There is a very large genus of rather small humming-bees, many of which are popularly mistaken for wasps, on account of their sharply pointed and yellow banded abdomen; they belong to the genus Crabro, and are extremely variable in the material into which they burrow, and the insects with which they feed their young. Some species burrow into dry bramble sticks. If the reader should wish to obtain specimens of them, as well as other burrowers, he will find bramble, rose, and jessamine sticks most prolific in them. The best plan is to collect a quantity of these sticks and put them into glass tubes, with the ends stopped with wire gauze; there is then an absolute certainty of identifying the insect with its habitation. The spring is the best time for collecting. Sometimes these creatures are afflicted with parasites, which also are detained in the tube, so as to yield valuable information to the captor.
Some species burrow in sand-banks and feed their young with gnats, others burrow into dead timber, and stock their tunnels with flies of various kinds. One very useful species, _Crabro lævis_, burrows in sand-banks, and provisions its nest with the noxious turnip-fleas (_Halticæ_), great numbers of which are needed to stock a single burrow. It is rather a social insect, many burrows being often found near each other. The turnip-flea has so hard a shell, that the young Crabro seems hardly capable of eating it. Mr. Smith, however, has remarked that another burrowing-bee stocks its nest with certain weevils that are almost too hard to be pierced with a pin, and that the shell is probably softened by the damp ground. The greater number of this group, however, are burrowers into the ground, and stock their nests with flies of some kind.
Another species of this large genus, _Crabro luteipalpis_, is fond of making its burrows in the mortar of old walls, preferring those spots where nails have been drawn, making the process of burrowing easier for the insect. It is not uncommon in the outskirts of London. All gardeners, especially those who cultivate roses, ought to encourage this very little insect, and welcome its presence, for it provisions its nest with the aphides, or green blight, which infect the rose-trees, and which have destroyed so many promising plants.