Carpenter bees are large, hairy bees distributed worldwide. There are some 500 species of carpenter bee in 31 subgenera. Their name comes from the fact that nearly all species build their nests in burrows in dead wood, bamboo, or structural timbers (except those in the subgenus Proxylocopa, which nest in the ground). Carpenter bees make nests by tunneling into wood, vibrating their bodies as they rasp their mandibles against the wood, each nest having a single entrance which may have many adjacent tunnels. Carpenter bees do not eat wood. They discard the bits of wood, or re-use particles to build partitions between cells. The tunnel functions as a nursery for brood and the pollen/nectar upon which the brood subsists. The provision masses of some species are among the most complex in shape of any group of bees; whereas most bees fill their brood cells with a soupy mass, and others form simple spheroidal pollen masses, Xylocopa form elongate and carefully sculpted masses that have several projections which keep the bulk of the mass from coming into contact with the cell walls, sometimes resembling an irregular caltrop. The eggs are very large relative to the size of the female, and are some of the largest eggs among all insects.