The Cimicidae are a family of small parasitic bugs that feed exclusively on the blood of warm-blooded animals. They are called cimicids or, loosely, bed bugs, though the latter term properly refers to the most famous member of the family, Cimex lectularius, the common bed bug and its tropical relation Cimex hemipterus. The family contains over 100 species. Cimicids appeared in the fossil record in the Cretaceous period. When bats evolved in the Eocene, they switched hosts and now feed mainly on bats or birds. Members of the group have colonised humans on three occasions. Cimicids usually feed on their host's blood every three to seven days, crawling away from the host and hiding while they digest the blood, which may take several days. This means that they specialise in vertebrate hosts that return regularly to particular sites to nest, roost or sleep. Birds and bats suit these specific requirements, as do humans now that they live in dwellings, and these are the main hosts used by the bugs. Most cimicids are able to go for long periods without feeding, over a year in some instances. Cimicids are typically small, oval, flattened, wingless insects. They are stimulated to appear from their hiding places by cues such as a slight rise in the temperature of their surroundings. Among the family's distinctive characteristics are traumatic insemination, in which the male fertilises the eggs by piercing the female's abdominal wall with his intromittent organ. They also have distinctive paired structures called mycetomes inside their bodies, in which they harbour bacterial symbionts: these may help them to obtain nutrients they cannot get from blood. Although the insects may acquire viruses and other pathogens while feeding, these do not normally replicate inside the insect, and the infections are not transmitted to new hosts. Biology All cimicids are small, oval-shaped, and flat in appearance, although their bodies bulge after feeding. They do not fly, but have small, non-functional wing pads. They have beak-like mouthparts with which they pierce the skin and suck the blood of their hosts. They are often considered to be ectoparasites because, although they move away from the host after feeding, they remain within the confines of their host's roost, nest or dwelling; however, under a different definition, they may be considered to be micropredatory bloodsuckers. Reproduction in cimicids involves traumatic insemination; although the female has a normal genital tract for laying eggs,