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Three keys are given here: the first to sub-families, the second to genera and the third to species. These are based on characteristics shown by workers and, in those species which lack them, queens; males help a great deal but for simplicity have been left out. To identify ants as far as the species is difficult; indeed, experts are often not in agreement about some very similar forms. Nevertheless the four main genera, Myrmica, Leptothorax, Lasius and Formica, have been here divided into their most common and easily identified species. The key is not based solely on structural features but includes in a few places reference to habitat, nest site and shape and other aspects of natural history. A low-powered stereoscopic microscope is best for assessing many characteristics but a hand lens may sometimes be adequate. Many distinctions are comparative, e.g., hairy or not hairy, and it is obvious that in these cases experience and reference to a reliable, modern collection is essential.

Key to sub-families

1	Waist of 2 small segments (fig. 1)	Myrmicinae
—	Waist of 1 segment	2
2	Gaster constricted between segments 1 and 2, with a well-developed sting	Ponerinae
—	No constriction, no sting	3
3	Five segments of the gaster visible from above, a circular orifice for ejecting venom fringed by guide hairs (fig. 7)	Formicinae
—	Four segments of gaster visible from above, no circular orifice but a slit through which viscous defensive fluid is passed, no hairs	Dolichoderinae

Key to genera

A	Ponerinae
	Only one certain British species, Ponera coarcta, exists; it is a slow, timid ant with small colonies
B	Myrmicinae
1	Queens black, with shallow, longitudinal trough on gaster, no workers; in Tetramorium caespitum nests	Anergates
—	Otherwise	2
2	No spines on the rear of the mesosoma, antennae of 10 segments, the last 2 forming a club	Solenopsis
—	Mesosoma toothed or spined at the rear	3
3	Second waist segment with a forwards-directed spine underneath	4
—	No such spine	5
4	Antennae 11-segmented, tibial spurs on legs 2 and 3, in Formica rufa nests	Formicoxenus
—	Antennae 12-segmented, no tibial spurs on legs 2 and 3, no workers; in Myrmica nests	Sifolinia
5	Jaw sickle-shaped; in nests of Tetramorium caespitum	Strongylognathus
—	Jaw with teeth	6
6	First waist segment elongate, eyes minute	Stenamma
—	First waist segment not elongate, eyes normal	7
7	First waist segment oblong from side with 2 tubercles above	Myrmecina
—	Segment not so	8
8	Front of first thoracic segment squared dorsally, small black ants	Tetramorium
—	Front of first thoracic segment rounded dorsally, reddish-brown ants	9
9	Last 3 antennal segments shorter than the rest of the funiculus	Myrmica
—	Last 3 antennal segments about as long as the rest of the funiculus	Leptothorax
C	Formicinae
I	Ocelli large and distinct, legs long, segments 2–5 of the funiculus longer than all the rest together	Formica
2	Ocelli very small and indistinct, legs short, segments 2–5 of the funiculus shorter than all the rest together	Lasius
D	Dolichoderinae
	Only one British species, Tapinoma erraticum, exists; it has small, black, agile workers which run about with their gaster raised

Key to species

A	Myrmica
O	Scape of antenna near point of attachment to head bent gradually and smoothly without ridges; head relatively shiny, especially the frontal area (fig. 6c)	1
—	Scape bent sharply through a right angle, with or without ridges, head dull, matt (fig. 6d)	2
1	Epinotal spines long in relation to body size; either workers large, queens larger than workers, fewer than 10 in a colony (macrogyna)
	or
	queens same size as workers, more than 10 in a colony (microgyna)	ruginodis
—	Epinotal spines short in relation to body size; workers small, queens much larger, up to 100 in a colony	rubra
2	Antennal scape without ridge or teeth; frontal area with marked striations; a dark ant in moorland	sulcinodis
—	Antennal scape with ridges or teeth at the bend	3
3	Scape with very characteristic transverse ridge or plate at bend, almost tooth-like from some aspects; a small, dark species	lobicornis
—	Scape with lateral ridge at bend, reddish-brown	scabrinodis
	(fig. 6d) and	sabuleti

FIG. 6. Worker of Myrmica rubra: a. head, b. foreleg, c. scape of antenna, d. Myrmica scabrinodis: scape of antenna. c. and d. are viewed from behind. Hairs are abundant on the head which is strongly corrugated.

	scabrinodis is a smaller ant and has a less pronounced lateral ridge than sabuleti; it also has more queens in each colony
B	Leptothorax
O	Antennae with 11 segments; a relatively large species	acervorum
—	Antennae with 12 segments; a relatively small species	1
I	Club of funiculus no darker than the rest of the antenna; a distinct dorsal groove or depression across the middle of the mesosoma; nests in tree stumps and wood	nylanderi
—	Club of funiculus darker than the rest of the antenna; no transverse groove on the mesosoma; rare species	tuberum
	and	interruptus

FIG. 7. Worker of Lasius niger: a. head; b. scale on petiole from behind; c. side view of tail segments to show ring of hairs around the circular orifice. The whole body is covered with a light pubescence and there are short, erect hairs on the scape of the antenna but none of these have been shown.

C	Lasius
O	Colour jet black, shiny, head heart-shaped	fuliginosus
—	Colour otherwise, head normal	1
1	Colour brown to dull black	2
—	Colour yellow	4
2	Scape of antenna and tibia of leg with short, upright hairs; body dark, almost black but hairy and matt	niger
—	No such hairs; body browner, less hairy	3
3	Frontal area indistinct; smaller, uniformly coloured, individuals living in open, sunny places	alienus
—	Frontal area distinct; larger individuals with gaster and head darker than the thorax; living in old trees	brunneus
4	Scape of antenna and tibia of leg with short, upright hairs	umbratus
	and	rabaudi
—	No such hairs	5
5	Hairs on top of gaster short, scale tapered above	mixtus
—	Hairs on top of gaster long, scale broad and low, not tapered above, no cheek hairs in front view; makes soil mounds in grassland	flavus
	Three of the yellow species, umbratus, rabaudi and mixtus, are very variable and intergrade in the worker caste.
D	Formica
O	Clypeus with central notch in lower margin; colour usually deep red	sanguinea
—	Clypeus without notch; colour reddish-brown to black	I
1	Back of head and top of scale notched	exsecta
—	Not so	2
2	Thorax reddish-brown, paler than head and gaster	3
—	Body black all over	6
3	Eyes with small hairs and back of head with prominent long hairs; wood ants making mound nests of vegetation near trees or in open moorland in northern Britain	4
—	Eyes and back of head bare	5
4	Thorax with many fine, long hairs	lugubris
—	Thorax with fewer, shorter hairs	aquilonia
5	Frontal area shiny, maxillary palp short and hairy; southern wood ants making large mound nests in open forest	rufa
—	Frontal area dull; individuals smaller, making very small mound nests or excavations in open, heathy places	cunicularia
6	Body shiny, black; building small vegetation mounds in wet heath and bog	transkaucasica
—	Body dull, black; excavating nests in drier places	lemani
	and	fusca