What are Gnats? Are Gnats Flies?

I received this question from my learning community on Facebook called the SciHive! We deep dive into topics and I get some really insightful questions! And I even love these kinds of questions that really show the dichotomy between colloquial language and scientific language. Because while it seems like a simple answer… it gets complicated quickly.

Heather asks, “My husband and I have a question. Are gnats different from flies? If so, what are the identifiers.”

So the short of it: Yes! Gnats are flies!
The complicated part is: but “gnat” doesn’t really have a taxonomic bearing so several families of small flies can be considered “gnats”. So pinpointing an identification of what is a “gnat” is not very easy.

So let’s get into it

Gnats *are* flies – but the grouping of flies has two suborders

Diptera: the order of flies are characterized by the presence of only two wings and a modified hind wing into a club shaped structure called a haltere. This haltere is used for stabilization and balance. 

So if your critter has two wings and halteres, it’s a fly! (there are some weird, parasitic, wingless flies, but we’re not going to get into those today.)

an orange fly with red striped eyes. below the transparent black striped wings you can see a small club structure - the haltere.

So we know that gnats are flies, and we can identify flies based on their two wings and halteres.  In the picture to the right (PC Nancy Miorelli), you can see the haltere (the little club sticking out) under the wing. 

But there’s two suborders of flies … so let’s get into them. 

The Thread-Horns and Branched-Horns

You’re probably familiar with house flies, or maybe the shiny bottle flies. Those are in the suborder of Brachycera – the branched-horns. This suborder name refers to their antennal structure. There’s kind of a bulb at the base and either a feather structure or a hair sticking out. While this group has a lot of characteristic flies that you’re likely familiar with – this is not where our gnats lie.

First step to identification – is determine what it’s not.

And gnats are not in the suborder Brachycera because they don’t have branched antennae.

a fly head highlighting the branched antennae. A bulb shape with a thin hair sticking out covered in smaller finer hairs.
Brachycera antennae.
PC: Nancy Miorelli

So now we get into the thread-horns which is where the gnats sit. The Nematocera suborder includes such characters like crane flies and mosquitoes and this midge pictured to the left (PC Nancy Miorelli).  Unlike their branch-horned counterparts, these Nematocerans have thread like antennae. Sometimes they’re fluffy – but they definitely don’t have an odd morphology like the Brachycerans.

Gnats are generally just considered to be small Nematocerans. Many have a swarming behavior but not all.

There’s a few families where their members are considered to be gnats. Identification of them really tends to come down to wing venation and other small morphological features like leg length, presence or absence of simple eyes, shape of the compound eyes, and hairs on the body. Some families of flies that can be considered gnats include the fugus gnats (Mycetophilidae), dark winged fungus gnats (Sciaridae) black flies (buffalo gnats Simuliidae), gall midges (Cecidomyiidae), no-see-ums (Ceratopogonidae), sand flies (Subfamily: Phlebotominae) and other small midges.

TL;DR

Gnats is a colloquial term for “small fly”. They belong to a suborder of flies called Nematocera which are characterized by their small antennae. Because there are several families of flies that can be considered gnats, there are not any good characteristics to identify them as a whole group. Each family has their own unique set of characteristics relying on subtle morphology.

Reference Section

CA Triplehorn, NF Johnson, DJ Borror 2005. Borror and DeLong’s Introduction to the Study of Insects.

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