The typical Genus _Iridomyrmex_ contains 18 distinct species, some of which have been subdivided into three or four varieties or races; most of them are small, except our “Mound Ant,” sometimes known as the “Meat Ant,” _Iridomyrmex detectus_, which is the commonest and most widely distributed ant in Australia. They construct large mounds a couple of feet above the surface of the ground, and two or three yards in diameter; they are formed of the soil excavated from beneath when forming their network of irregular open galleries; the upper surface is pierced with numbers of rounded vertical shafts, up which they swarm in countless thousands and attack any intruder, biting savagely with their stout sharp jaws and making things generally unpleasant for the stranger. When a mound is situated in open grassed country, one can trace regular bare roads leading off from the nest, worn smooth by the regular stream of ants passing backward and forward day after day. It is too well known to need description, but measures about ⅓ of an inch in length, and is of a general brownish purple tint, with the head light reddish brown. Smith described the worker under the name of _Formica purpurea_, and the male as _F. detectus_, so in most Museum collections it will be found under the former specific name. Forel has made a new variety, which he calls _Var sanguineus_, of the coastal form found in North Queensland, with the head and thorax light reddish brown. Most of the other species are small black ants: _Iridomyrmex rufoniger_ is very common in the bush and in the gardens; a variety which Forel has called _domesticus_ is the common black ant that comes into the house in Sydney, and is a regular pest in the summer in many districts. _Tapinoma minutum_ is about as large as the “Mound Ant”; black; the head and thorax deeply pitted and corrugated; the abdomen slightly constricted in the centre, smooth and shining. It comes from Townsville, N.Q.: a second species, _T. melanocephalum_, taken in Cairns, is also found in Samoa and the Tonga Islands. * * * * * The last sub-family, CAMPONOTINAE, is a large division well represented in Australia; they live in more or less large communities, and with a few exceptions have the pupae enclosed in cocoons. The base of the abdomen forms a single node, and there is no constriction between the second and third abdominal segments; the sting is wanting, and the anal orifice is fringed with hairs. In the works of early entomologists a number of our ants were described under the Genus _Formica_, but they have been gradually identified and placed in their proper genera, until we only have about half a dozen still remaining in this genus, probably more on account of the difficulty of identifying them than because they really belong here.