• Characteristics Adult fleas are small insects usually measuring 0.04 to 0.3 inches (1.0 to 8 millimeters). The females have shorter antennae (an-TEH-nee) or sense organs and are usually larger than the males. The bodies of fleas are flat from side to side, allowing them to move easily between the fur and feathers of their mammal or bird hosts. Their tubelike mouthparts are used to pierce skin and suck blood. The eyes, if present, are simple and made up of a single lens each. The head and thorax, or midsection, sometimes have special comblike structures, while the legs are spiny. These features help to protect their bodies from damage, allow the fleas to cling to hair and feathers, and prevent them from being removed by the grooming activities of their hosts. The spiny and hairlike structures of bird fleas are longer, more slender, and more numerous than those of mammal fleas. The legs of fleas are used for moving through hair and feathers. Fleas are excellent jumpers and use this method to find new hosts. The abdomen is ten-segmented. In some species the abdomen of the females expands up to one hundred times its original size to accommodate either a blood meal or hundreds of developing eggs. They actually grow new tissue to allow their abdomens to expand without molting, or shedding their exoskeletons or hard outer coverings.