(work in progress)
Whether the eyes are hairy (covered in ommatricthia) or not (bare) is a very important and frequently encountered feature when identifying tachinids. The keys can go into great detail, describing whether the hairs are as long or longer than 3 eye-facets or trying to relate to how dense the hairs are. But in general the feature should be very obvious in most specimens … so if you see lots of hairs all over the eyes then it is hairy and if you can’t see any – or it is just dusty – or you get a few tiny hairs spaced widely around the eye then it is bare. Saying that, you can have problems when specimens are old and tatty and this is made worse if you are examining material that has come out of liquid, such as from malaise traps and pan-traps.
Be careful to orient the specimen so that you see the edge of the eye against a dark background … and be very careful not to get confused by the hairs on the back of the head, which from the wrong angle can look like they are along the edge of the eye! Here are some examples, first with and then without hairs:
If your specimen has come out of a liquid, such as alcohol, then be aware that some hairs might have been rubbed off as they sloshed about with the other trapped insects. In this case focus on the base of the eye because I have found this to be the last place that hairs remain after being thoroughly washed. The exposed front and top of the eye loose their hairs first.
[insert a photo of a rough specimen with eye hairs at the bottom]