Because of their complex life cycle, a single species of aphid can have different body forms, some four-winged, but mostly wingless. Overall, they are soft-bodied, plump, pear-shaped, and tiny. All aphids have two tubelike projections on the hind end of the body, called cornicles or siphunculi, which apparently emit scents that warn other aphids of danger or can emit a defensive goo to deter predators. Aphids are commonly green, yellow, or brown, but the color is quite variable among the many, many species. Most aphids are slow-moving and sedentary.