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

There are about 4,000 species of cockroach, of which 30 species are associated with human habitations and about four species are well known as pests.

Among the best-known pest species are the American cockroach, Periplaneta americana, which is about 30 millimetres (1.2 in) long, the German cockroach, Blattella germanica, about 15 millimetres (0.59 in) long, the Asian cockroach, Blattella asahinai, also about 15 millimetres (0.59 in) in length, and the Oriental cockroach, Blatta orientalis, about 25 millimetres (0.98 in). 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.

  • Blaptica dubia, South American Dubia cockroach
  • Blatta orientalis, Oriental cockroach 
    Oriental cockroach (Blatta orientalis). a, female; b, male; c, side view of female; d, young.
  • Blattella asahinai, Asian cockroach
  • Blattella germanica, German cockroach
  • Blaberus craniifer, true death`s head cockroach
  • Blaberus discoidalis, discoid cockroach or false death`s head
  • Eurycotis floridana, Florida woods cockroach
  • Gromphadorhina portentosa, Madagascar hissing cockroach
  • Parcoblatta pennsylvanica, Pennsylvania woods cockroach
  • Periplaneta americana, American cockroach
  • Periplaneta australasiae, Australian cockroach
  • Periplaneta brunnea, black Mississippi cockroach
  • Periplaneta fuliginosa, smokybrown cockroach
  • Pycnoscelus surinamensis, Surinam cockroach
  • Supella longipalpa, brown-banded cockroach

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.

The spines on the legs were earlier considered to be sensory, but observations of their locomotion on sand and wire meshes has demonstrated that they help in locomotion on difficult terrain. The structures have been used as inspiration for robotic legs.

Cockroaches leave chemical trails in their feces as well as emitting airborne pheromones for swarming and mating. Other cockroaches will follow these trails to discover sources of food and water, and also discover where other cockroaches are hiding. Thus, cockroaches can exhibit emergent behavior, in which group or swarm behavior emerges from a simple set of individual interactions.

Research has shown that group-based decision-making is responsible for complex behavior such as resource allocation. In 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.

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

Description
Cockroaches are rather large insects. Most species are about the size of a thumbnail, but several species are bigger. The world`s largest cockroach is the Australian giant burrowing cockroach, which can reach 9 centimetres (3.5 in) in length and weigh more than 30 grams (1.1 oz). Comparable in size is the Central American giant cockroach Blaberus giganteus, which grows to a similar length but is not as heavy.

Cockroaches have a broad, flattened body and a relatively small head. They are generalized insects, with few special adaptations, and may be the most primitive living neopteran insects. The mouthparts are on the underside of the head and include generalised chewing mandibles. They have large compound eyes, two ocelli, and long, flexible, antennae.
The first pair of wings are tough and protective, lying as a shield on top of the membranous hind wings. All four wings have branching longitudinal veins, and multiple cross-veins. The legs are sturdy, with large coxae and five claws each. The abdomen has ten segments and several cerci

Eggs and egg capsules
Female Blatella germanica with ootheca.
Female cockroaches are sometimes seen carrying egg cases on the end of their abdomen; the egg case of the German cockroach holds about 30 to 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 claims of glimpses of an albino cockroach.

Common household roaches A. German cockroach, B. American cockroach, C. Australian cockroach, D&E. Oriental cockroach (Š & ‰).

A female German cockroach carries an egg capsule containing around 40 eggs. She drops the capsule prior to hatching, though live births do rarely occur. Development from eggs to adults takes 3 to 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 to 400 offspring. Other species of cockroach, however, can produce an extremely high number of eggs in a lifetime, but in some cases a female needs to be impregnated only once to be able to lay eggs for the rest of her life.

Nymph of a cockroach, 3 mm in length.
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 cockroaches, like all insects, are not dependent on the mouth and windpipe to breathe. 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. Unlike in vertebrates that depend on blood for transporting O2 and CO2, the tracheal system brings the air directly to cells, the tracheal tubes branching 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.

While cockroaches do not have lungs and thus do not actively breathe in the vertebrate lung manner, in some very large species the body musculature may contract rhythmically to forcibly move air out and in the spiracles; this may be considered a form of breathing.

Reproduction
Cockroaches use pheromones to attract mates, and the males practice courtship rituals such as posturing and stridulation. Like many insects, cockroaches mate facing away from each other with their genitalia in contact, and copulation can be prolonged. A few species are known to be parthenogenetic, reproducing without the need for males.

The female usually attaches the egg-case to a substrate, inserts it into a suitably protective crevice, or carries it about until just before the eggs hatch. Some species, however, are ovoviviparous, keeping the eggs inside their bodies, with or without an egg-case, until they hatch. At least one genus, Diploptera is fully viviparous.

Cockroach nymphs are generally similar to the adults, except for undeveloped wings and genitalia. Development is generally slow, and may take anything from a few months to over a year. The adults are also long-lived, and have been recorded as surviving for four years in the laboratory.

Hardiness
Cockroaches are among the hardiest insects on the planet. Some species are capable of remaining active for a month without food and are 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. In one experiment, cockroaches were able to recover from being submerged underwater for half an hour.

Ootheca of Periplaneta americana;
It is popularly suggested that cockroaches will "inherit the earth" if humanity destroys itself in a nuclear war. Cockroaches are only 5 times stronger than humans. 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 cockroach`s ability to withstand radiation better than human beings can be explained through the cell cycle. Cells are most vulnerable to the effects of radiation when they are dividing. A cockroach`s cells divide only once each time it molts, which is weekly at most in a juvenile roach. Since not all cockroaches would be molting at the same time, many would be unaffected by an acute burst of radiation, but lingering radioactive fallout would still be harmful.

Role as pests
Cockroaches are one of the most commonly noted household pest insects. They feed on human and pet food, and can leave an offensive odor.They can also passively transport microbes on their body surfaces including those that are potentially dangerous to humans, particularly in environments such as hospitals.Cockroaches have been shown to be linked with allergic reactions in humans. One of the proteins that triggers allergic reactions has been identified as tropomyosin.These allergens have also been found to be linked with asthma.

General preventive measures against household pests include keeping all food stored away in sealed containers, using garbage cans with a tight lid, frequent cleaning in the kitchen, and regular vacuuming. Any water leaks, such as dripping taps, should also be repaired. It is also helpful to seal off any entry points, such as holes around baseboards, in between kitchen cabinets, pipes, doors, and windows with some steel wool or copper mesh and some cement, putty or silicone caulk.

American cockroaches have been known to live up to three months without food and a month without water. Frequently living outdoors, although preferring warm climates and considered "cold intolerant," they are resilient enough to survive occasional freezing temperatures. This makes them difficult to eradicate once they have infested an area.

  
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