocelli) located in a triangle like we see on your fly. See for example
<http://en.wikipedia.org/wiki/Simple_eye_in_invertebrates>
for more details on simple eyes. However, many insect taxa do not have simple eyes, or have only 2, sometimes very hard to see.

"By contrast, typical spiders have 8 eyes (or 6 in many groups), all simple, NEVER compound." Also note the pollen on the head hairs behind the left compound eye.