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Hornets vs Wasps

What is the difference between a wasp and a hornet? Are yellowjackets hornets, for example? A friend of mine asked me this and I had no answer!

Common or scientific meaning?
Formal naming does not recognize "wasps", "yellowjackets", and "hornets" but uses Latin (or Latinized) names to get around confusion from using imprecise language.

Informally I don't know how to distinguish a yellowjacket and a hornet. There is a conventional list of English names for species of Vespinae that mixes "hornet" and "yellowjacket" in the same genus. For example Dolichovespula maculata is the "bald-faced hornet" while D. arenaria is the "aerial yellowjacket".

In common use I would say wasp means Vespidae, yellowjacket means Vespinae, and hornet is a synonym of yellowjacket. Add ants and bees with the obvious meanings. The rest of Hymenoptera doesn't really come up in everyday conversation.

Hornet may be a synonym for yellowjacket...
but that is incorrect and many people do not realize it. True hornets have their own Genus (acutally, there are more than one) and are different from yellowjackets both in terms of nesting biology and appearance. For instance, we only have one species of true hornet in North America, which is Vespa crabro

> ... in terms of nesting biology and appearance.

Are there any more specific anatomical or diagnostic features used to separate wasps from hornets?

hornets are a type of wasp
I think you mean hornets vs. yellowjackets

My impression is that smaller black-and-yellow species (Vespula and some Dolichovespula) are called yellowjackets whereas certain larger species (Vespa and some Dolichovespula) with different coloration are called hornets. However my speciality is Apoidea, not Vespoidea, so I may be unaware of some relevant considerations

"Hornet" is a common name pertaining to a group diagnosed on the basis of superficial characters such as size and coloration. It does not refer to a monophyletic group, so don't expect an efficient technical diagnosis

Not completely baseless.
As I understand it, the word "hornet" originally referred to Vespa crabro, and was broadened to include other Vespa species, generally with a qualifier to designate which species.
As the English language spread to other continents, there were no Vespa species to take the name, so it was applied to various insects that resembled Vespa crabro in characteristics such as size and coloration (Vespa spp. have since been introduced, but the names were already in use).
This is the same process by which the name "robin" got applied in the US to a species that would probably be considered a blackbird if it were native to Europe, and by which the word "elk" was given in the US to a close relative of the Red Deer even though the European elk is what Americans would call a moose.
The usage would then seem to fall into the following categories:
  • In Europe and among those trying to enforce a more taxonomically-based usage, "hornet" is synonymous with "Vespa spp.". In that case, one could define it by using the diagnostic characteristics for the genus Vespa.
  • In overseas English-speaking countries like the US and Australia, certain species have been given the name with a qualifier, such as "Bald-faced Hornet" or "Australian Hornet". In those cases, they can be dealt with as limited exceptions to the hornet = Vespa rule above.
  • In careless American usage (I don't know about Canadian usage), "hornet" is often used to apply to any social wasp perceived as particularly big or aggressive. In this sense, there's very little taxonomic logic behind the term.
As you can see, in different places or different semantic registers, the word can be a taxonomically-rigorous or a taxonomically-nebulous term- you just have to determine the context.

To my simple way of seeing it
All are wasps - yellow jacket or hornet, they are all wasps.

That said, a wasp is much bigger overall, far more scary to look at and packs more punch in its sting; a hornet has a longer abdomen/stinger end when compared to a yellow jacket and has a sharp bite to boot. A yellow jacket is a wee bit smaller and more bee looking.

And as Chuck stated, its all about context. So at your next family picnic, when someone says they have a yellow jacket in their pop, toss the can. If there is a hornet in the hot-dogs, chuck that hot-dog it outta there quick like. And if there is a wasp in the porta potty, run like the dickens and head for a bush and keep head low.

Another context:

Yellow jacket -OWCH! (Rubs swelling welt).

Hornet - YOWZA!! (Hand on welt, seeking asprin).

Wasp - ARAHHAAA!!! ARRRHAHAHA!!!!EEEEEEEEEEE!!! (Limps away with swollen limbs and flesh missing).

All hornets are wasps, but
not all wasps are hornets. Hornets and Yellowjackets are closely related (same subfamily, but different genera). I'll leave it to others more knowledgeable to describe the anatomical and/or behavioral differences between the two.

EDIT: Scratch that bit about the different genera (gotta be more careful about the sources I'm consulting). Listen to John; he knows more. :)

EDIT #2: Now I'm hopelessly confused. I'm just going to have to let John and Vespula fight it out, I guess. :)

I am not a taxonomist, but if someone says to me "I study Hornets" it means they study Vespa. If another person says to me "I have hornets in my back yard" I would not know what to conclude (unless I am in BC, in which case I would conclude the person has some kind of Vespula or Dolichovespula...Although I have heard a rumour that there was a small introduction of Vespa to BC).
I tell people that I study social wasps, which means that I am studying Vespinae and Polistinae. If I had the opportunity I would study Stenogastrinae as well! I have seen the nests of the social sphecids, but have never encountered an adult.
There is really no good system for applying common names to these insects. Yellowjackets are polyphyletic and the Bald-Faced hornet is just a big Dolichovespula. "Wasp" can mean many Hymenoptera depending on the circumstances.