Grasshoppers are herbivorous insects in the order Orthoptera. To distinguish them from bush crickets or katydids, they are sometimes referred to as short-horned grasshoppers. Species that change colour and behaviour at high population densities are called locusts. The Grasshoppers have antennae that are almost always shorter than the body (sometimes filamentous), and short ovipositors. Those species that make easily heard noises usually do so by rubbing the hind femurs against the forewings or abdomen, or by snapping the wings in flight. Tympana, if present, are on the sides of the first abdominal segment. The hind femora are typically long and strong, fitted for leaping. Generally they are winged, but hind wings are membranous while front wings are coriaceous and not fit for flight. Females are normally larger than males, with short ovipositors. Recent estimates indicate some 11,000 valid species described to date. Many undescribed species exist, especially in tropical wet forests.