All bugs are separated into two distinct groups. The plant bugs (Homoptera) have a sucking tube (rostrum or proboscis) held permanently out from beneath the head. In the true bugs (Heteroptera), which include both plant feeders and feeders on other insects and spiders, the rostrum can be tucked back under the body under the legs. Another way to distinguish them is by the way they hold their wings. Both the plant and the true bugs have two pairs of wings, the hind membranous pair that is used for flying, and the front pair stiffer, often brightly pigmented, and used to protect the hind pair. In plant bugs, the forewings are held tent-like above the body and covering the hindwings, but in the true bugs, the forewings are held flat over the body, with the ends overlapping and the hindwings folded up beneath them. About 25,000 species of true bugs are known, though there are more plant bugs with over 40,000 species described. These include the familiar aphids, cicadas, treehoppers, froghoppers, and leafhoppers.