The females closely resemble in form those of the genus _Andrena_, but the males are very unlike both those of that genus and their own females, for they all have long cylindrical bodies and very long antennæ, much longer relatively than those of the former genus. Although none of the species approach in size the larger ones of the preceding genus, their extremes of specific size are as distant apart as they are in that genus, the smallest being extremely minute. Some of even the commoner species are very pretty when in fine condition, and several of them have a rich metallic green or blue tint, and in the majority the wings are iridescent with the brightest and gayest colours of the rainbow. The numbers in which they associate together upon the same spot varies considerably, and a very few indeed burrow solitarily and apart from their congeners. In burrowing they form a tunnel which branches off to several cells, the excavations being as inartificial as are those of _Andrena_. Walkenaer tells us in his memoir upon the genus _Halictus_, that they line their cells with a kind of glaze, that they burrow in horizontal surfaces to a depth of about five inches, and which they polish very smoothly previous to covering it with their viscous secretion, and that the cells are all oval, the largest end being at the bottom. He says also that they burrow solely during the night, especially when the moon is shining, when it is difficult to walk without treading upon them; so numerous are they, indeed, that they look like a cloud floating close to the surface of the ground. Although burrowing thus at night, it is only during the day that they supply their nests with their provision of pollen and lay their eggs. Each of their cells is furnished with a small ball of pollen, varying in size with the species, but which never entirely fills the cell, and is affixed intermediately between both extremities, and upon the mass contained in each cell they deposit their small egg, which is placed at the extremity of the lump of pollen most distant from the entrance. The larva is hatched in about ten days, when it changes into the pupa. Some doubt attaches as to the length of time that the pupa remains before its transformation into the imago, and also as to the period at which this takes place. A peculiarity attends the appearance of the larger species. Some are very early spring insects, among which is the _Halictus rubicundus_; this I have seen in abundance on the first fine spring days collecting its stores on the flowers of the chickweed. It is then in the very finest condition, and it is really a very beautiful although a very common insect, having a richly golden fulvous pubescence on the thorax, an intensely black and glabrous abdomen, the apex of which is fringed with golden hair. No males are now to be found at all. Yet it is only some species, and these the larger ones, which are subject to this