Among the most well-known species are Periplaneta americana, which is about 3 cm long, the German cockroach, Blattella germanica, about 1½ cm long, the Asian cockroach, Blattella asahinai, also about 1½ cm in length, and the Oriental cockroach, Blatta orientalis, about 2½ cm. Tropical cockroaches are often much bigger, and extinct cockroach relatives such as the Carboniferous Archimylacris and the Permian Apthoroblattina were several times as large as these. Cockroaches are often considered pests, infesting urban habitats. Of thousands of species, about 30 (less than 1%) do so.
Cockroaches have been shown to exhibit emergent behaviour. Research being conducted at the University of Florida shows that cockroaches leave chemical trails in their feces. Other cockroaches will follow these trails to discover sources of food and water, and also discover where other cockroaches are hiding.
Research has shown that group-based decision making is responsible for more complex behavior such as resource allocation. A study where 50 cockroaches were placed in a dish with three shelters with a capacity for 40 insects in each, the insects arranged themselves in two shelters with 25 insects in each, leaving the third shelter empty. When the capacity of the shelters was increased to more than 50 insects per shelter, all of the cockroaches arranged themselves in one shelter. Researchers found a balance between cooperation and competition exists in group decision-making behavior found in cockroaches.
A 2005 US national study on factors that affect asthma in inner-city children shows that cockroach allergens appear to worsen asthma symptoms more than other known triggers. This study, funded by the US National Institute of Environmental Health Sciences (NIEHS) and the US National Institute of Allergy and Infectious Diseases (NIAID), is the first large-scale study to rank asthma triggers according to severity.
Additionally, a 2005 research study, sponsored by the National Pest Management Association (NPMA), shows a disparity in homeowner knowledge about this link. Only 10% of homeowners in the USA feel that cockroaches are a threat to their family's health.
In popular culture, cockroaches are often regarded as vile and extremely resilient pests, due to their shiny exoskeletons (making them look slimy, contrary to their fastidious habits), and their size, unusual for a scavenging insect.