It was in the early days of August, as I approached the armory at Boulder, that I came across the nests of Melissodes obliqua Say. Careful examination disclosed eight entrances in an area of four square meters. This had been traversed by automobiles and so was without vegetation. The bees had constructed tunnels which led at first straight down and then suddenly turned off to one side. Ceaselessly, the males of the colony were darting back and forth. Occasionally, a female circled her way in from the fields, her hind legs bright yellow with their heavy loads of pollen. Then, almost inevitably, the males rushed in on all sides. There was a collision in mid-air and a sudden descent to the ground. Before the males could gather themselves together, the female was on her way. Quickly she made her way to the nest, hovered over the entrance, dropped in head first and was gone.