The habits of the little blue or green Mason bees (Osmia) are quite varied. They construct their cells in the stems of plants, and in rotten posts and trees, or, like Andrena, they burrow in sunny banks. A European species selects snail shells for its nest, wherein it builds its earthen cells, while other species nidificate under stones. Curtis found two hundred and thirty cocoons of a British species (Osmia paretina), placed on the under side of a flat stone, of which one-third were empty. Of the remainder, the most appeared between March and June, males appearing first; thirty-five more bees were developed the following spring. Thus there were three successive broods, for three succeeding years, so that these bees lived three years before arriving at maturity. This may partly account for _insect years_, which are like "apple years," seasons when bees and wasps, as well as other insects, abound in unusual numbers. [Illustration: 26. Nest of Osmia.] Mr. G. R. Waterhouse, in the Transactions of the Entomological Society of London, for 1864, states that the cells of Osmia leucomelana "are formed of mud, and each cell is built separately. The female bee, having deposited a small pellet of mud in a sheltered spot between some tufts of grass, immediately begins to excavate a small cavity in its upper surface, scraping the mud away from the centre towards the margin by means of her jaws. A small, shallow mud-cup is thus produced. It is rough and uneven on the outer surface, but beautifully smooth on the inner. On witnessing thus much of the work performed, I was struck with three points: first, the rapidity with which the insect worked; secondly, the tenacity with which she kept her original position whilst excavating; and thirdly, her constantly going over work which had apparently been completed.... The lid is excavated and rendered concave on its outer or upper surface, and is convex and rough on its inner surface; and, in fact, is a simple repetition of the first-formed portion of the cell, a part of a hollow sphere." The largest species of Osmia known to us is a very dark-blue species (O. lignivora). We are indebted to a lady for specimens of the bees with their cells, which had been excavated in the interior of a maple tree several inches from the bark. The bee had industriously tunnelled out this elaborate burrow (Fig. 26), and, in this respect, resembled the habits of the Carpenter bee more closely than any other species of its genus.