The insects are 3 to 9 millimetres (0.12 to 0.35 in) long and have flattened reddish-brown bodies with small nonfunctional wings.

[4] Newly hatched nymphs are translucent, light in color at first, becoming browner as they moult and approach maturity.

[3] Cimex may be mistaken for other insects, such as booklice, small cockroaches, or carpet beetles; however, when warm and active, their movements are more ant-like, and like most other true bugs, they emit a characteristic disagreeable odor when crushed.

Research on C. lectularius shows that it can survive a wide range of temperatures and atmospheric compositions.

[12] Common commercial and residential freezers reach temperatures low enough to kill most life stages of bed bug, with 95% mortality after 3 days at −12 °C (10 °F).

[13] They show high desiccation tolerance, surviving low humidity and a 35–40 °C range even with loss of one-third of body weight; earlier life stages are more susceptible to drying out than later ones.

[14] The thermal death point for C. lectularius is 45 °C (113 °F); all stages of life are killed by 7 minutes of exposure to 46 °C (115 °F).

[12] Bed bugs apparently cannot survive high concentrations of carbon dioxide for very long; exposure to nearly pure nitrogen atmospheres, however, appears to have relatively little effect even after 72 hours.

[19] Cimex are attracted to their hosts primarily by carbon dioxide, secondarily by warmth, and also by certain chemicals.

At the 57th annual meeting of the Entomological Society of America in 2009, newer generations of pesticide-resistant C. lectularius in Virginia were reported to survive only two months without feeding.

[26] DNA from human blood meals can be recovered from Cimex for up to 90 days, which means they can be used for forensic purposes in identifying on whom the bed bugs have fed.

The rostrum is composed of the maxillae and mandibles, which have been modified into elongated shapes from a basic, ancestral style.

[30] In all, the insect may spend less than 20 minutes in physical contact with its host, and does not try to feed again until it has either completed a moult or, if an adult, has thoroughly digested the meal.

Since males are attracted to large body size, any Cimex with a recent blood meal can be seen as a potential mate.

[4] North Carolina State University found that bed bugs in contrast to most other insects tolerate incest and are able to genetically withstand the effects of inbreeding quite well.

[32] This behaviour occurs because sexual attraction in bed bugs is based primarily on size, and males mount any freshly fed partner regardless of sex.

In all bed bug species except Primicimex cavernis, sperm are injected into the mesospermalege,[34] a component of the spermalege,[34] a secondary genital structure that reduces the wounding and immunological costs of traumatic insemination.

Due to these dangerous microbes, males have evolved antimicrobial ejaculate substances that prevent sperm damage.

When the microbes contact sperm or the male genitals, the bed bug releases antimicrobial substances.

Due to these findings, Reinhard et al. proposed that multiple mating is limited by seminal fluid and not sperm.

Due to egg production, females may refrain from spending additional energy on alarm pheromones.

[49] Cimex lectularius only feeds every five to seven days, which suggests that it does not spend the majority of its life searching for a host.

After searching—regardless of whether or not it has eaten—the Cimex returns to the shelter to aggregate before the photophase (period of light during a day-night cycle).

Reis argues that two reasons explain why C. lectularius would return to its shelter and aggregate after feeding.

Cimex may choose to aggregate because of predation, resistance to desiccation, and more opportunities to find a mate.