Humble-Bees and other Matters. 155
served them, there has occurred no change in their relative positions; though both have greatly increased in numbers during that time, owing to the spread of cultivation. And yet it would scarcely be too much to expect some marked change in a period so long as that, even through the slow-working agency of natural selection; for it is not as if there had been an exact balance of power between them. In the same period of time I have seen several species, once common, almost or quite disappear, while others, very low down as to numbers, have been exalted to the first rank. In insect life especially, these changes have been numerous, rapid, and widespread.
In the district where, as a boy, I chased and caught tinamous, and also chased ostriches, but failed to catch them, the continued presence of our two humble-bees, sucking the same flowers and making their nests in the same situations, has remained a puzzle to my mind.
The site of the nest is usually a slight depression in the soil in the shelter of a cardoon bush. The bees deepen the hollow by burrowing in the earth; and when the spring foliage sheltering it withers up, they construct a dome-shaped covering of small sticks, thorns, and leaves bitten into extremely minute pieces. They sometimes take possession of a small hole or cavity in the ground, and save themselves the labour of excavation.
Their architecture closely resembles that of B. terrestris. They make rudely-shaped oval honey-cells, varying from half an inch to an inch and a half in length, the smaller ones being the first
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The Naturalist in La Plata.
made; later in the season the old cocoons are utilized for storing honey. The wax is chocolate-coloured, and almost the only difference I can find in the economy of the two species is that the black bee uses a large quantity of wax in plastering the interior of its nest. The egg-cell of the yellow bee always contains from twelve to sixteen eggs; that of the black bee from ten to fourteen; and the eggs of this species are the largest though the bee is smallest. At the entrance on the edge of the mound one bee is usually stationed, and, when approached, it hums a shrill challenge, and throws itself into a menacing attitude. The sting is exceedingly painful.
One summer I was so fortunate as to discover two nests of the two kinds within twelve yards of each other, and I resolved to watch them very carefully, in order to see whether the two species ever came into collision, as sometimes happens with ants of different species living close together. Several times I saw a yellow bee leave its own nest and hover round or settle on the neighbouring one, upon which the sentinel black bee would attack and drive it off. One day, while watching, I was delighted to see a yellow bee actually enter its neighbour's nest, the sentinel being off duty. In about five minutes' time it came out again and flew away unmolested. I concluded from this that humble-bees, like their relations of the hive, occasionally plunder each other's sweets. On another occasion I found a black bee dead at the entrance of the yellow bees' nest; doubtless this individual had been caught in the act of stealing honey, and, after