Cimex lectularius, the common bed bug, is a flattened, oval, wingless insect that feeds exclusively on blood. Adults are approximately 5–7mm long — roughly the size of an apple seed — and are reddish-brown when unfed, swelling and darkening to a mahogany red after feeding. The species has six life stages: egg, five nymphal instars, and adult.
Life Stage Identification
| Stage | Size | Color | Visibility |
|---|---|---|---|
| Egg | ~1mm | Pearlescent white | Difficult — tiny, laid in clusters in cracks |
| 1st instar nymph | ~1.5mm | Nearly translucent/pale yellow | Very difficult without magnification |
| 2nd–3rd instar nymph | 2–3mm | Pale yellow to tan | Difficult — size of a sesame seed |
| 4th–5th instar nymph | 3–4.5mm | Tan to light brown | Visible to careful inspection |
| Adult (unfed) | 5–7mm | Reddish-brown, flat | Clearly visible |
| Adult (fed) | 6–9mm | Dark mahogany, swollen | Clearly visible |
Where to Inspect
Bed bugs shelter within 5–8 feet of the host during the day and emerge at night to feed. The box spring is the most common harborage site — specifically the fabric-covered corners and the interior framework accessible through the dust cover. The seams and tufts of mattresses, the joints of the bed frame and headboard, and the space behind a headboard mounted to the wall are the next most common locations. In established infestations, harborage expands to electrical outlets, baseboard crevices, picture frame backs, and upholstered furniture within the room.
Common Lookalikes to Rule Out
Bat bugs (Cimex adjunctus) are nearly identical to bed bugs — the distinction requires microscopic examination of body hairs. Spider beetle nymphs, booklice, and carpet beetle larvae are occasionally misidentified as bed bugs but are readily distinguished by shape and behavior. When in doubt, capture a specimen and have it identified by a licensed pest management professional before beginning treatment. For the St. Louis area and rural Missouri, stlouisbedbugcontrol.com provides inspection and identification services, and D&D Pest Control serves rural Franklin County at ddpestcontrolmo.com.