The common bed bug, Cimex lectularius, is a resilient parasitic insect that feeds on human blood and has achieved global distribution. These pests hide in tiny crevices and emerge at night to feed, causing stress and discomfort. Understanding the bed bug’s appearance and the evidence it leaves behind is the first step in managing an infestation. This knowledge is important because many small insects are mistaken for bed bugs, leading to misdirected treatment efforts.
The Actual Appearance and Color of Bed Bugs
Adult bed bugs are not green. When unfed, an adult bed bug is mahogany or reddish-brown. They are about the size and shape of an apple seed, measuring 5 to 7 millimeters in length, with a flat, oval body. This flatness allows them to hide easily in narrow cracks and seams.
After a successful blood meal, the insect’s appearance changes, becoming balloon-like and elongated. The color shifts to a deep purple or red as the ingested blood fills the abdomen. Bed bugs are true bugs, possessing a three-segmented beak and four-part antennae.
Young bed bugs, called nymphs, pass through five stages before reaching adulthood and look like smaller versions of the adult. Newly hatched nymphs are nearly colorless or whitish-yellow. After feeding, the blood meal is visible as a bright red or black spot within their translucent bodies. Bed bug eggs are tiny, about 1 millimeter long, pearl-white, and cemented onto rough surfaces within hiding spots.
Tell-Tale Signs of an Infestation
Since bed bugs are nocturnal and hide well, infestations are often identified by the physical evidence they leave behind rather than by seeing the live insects. One common sign is the presence of small, dark fecal spots. These spots are digested blood and look like tiny ink stains or specks of black pepper on bedding, mattress seams, or near their harborages.
Fecal spots are dark brown or black because the blood meal has been digested. On fabric, these spots will often bleed or smear if wiped with a damp cloth, which distinguishes them from dirt. Small, rusty or reddish stains may also appear on sheets and pillowcases. These blood stains are usually caused by an engorged bug being accidentally crushed while a person sleeps.
Another piece of evidence is the presence of shed skins, also known as exuviae or cast skins. Bed bugs must shed their exoskeleton five times to reach maturity, leaving behind translucent, hollow shells that resemble the insect. These skins vary in size depending on the nymphal stage and are often found clustered in crevices near the feeding area. In heavy infestations, a faint, sweet, or musty odor may be noticeable. This distinct smell is produced by the bugs’ scent glands and confirms a long-standing problem.
Common Insects Mistaken for Bed Bugs
Many small household insects are mistaken for bed bugs, causing confusion and misdiagnosis. One similar-looking pest is the Bat Bug (Cimex adjunctus), a close relative of the common bed bug. Bat bugs look nearly identical in shape and color, but a key difference is the length of the hairs on their upper thorax. Bat bug hairs are visibly longer than the width of their eye, a feature that requires magnification to observe.
Carpet beetles are also confused with bed bugs, particularly their larvae. Adult carpet beetles are smaller than bed bugs and have a rounded, convex body shape, sometimes mottled with white, brown, or yellow scales. Carpet beetle larvae appear fuzzy or bristly, resembling tiny, hairy worms, unlike the flatter bed bug nymphs. Carpet beetles consume natural fibers like wool and hair, while bed bugs are strictly blood feeders.
Booklice, also called psocids, are often mistaken for bed bug nymphs due to their small size and pale color. Booklice are much smaller, typically 1 to 2 millimeters in length, and have a proportionally larger head compared to the rest of their body. They are pale or translucent and feed on mold, fungi, and starchy materials, meaning they do not feed on humans. Finding these insects in damp areas, old books, or stored food indicates booklice, not bed bugs.