Blattodea (Roaches)

**Cockroaches** are insects of the order Blattodea, sometimes called Blattaria, of which about 30 species out of 4,600 total are associated with human habitats. About four species are well known as pests.

Among the best-known pest species are the American cockroach, *Periplaneta americana*, which is about 30 mm (1.2 in) long; theGerman cockroach, *Blattella germanica*, about 15 mm (0.59 in) long; the Asian cockroach, *Blattella asahinai*, also about 15 mm (0.59 in) in length; and the Oriental cockroach, *Blatta orientalis*, about 25 mm (0.98 in). Tropical cockroaches are often much bigger, and, contrary to popular opinion, extinct cockroach relatives and 'roachoids' such as the Carboniferous *Archimylacris* and the Permian*Apthoroblattina* were not as large as the biggest modern species.

Several cockroach species, notably *Blaptica dubia*, are raised as feeder insects for insectivorous pets. A few cockroach species are raised as pets, most commonly the GiantMadagascar Hissing Cockroach, *Gromphadorhina portentosa*.

## Behavior

Cockroaches live in a wide range of environments around the world. Pest species 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.

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

The spines on the legs were earlier considered to be sensory, but observations of their locomotion on sand and wire meshes have demonstrated 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. These chemical trails transmit bacteria onto surfaces. 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.

Daily rhythms may also be regulated by a complex set of hormonal controls of which only a small subset have been understood. In 2005, the role of one of these proteins, pigment dispersing factor (PDF), was isolated and found to be a key mediator in the circadian rhythms of the cockroach.

### Collective decision-making

Gregarious cockroaches display collective decision-making when choosing food sources. In particular, it appears that when a sufficient number of individuals (a "quorum") exploits a food source, this signals to newcomer cockroaches that they should stay there longer rather than leave for elsewhere. Other mathematical models have been developed to explain aggregation dynamics and conspecific recognition.

Research has shown group-based decision-making is responsible for complex behaviors 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 the 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 other cockroaches there are. The study 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 they would be accepted by the real roaches.

### Social behavior

Gregarious German cockroaches show different behavior when reared in isolation than when reared in a group. In one study, isolated cockroaches were less likely to leave their shelters and explore, spent less time eating, interacted less with conspecifics when exposed to them, and took longer to recognize receptive females. Because these changes occurred in many contexts, the authors suggested them as constituting a behavioral syndrome. These effects might have been due either to reduced metabolic and developmental rates in isolated individuals or the fact that the isolated individuals hadn't had a training period to learn about what others were like via their antennae.

Individual American cockroaches appear to have consistently different "personalities" regarding how they seek shelter. In addition, group personality is not simply the sum of individual choices but reflects conformity and collective decision-making.

A number of studies have explored the social structure, chemical signaling, and "social herd" characteristics of gregarious cockroaches, including German and American cockroaches. According to one review, these two model species suggest that "the social biology of domiciliary cockroaches so far can be characterised by a common shelter, overlapping generations, non-closure of groups, equal reproductive potential of group members, an absence of task specialisation, high levels of social dependence, central place foraging, social information transfer, kin recognition, and a meta-population structure [...]." One BBC article summarized these findings with the title, "Why cockroaches need their friends."

## Description

Cockroaches are generally rather large insects. Most species are about the size of a thumbnail, but several species are bigger. The world's heaviest cockroach is the Australian giant burrowing cockroach *Macropanesthia rhinoceros*, which can reach 9 cm (3.5 in) in length and weigh more than 30 g (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. According to the Guinness World Records, the longest cockroach species is *Megaloblatta longipennis*, which can reach 97 mm (3.8 in) in length and 45 mm (1.8 in) across. A Central and South American species, *Megaloblatta blaberoides*, has the largest wingspan of up to 185 mm (7.3 in).

