Cockroaches

Cockroaches (or simply "roaches") are insects of the order Blattodea. This name derives from the Latin word for "cockroach", blatta.

Among the most well-known species are the American cockroach, Periplaneta americana, which is about 30 mm long, the German cockroach, Blattella germanica, about 15 mm long, the Asian cockroach, Blattella asahinai, also about 15 mm in length, and the Oriental cockroach, Blatta orientalis, about 25 mm. 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 generally considered pests; however, only about 30 species (less than 1%) infest urban habitats.

Evolutionary history and relationships
The earliest cockroach-like fossils are from the Carboniferous period between 354–295 million years ago. However, these fossils differ from modern cockroaches in having long ovipositors and are the ancestors of mantids as well as modern cockroaches. The first fossils of modern cockroaches with internal ovipositors appear in the early Cretaceous.

Mantodea, Isoptera, and Blattodea are usually combined by entomologists into a higher group called Dictyoptera. Current evidence strongly suggests that termites have evolved directly from true cockroaches. If this is the case, then Blattodea excluding Isoptera is not a monophyletic group and the Isoptera are actually a family (or epifamily) of cockroaches. .

Biology
Cockroaches are generally omnivores. An exception to this is the wood-eating genus Cryptocercus, with various species found in Russia, China, Korea and the United States. Although they are incapable of digesting the cellulose themselves, they have a symbiotic relationship with a protozoan that digests the cellulose, allowing them to extract the nutrients. In this, they are similar to termites and current research suggests that the genus Cryptocercus is more closely related to termites than it is to other cockroaches. Cockroaches are most common in tropical and subtropical climates. Some species are in close association with human dwellings and widely found around garbage or in the kitchen.

Cockroaches, like all insects, breathe through a system of tubes called tracheae. The tracheae of insects are attached to the spiracles, excluding the head. Thus, all insects, including cockroaches, can breathe without a head. The valves open when the CO2 level in the insect rises to a high level; then the CO2 diffuses out of the tracheae to the outside and fresh O2 diffuses in. The tracheal system brings the air directly to cells because they branch continually like a tree until their finest divisions tracheoles are associated with each cell, allowing gaseous oxygen to dissolve in the cytoplasm lying across the fine cuticle lining of the tracheole. CO2 diffuses out of the cell into the tracheole.

Insects do not have lungs and thus do not actively breathe in the vertebrate lung manner. However, in some very large insects the diffusion process may not be sufficient to provide oxygen at the necessary rate and body musculature may contract rhythmically to forcibly move air out and in the spiracles and one can actually call this breathing.

Cockroaches can survive sterile surgical decapitation for a very long period, especially if recently fed, but of course become unable to feed and die within a few weeks.

Female cockroaches are sometimes seen carrying egg cases on the end of their abdomen; the egg case of the German cockroach holds about 30–40 long, thin eggs, packed like frankfurters in the case called an ootheca. The eggs hatch from the combined pressure of the hatchlings gulping air and are initially bright white nymphs that continue inflating themselves with air and harden and darken within about four hours. Their transient white stage while hatching and later while molting has led to many individuals to claim to have seen albino cockroaches.

A female German cockroach carries an egg capsule containing around 40 eggs. She drops the capsule prior to hatching. Development from eggs to adults takes 3-4 months. Cockroaches live up to a year. The female may produce up to eight egg cases in a lifetime; in favorable conditions, it can produce 300-400 offspring. Other species of cockroach, however, can produce an extremely high number of eggs in a lifetime, but only needs to be impregnated once to be able to lay eggs for the rest of its life.

The world's largest cockroach is the Australian giant burrowing cockroach, which can grow to 9 cm in length and weigh more than 30 grams. Comparable in size is the giant cockroach Blaberus giganteus, which grows to a similar length but is not as heavy.

Cockroaches are mainly nocturnal and will run away when exposed to light. A peculiar exception is the Oriental cockroach, which is attracted to light.

Cockroaches are among the hardiest insects on the planet, some species capable of remaining active for a month without food, or being able to survive on limited resources like the glue from the back of postage stamps. Some can go without air for 45 minutes or slow down their heart rate. It is popularly suggested that cockroaches will "inherit the earth" if humanity destroys itself in a nuclear war. Cockroaches do indeed have a much higher radiation resistance than vertebrates, with the lethal dose perhaps 6 to 15 times that for humans. However, they are not exceptionally radiation-resistant compared to other insects, such as the fruit fly. The MythBusters of Discovery Channel had tested this popular belief in an episode aired on January 30, 2008, and had confirmed that fruit flies do indeed have a higher resistance to radiation than cockroaches.

The cockroach's ability to withstand radiation better than human beings can be explained in terms of the cell cycle. Cells are more vulnerable to effects of radiation when they are dividing. A cockroach's cells divide only once when in its molting cycle, which at most happens weekly in a juvenile roach. The cells of the cockroach take roughly 48 hours to complete a molting cycle, which would give time enough for radiation to affect it but not all cockroaches would be molting at the same time. This would mean some would be unaffected by the initial radiation and thus survive, at least until the fallout arrived.

Selected species

 * Periplaneta americana, American cockroach
 * Eurycotis floridana, Florida woods cockroach
 * Blatta orientalis, Oriental cockroach
 * Blattella germanica, German cockroach
 * Blattella asahinai, Asian cockroach
 * Pycnoscelus surinamensis, Surinam cockroach
 * Supella longipalpa, brown-banded cockroach
 * Periplaneta australasiae, Australian cockroach
 * Periplaneta fuliginosa, smokybrown cockroach
 * Parcoblatta pennsylvanica, Pennsylvania woods cockroach
 * Periplaneta brunnea, brown cockroach
 * Blaberus discoidalis, discoid cockroach or false death's head
 * Blaberus craniifer, true death's head cockroach
 * Gromphadorhina portentosa, Madagascar hissing cockroach

Behavior
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. The models used in this research can also explain the group dynamics of other insects and animals.

Another study tested the hypothesis that cockroaches use just two pieces of information to decide where to go under those conditions: how dark it is and how many of their friends are there. The study conducted by José Halloy and colleagues at the Free University of Brussels and other European institutions created a set of tiny robots that to the roaches appear to be other roaches and can thus alter the roaches perception of critical mass. The robots were also specially scented so that they would be accepted by the real roaches.

Additionally, researchers at Tohoku University engaged in a Classical Conditioning experiment with cockroaches and discovered that the insects were able to associate the scent of vanilla and peppermint with a sugar treat.

Cockroaches and health risks
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.

Habitat
Cockroaches live in a wide range of environments around the world. Pest species of cockroaches adapt readily to a variety of environments, but prefer warm conditions found within buildings. Many tropical species prefer even warmer environments and do not fare well in the average household. Only about 20 species of cockroaches out of the known 3,500 are suited to thrive in the typical home.

Free-standing houses sited near wilderness areas or those with elaborate gardens often remain strikingly cockroach-free without chemical control or any particularly rigorous attention to hygiene. The explanation appears to be the surrounding habitat carries a healthy population of inoffensive non-invasive species of cockroaches, in turn supporting a population of cockroach predators, which do not discriminate between invasive and non-invasive species. Often when such houses are sprayed with insecticide for the first time, the most noticeable effect is the appearance of large numbers of dead predatory species such as centipedes.