_Structure._--The head of an ant carries a pair of elbowed feelers, each
consisting of a minute basal and an elongate second segment, forming the
stalk or "scape," while from eight to eleven short segments make up the
terminal "flagellum." These segments are abundantly supplied with
elongate tooth-like projections connected with nerve-endings probably
olfactory in function. The brain is well developed and its
"mushroom-bodies" are exceptionally large. The mandibles, which are
frequently used for carrying various objects, are situated well to the
outside of the maxillae, so that they can be opened and shut without
interfering with the latter. The peculiar form and arrangement of the
anterior abdominal segments have already been described. The fourth
abdominal segment is often very large, and forms the greater part of the
hind-body; this segment is markedly constricted at its basal (forward)
end, where it is embraced by the small third segment. In many of those
ants whose third abdominal segment forms a second "node," the basal
dorsal region of the fourth segment is traversed by a large number of
very fine transverse striations; over these the sharp hinder edge of the
third segment can be scraped to and fro, and the result is a
stridulating organ which gives rise to a note of very high pitch. For
the appreciation of the sounds made by these stridulators, the ants are
furnished with delicate organs of hearing (chordotonal organs) in the
head, in the three thoracic and two of the abdominal segments and in the
shins of the legs.

The hinder abdominal segments and the stings of the queens and workers
resemble those of other stinging Hymenoptera. But there are several
subfamilies of ants whose females have the lancets of the sting useless
for piercing, although the poison-glands are functional, their secretion
being ejected by the insect, when occasion may arise, from the greatly
enlarged reservoir, the reduced sting acting as a squirt.

_Nests._--The nests of different kinds of ants are constructed in very
different situations; many species (_Lasius_, for example) make
underground nests; galleries and chambers being hollowed out in the
soil, and opening by small holes on the surface, or protected above by a
large stone. The wood ant (_Formica rufa_, fig. 1) piles up a heap of
leaves, twigs and other vegetable refuse, so arranged as to form an
orderly series of galleries, though the structure appears at first sight
a chaot