of the abdomen; their other end passes as a thin filament right down to the genital buds just in front of the anus.
When the chemical signal for metamorphosis is given these buds grow more quickly and begin to join up and assume their adult shape. Even the sheaths spread out and form, at least in the legs and antennae, some of the basal segments. The wings elongate but never split. The ovary, in the case of a queen-forming larva, splits longitudinally into about eight parts which grow along the filament towards the genital buds where they meet the oviduct as it grows upwards. In the worker this is a narrow tube and a broader one in the queen.
All this takes place in the larval skin after the larva has stopped feeding and ejected its food residues through the hindgut which breaks down where it touches the midgut. The membrane enclosing all the residues not only makes this easier but probably prevents gut bacteria getting into the body and starting an infection. The larval head is not big enough to hold the new pupal head and so this is formed in the first body segment; only the tips of the pupal antennae lie in the larval head. The young pupa escapes through a split in the larval skin which starts dorsally just behind the head. Somehow or other it manages to push off the larval skin as far as the tip of its gaster without any help from workers. Simultaneously the petiole is formed by a contraction of the second and third abdominal segments. This together with the inward telescoping of the last body segments seems to reduce the body volume and create enough blood pressure to inflate and smooth out the surface just before it sets.
Only one more moult is needed before the adult is produced. This forms with much less change in shape inside the pupal skin. The wings flatten, elongate and fold up and the legs and antennae and petiole segments narrow a little more. Colour appears as a gradually increasing brownness in the general body but the eyes change from pink to black. The adult emerges with a very soft flexible skin and the wings, if any, inflate, spread out and set hard.
The caste differences in the female do not develop until very near the end of larval life. In workers the wing buds stop growing when they are quite small and all traces vanish by the time the pupa is formed; their ovary never thickens and splits into egg tubes but simply elongates to meet the oviduct. Caste differences in ants thus depend quite simply on whether buds grow or not; there is no degeneration and reorganization as happens in the honeybee. It seems likely that the growth of wings and ovaries is specially delayed during the development of females so that caste determination can be left to the last moment.
In males, which have only one caste, the wing grows earlier and the testis is split longitudinally very early in the larval stage. Late caste determination presumably gives a more sensitive response to social conditions.
Males are either the same size or smaller than the females and they are usually darker. Their antennae are straighter and clumsier and cannot be folded back against the head. They have well-developed compound eyes and ocelli. Their thorax, like that of the sexual females, carries two pairs of wings linked by hooks and it is composed of plates separated by sutures but their petiole is not quite so well-developed. There is no sting at the end of the abdomen; in its place are appendages for locking on to the female during copulation. They do not possess large body reserves like the sexual females.
IDENTIFICATION
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 (
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