Bees (a lineage within the superfamily Apoidea) are flying insects, closely related to wasps and ants. There are approximately 20,000 species of bees, and they may be found on every continent except Antarctica. Bees are adapted for feeding on nectar and pollen, the former primarily as an energy source, and the latter primarily for protein and other nutrients. Most pollen is used for food for the brood. Bees have a long proboscis that enables them to obtain the nectar from flowers. Bees have antennae almost universally made up of thirteen segments in males and twelve in females. They all have two pairs of wings, the back pair being the smaller of the two; in a very few species, one sex or caste has relatively short wings that make it difficult or impossible to fly. Many species of bees are poorly known. The smallest bee is a dwarf bee (Trigona minima) and it is about 2.1 mm (5/64") long. The largest bee in the world is Megachile pluto, which can be as large as 39 mm (1.5"). The most common type of bee in North America is the Halictidae, or sweat bee, though this may come as a surprise to natives, as they are so small, and often mistaken for wasps.