Cockroaches have broad, flattened bodies and relatively small heads. They are generalized insects, with few special adaptations, and may be among 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 (the tegmina) 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

Female cockroaches are sometimes seen carrying egg cases on the end of their abdomens; the egg case of the German cockroach holds about 30 to 40 long, thin eggs in a case called an ootheca. The egg capsule may take more than five hours to lay and is initially bright white in color. The eggs are hatched from the combined pressure of the hatchlings gulping air. The hatchlings are initially bright whitenymphs and continue inflating themselves with air, becoming harder and darker within about four hours. Their transient white stage while hatching and later while molting has led many to claim the existence of albino cockroaches.

A female German cockroach carries an egg capsule containing around 40 eggs. She drops the capsule prior to hatching, though live births do occur in rare instances. Development from eggs to adults takes three to four months. Cockroaches live up to a year. The female may produce up to eight egg cases in a lifetime; in favorable conditions, she can produce 300 to 400 offspring. Other species of cockroaches, however, can produce an extremely high number of eggs in a lifetime; in some cases a female needs to be impregnated only once to be able to lay eggs for the rest of her life.

### Sounds

Aside from the famous hissing noise, some cockroaches (including a species in Florida) will make a chirping noise. Several Australian species practice acoustic and vibration behaviour as an aspect of courtship. They have been observed producing hisses and whistles from air forced through the spiracles. Furthermore, in the presence of a potential mate some cockroaches will tap the substrate in a rhythmic, repetitive manner. It is hypothesized that acoustic signals may be of greater prevalence amongst perching species, particularly those that live on low vegetation in Australia's tropics.

### Digestive tract

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 are generally omnivorous; with the exception of the wood-eating species such as *Cryptocercus*, these roaches are incapable of digesting cellulose themselves, but have symbiotic relationships with various protozoans and bacteria that digest the cellulose, allowing them to extract the nutrients.

The similarity of these symbionts in the genus *Cryptocercus* to those in termites are such that they have been suggested to be more closely related to termites than to other cockroaches, and current research strongly supports this hypothesis of relationships. All species studied so far carry the obligate mutualistic endosymbiont bacterium *Blattabacterium*, with the exception of *Nocticola australiensise*, an Australian cave-dwelling species without eyes, pigment or wings, and which recent genetic studies indicate are very primitive cockroaches.

### Breathing

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 invertebrates that depend on blood for transporting O2 and CO2, the tracheal system brings the air directly to cells, with 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 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. Some species are capable of remaining active for a month without food and are able to survive on limited resources, such as the glue from the back of postage stamps. Some can go without air for 45 minutes. In one experiment, cockroaches were able to recover from being submerged underwater for half an hour. Japanese cockroach (*P. japonica*) nymphs, which hibernate in cold winters, survived twelve hours at -5 °C to -8 °C in laboratory experiments.

Experiments on decapitated specimens of several species of cockroach found a variety of behavioral functionality remained, including shock avoidance and escape behavior, although many insects other than cockroaches are also able to survive decapitation, and popular claims of the longevity of headless cockroaches do not appear to be based on published research. The severed head is able to survive and wave its antennae for several hours, or longer when refrigerated and given nutrients.

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 six 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.

## Notable species

* *Blattella germanica*, German cockroach
* *Blaptica dubia*, South American/Peruvian Dubia cockroach
* *Blatta orientalis*, Oriental cockroach
* *Blattella asahinai*, Asian 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
* *Laxta granicollis*, bark cockroach
* *Parcoblatta pennsylvanica*, Pennsylvania woods cockroach
* *Periplaneta americana*, American cockroach
* *Periplaneta australasiae*, Australian cockroach
* *Periplaneta brunnea*, brown cockroach
* *Periplaneta fuliginosa*, smokybrown cockroach
* *Periplaneta japonica*, Japanese cockroach, a freeze-tolerant species
* *Pycnoscelus surinamensis*, Surinam cockroach, a greenhouse pest
* *Saltoblattella montistabularis*, leaproach, the only jumping cockroach
* *Supella longipalpa*, brown-banded cockroach