Bed bugs are small, parasitic insects that feed exclusively on the blood of humans and animals. They hide in mattresses, furniture, and cracks, emerging primarily at night to feed. Adult bed bugs possess only six legs, which immediately places them in a different biological class than eight-legged creatures often mistaken for them.
Six Legs Confirm Bed Bugs Are Insects
The presence of six jointed legs classifies the bed bug (Cimex lectularius) within the Class Insecta. All six appendages are attached to the thorax, the middle section of the body. Like all true insects, the bed bug’s body is segmented into the head, the thorax, and the abdomen.
This body plan is consistent across all developmental stages, including the five nymphal stages and the adult form. Each leg is equipped with small claws, which help the flightless insect grip surfaces like fabric and wood as it crawls.
Why Bed Bugs Are Often Mistaken for Arachnids
The confusion about the number of legs often stems from misidentifying bed bugs as arachnids, a class of animals that includes spiders, ticks, and mites. Arachnids are defined by having eight legs, which is the primary distinction from the six-legged insect body plan. Small arachnids like mites and ticks frequently share environments with bed bugs, leading to incorrect identification.
Ticks are also blood feeders, and their engorged appearance after a meal can visually resemble a fed bed bug. Mites are generally microscopic but can cause similar skin irritation. This overlap in habitat and feeding behavior causes people to mistakenly attribute the eight-legged structure of arachnids to the bed bug.
Other Key Physical Traits for Identification
Adult bed bugs are roughly the size and shape of an apple seed, growing to about five to seven millimeters in length. When unfed, their bodies are notably flat and broad-oval, allowing them to squeeze into tight crevices, such as mattress seams and cracks in furniture. Their color is usually a rusty or mahogany brown, but this changes dramatically after a blood meal.
After feeding, the body becomes swollen and elongated, taking on a more reddish hue as it fills with blood. Bed bugs are wingless and possess a pair of antennae on their heads, which they use as sensory organs to locate a host. Identifying this combination of flat, oval shape, reddish-brown color, and two antennae is important for confirmation beyond counting the six legs.