NO.2 Eye bare or pubescent?

This is the second article where I'll introduce tips for identifying tachinids. Today's theme is "Eye bare or pubescent?"

Here, "eye" refers to the "compound eye," not an ocellus.

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NO.2

Eye (nearly) bare or pubescent ?

Importance：　★★★★★

　　Difficulty：　Easy

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This topic is quite similar to NO.1. Today what we examine is whether there's a hair on compound eyes.

You might find it strange or bizzare that eyes can have hairs, but in tachinids, it's not unusual at all.

Let's take a look at some actual specimens.

⚫︎ Case : "Eye (nearly) bare"

At first glance the eyes seem to have no hairs...

But let's observe them from upper view and magnify them using a microscope.

You'll find a few very short hairs.

However, these hairs are very insignificant. In this case, we classify it as "eye bare."

Now, let's move on to the next example.

If you enlarge the image, you'll see more visible and denser short hairs than in previous one. But, they're after all still trivial.

When short hairs are sparsely distributed across the entire eye, we often describe it as "Eye sparsely haired" or "Eye with scattered hairs."

However, this is still considered "eye nearly bare" and falls under the same category as "eye bare."

These short hairs are called ommatrichia.

It is best to observe ommatrichiae against a black background as they are often difficult to see against a white one. Even with a black background, it can sometimes be hard to see depending on the angle and lighting.

Now, similar to the NO.1, the question arises: How long must the hairs be for us to classify an eye as "pubescent"? Where is the boundary between "Eye (nearly) bare" and "Eye pubescent"?

Conventionally, we often divide them by the following criteria.

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Length of eye hairs ≦ Length of two facets ・・・ eye (nearly) bare

Length of eye hairs ＞Length of three facets ・・・ eye pubescent

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A facet is a tiny individual unit that forms a compound eye.

⚫︎ Case : "Eye pubescent"

The hairs are long enough for us to recognize them easily even against a white background. When long hairs are densely distributed across the entire eye, we often describe it as "Eye densely haired."

Genus Senometopia is typically characterized by such densely haired eyes.

Hairs are also obvious in the next species.

The hairs around compound eyes look whitish and smokey when seen from a dorsal view.

The hairs, if pubescent, are often easy to recognize and are frequently visible even in ecological photographs taken in the field.

How was today's article?

Today I introduced the difference between "eye (nearly) bare" and "eye pubescent." In Tachinidae, the former is more common than the latter.

According to Mosch Web, among the 423 genera that inhabit the Palaearctic region, 309 genera can have (nearly) bare eyes, while 148 genera can have pubescent eyes (including overlapping cases).

However, tachinidae with pubescent eyes are also common, and the trait "eye (nearly) bare or eye pubescent" can be said as one of the most important characteristics for identifying Tachinidae.

For example, these two individuals above are very similar in appearance, but they belong to different genera. There are some differences between the genus Vibrissina and Compsilura, but the easiest one would be the presence of hairs on the eyes. The former has "nearly bare" eyes, and the latter has "pubescent" eyes.

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