How to tell a queen ant from a big worker

Every August someone posts a photo of an enormous ant they scooped off the patio, certain they have found a queen, and every August somebody else posts a photo of a genuinely small queen that got walked past because she looked like nothing much. Both mistakes come from the same habit: judging by length.

Length is the first thing anyone reaches for, because it's the easiest thing to see. It's also the least reliable tell in the whole business, and once you know why, the better one is right there in front of you.

Ants build big bodies for more than one reason, and only one of them is being a queen.

Some species run polymorphic worker castes, where the biggest workers are built as soldiers or specialist foragers rather than as anything to do with reproduction. Nothing in Britain gets as dramatic about it as a leafcutter or a big-headed ant elsewhere, but even here the spread within a nest of ordinary workers can be wide enough to fool you. A large Formica fusca worker runs to about 7 mm. A Myrmica rubra queen, by contrast, is only around 6.5 to 7.5 mm. Put those two ants side by side and the "big" one is the worker.

It runs the other way too. Some queens are barely larger than their own workers at all. The thief ant, Solenopsis fugax, has queens of 6.0 to 6.5 mm next to workers of 1.5 to 3.0 mm, a clear gap, but her males are 4.0 to 4.8 mm, closer to the workers than to her. Anyone hunting for "the biggest one in the group" on a thief-ant nest would do fine. Anyone applying the same instinct across species, where the gaps aren't consistent, will get it wrong on a regular basis.

Length isn't useless so much as species-specific, and most people trying to identify an ant on their patio don't know the species yet, which is exactly the problem it can't solve.

Look at the mid-section instead, the part behind the head and in front of the narrow waist. This is the thorax, and its shape is set by what it used to carry rather than by overall body length, which is why it works across species in a way raw size doesn't.

A queen's thorax is deep and boxy, humped noticeably above the line of her head and abdomen when you look at her from the side. That bulk is architectural, not fat: it's built to house the muscle that powered her wings during the flight. A worker of the same species has never had wings and never will, so her thorax stays slight and flat by comparison, barely rising above the rest of her body.

Once you've compared the two side by side, on the same species, the difference stops being subtle. It's the single most reliable structural tell there is, and it's the one that survives across the whole range of sizes above, from a 6.5 mm Myrmica queen to an 8 to 9 mm Lasius niger one.

A queen who has recently flown carries a second, even more specific tell: two small marks on the sides of the thorax, right where the wings used to attach.

After mating, a queen doesn't keep her wings. She reaches round and snaps them off close to the base, leaving a pair of small scars or stubs that are visible if you look closely, sometimes with a tiny sliver of wing still attached in the first hour or two. A worker has no such marks anywhere on her body, because she never had anything there to remove. If you can get a clear look at the sides of the thorax and see those two scars, you are looking at a mated queen, full stop, whatever her length happens to be.

This tell fades with time, though not from the body itself. As days pass the scars can be harder to spot without a hand lens, simply because you're no longer looking at a fresh specimen sitting still for you. Photograph her early if you want this one on record.

Here's a detail that trips people up if they compare a queen on day one against the same queen a few weeks into founding and conclude they must be different animals.

That flight muscle packed into her thorax isn't just sitting there once she lands. It gets broken down and used, the reserves inside it converted into the eggs and the feeding of her first larvae, since a fully claustral founding queen takes in nothing from outside while she does it. Wheeler and Buck documented exactly this kind of reserve depletion in founding queens in a 1996 paper, and the mechanism behind it, the flight muscle itself being histolysed after mating, was worked out earlier still in the fire ant Solenopsis invicta by Barker in 1979.

Practically, that means the boxy thorax is at its most obvious right after the flight, before she's drawn much down, and can look visibly slimmer weeks later once she's spent a chunk of what she came down with. None of that makes her any less a queen. It's the expected shape of the process, not a sign anything has gone wrong.

Males cause a different kind of confusion, because they've flown too and can turn up in the same place at the same time, and some of them are large enough to give a beginner pause.

The reliable differences are in the head. A male's head is noticeably small relative to his body, with comparatively weak jaws he has no real use for, and large compound eyes built for one job: spotting a female in a moving swarm. His thorax, unlike a worker's, does carry flight muscle, so it won't look as flat as hers does, but it tends to be slimmer and less boxy than a queen's overall, and his abdomen is narrower and less capable of the swelling a queen's develops once she's laying. A study of eight North American ant species found females weighing anywhere from 4 to 26 times what the males of the same species did, which is a genuinely large range but consistently in one direction: whatever the species, she outweighs him by a wide margin.

The other useful fact about a male is behavioural rather than anatomical: alone in a container, nothing will ever happen with him. He cannot found a colony under any circumstances, so if the only ant you have is one you now suspect is male, there's no husbandry question left to answer, only an identification one.

Get the picture before you get attached to an identification. From directly above first, for overall shape and colour, then a clean side view of the thorax, since that's where the whole argument in this post lives, then the head if you can manage it. Note where and when you found her too. Month and time of day narrow a species down faster than most photographs taken by a beginner do, since flight periods for common British species are well recorded even where exact size ranges are not.

None of this is a substitute for knowing the species, which decides everything from feeding to whether she can found alone in the first place. It's a starting filter, so that the ant in your hand is at least sorted correctly into queen, worker or male before you go looking for anything more specific. How to find a queen ant after a flight covers where they land in the first place, you caught a queen, now what covers the days right after, and fully claustral or not covers what she actually needs once you know what you're holding.

That's the same underlying idea behind the forecast: read the animal and the conditions properly rather than going on a first impression. Dealate scores flight odds by species from live weather, so the evening you go looking is one worth being out for. A reason to look up, not a guarantee.

British flight periods and species descriptions are from the species accounts of the Bees, Wasps and Ants Recording Society, the national recording scheme for British ants, including the size figures for Solenopsis fugax (Collingwood, 1979) and Formica fusca. The queen sizes for Myrmica rubra, Lasius niger and Lasius umbratus are the figures this site already uses in its founding-strategy coverage. The mechanism of post-mating flight-muscle depletion is from Wheeler & Buck (1996), Depletion of reserves in ant queens during claustral colony founding, Insectes Sociaux 43(3): 297 to 302, and from Barker (1979) in Experientia, who traced the underlying muscle histolysis in the fire ant Solenopsis invicta. The female-to-male weight range is from Helms et al. (2016), Predator foraging altitudes reveal the structure of aerial insect communities, Scientific Reports 6: 28670, an open-access study built on prey identified from Purple Martin nest deliveries in Oklahoma.