Adult bedbugs are reddish-brown, flattened, oval, and wingless, with microscopic hairs that give them a banded appearance. A common misconception is that they are not visible to the naked eye. Adults grow to 4–5 mm (1/8th – 3/16th of an inch) in length and do not move quickly enough to escape the notice of an attentive observer. Newly hatched nymphs are translucent, lighter in color and become browner as they moult and reach maturity. In size, they are often compared to lentils or apple seeds.
In most observed cases, bites consist of a raised red bump or flat welt, and are often accompanied by very intense itching. The red bump or welts are the result of an allergic reaction to the anesthetic contained in the bedbug's saliva, which is inserted into the blood of its victim. Bedbug bites may appear indistinguishable from mosquito bites, though they tend to last for longer periods. Bites may not become immediately visible, and can take up to nine days to appear. Bedbug bites tend to not have a red dot in the center such as is characteristic of flea bites. A trait shared with flea bites, however, is tendency towards arrangements of sequential bites. Bites are often aligned three in a row, giving rise to the colloquialism "fleas bite in threes." This may be caused by the bedbug being disturbed while eating, and relocating half an inch or so farther along the skin before resuming feeding. Alternatively, the arrangement of bites may be caused by the bedbug repeatedly searching for a blood vein. People react very differently to bedbugs, and individual responses vary with factors including skin type, environment, and the species of bug. In some rare cases, allergic reactions to the bites may cause nausea and illness. In a large number of cases, estimated to 50% of all people, there is no visible sign of bites whatsoever, greatly increasing the difficulty of identifying and eradicating infestations