The size of hover flies varies depending on the species. Some, such as members of the genus Baccha, are small, elongated, and slender, while others, such as members of Criorhina, are large, hairy, and yellow and black. As members of the Diptera, all hover flies have a single functional pair of wings (the hind wings are reduced to balancing organs). Many species are brightly colored, with spots, stripes, and bands of yellow or brown covering their bodies. Due to this coloring, they are often mistaken for wasps or bees; they exhibit Batesian mimicry. Despite this, hover flies are harmless to humans. Drone flies, E. tenax, are an example of a species of hover fly who exhibit Batesian mimicry. With a few exceptions, hover flies are distinguished from other flies by having a spurious vein, located parallel to their fourth longitudinal wing vein. Adults feed mainly on nectar and pollen. Many species also hover around flowers, lending to their comm on name. Bee flies of the family Bombyliidae often mimic Hymenoptera and hover around flowers, as well, rendering some bombyliid species hard to tell apart from Syrphidae at first glance. Hover flies can, nevertheless, be distinguished in the field by anatomical features such as: The legs and mouthparts of hover flies are usually not particularly long and thin (some bombyliids have a long and needle-like proboscis, many have legs that are noticeably longer and thinner than in similar-sized syrphids) Their facial cuticle often has prominent bulges and/or beak- to knob-like projections (most bee flies have an evenly curved or sloping face). The wings are often clear or have smooth gradients of tinting, and their veins merge posteriorly into a "false edge" that runs parallel to the wing's true rear edge and extends along half or more of the wing length (bombyliid wings lack a "false rear edge" and often have large dark areas with sharp boundaries, or complex pat terns of spots). Their abdomens and thoraces often have glossy cuticular body surfaces, abdominal colors are usually mainly due to cuticular pigments (bee flies are usually very hairy, their abdominal colors are almost always due to pigmentation of hairs and not the underlying cuticle