Blattodea

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

Though cockroaches are generally considered pests, only about 30 species (less than 1%) infest urban habitats^{citation needed}; 55 species in total live in the U.S.^[1]

Selected species

Evolutionary history and relationships

Mantodea, Isoptera, and Blattaria are usually combined by entomologists into a higher group called Dictyoptera. Current evidence strongly suggests that termites have evolved directly from true cockroaches, and many authors now consider termites to be a family of cockroaches^[2]^[3], as Blattaria excluding Isoptera is not a monophyletic group[1].

Historically, the name Blattaria has been used largely interchangeably with the name Blattodea, though in most recent treatments, the latter name refers to a larger grouping that includes numerous fossil groups that were related to roaches, but not true cockroaches themselves. Another name, Blattoptera has come into use for this same paraphyletic group^[4]. These earliest cockroach-like fossils ("Blattopterans" or "roachids") 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.

Behavior

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.

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,^[5] 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.^[5]

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 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.^[6]

Physiology

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 genus 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 to those in termites are such that the genus Cryptocercus has been believed to be more closely related to termites than to other cockroaches^[8], and current research strongly supports this hypothesis of relationships^[9].

Trachea and 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 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. [2].

Nervous system

Like all insects, the cockroach nervous system is decentralized, with some functions distributed in the ventral ganglia. Though decentralized, the insect nervous system is fundamentally similar to that of vertebrates in terms of development, structure, and mechanisms of function. A decapitated cockroach can still walk and show responses to stimulation of its legs, as can a spinal-transected cat on a treadmill. However, a cockroach can survive complete decapitation for up to several weeks before dying of starvation or dehydration^[10].

Eggs and egg capsules

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 claims of glimpses of an albino cockroach.

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 3-4 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 in some cases a female only needs to be impregnated 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.^{citation needed}

Size

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 cm in length and weigh more than 30 grams. Comparable in size is the Central American giant cockroach Blaberus giganteus, which grows to a similar length but is not as heavy.

Hardiness

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.^[11] 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.^[12] The MythBusters of Discovery Channel tested this popular belief in an episode aired on January 30, 2008, and 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 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.^[13]

Pest control

Cockroaches are one of the most commonly noted pest insects due to their large size. Since they feed on human and pet food, they can also serve as vectors for disease transmission. In addition, a 2005 study on factors that affect asthma in U.S. inner-city children shows that cockroach allergens appear to trigger asthma symptoms more severely than other known factors. This study, funded by the US National Institute of Environmental Health Sciences (NIEHS) and the US National Institute of Allergy and Infectious Diseases (NIAID), was the first large-scale study to rank asthma triggers according to severity.

General preventative 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.

There are numerous parasites and predators of cockroaches, but few of them have proven to be highly effective for biological control. Wasps in the family Evaniidae are perhaps the most effective insect predators, as they attack the egg cases, and wasps in the family Ampulicidae are predators on adult and nymphal cockroaches (e.g., Ampulex compressa). The house centipede is probably the most effective control agent of cockroaches, though many homeowners find the centipedes themselves objectionable.

Bait stations, gels containing hydramethylnon or fipronil, as well as boric acid powder, are toxic to cockroaches.^[14] Baits with egg killers are also quite effective at reducing the cockroach population. Additionally, pest control products containing deltamethrin or pyrethrin are very effective.^[14]

In Singapore and Malaysia, taxi drivers use Pandan leaves as a cockroach repellent in their vehicles.^[15] In Russia, some people, after opening all cupboards, leave a saucepan half-full of sulfur burning on a gas ring, then quickly come back in to switch it off (with gas mask) and leave the residence for a few days. This is said to be protective for ten years.^{citation needed}

Recently, a method called the Vegas roach trap has gained popularity as a successful way to control a cockroach infestation.

Pest control is cited as one of the reasons for reduced populations of cockroaches in ex-USSR countries[3].

Popular culture

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.

References

Notes

^ Cockroaches, Recognition and Control B-1458
^ "Termites are 'social cockroaches'" (in English), BBC News (13 April 2007).
^ Eggleton, P. &al. (2007), Biological Letters, June 7, cited in Science News vol. 171, p. 318
^ Grimaldi, D (1997): A fossil mantis (Insecta: Mantoidea) in Cretaceous amber of New Jersey, with coments on early history of Dictyoptera. American Museum Novitates 3204: 1-11
^ ^a ^b Jennifer Viegas. "Cockroaches Make Group Decisions". Discovery Channel. Retrieved on 10 June, 2006.
^ Lemonick, Michael D. (2007-11-15). "Robotic Roaches Do the Trick". Time Magazine.
^ Wynne Parry. "Pavlovian Cockroaches Learn Like Dogs (and Humans)". Discover Magazine. Retrieved on 5 September, 2007.
^ Eggleton, P. (2001). Termites and trees: a review of recent advances in termite phylogenetics. Insectes Sociaux 48: 187-193.
^ Evidence for Cocladogenesis Between Diverse Dictyopteran Lineages and Their Intracellular Endosymbionts
^ Choi, Charles (2007-03-15). "Fact or Fiction?: A Cockroach Can Live without Its Head". Scientific American. Retrieved on 2007-06-29.
^ Mullen, Gary; Lance Durden. Medical and Veterinary Entomology, 2002, p.32. ISBN 0-12510-451-0.
^ "Cockroaches & Radiation". Retrieved on 10 June, 2006.
^ Joseph G. Kunkel. "Are cockroaches resistant to radiation?". Retrieved on 10 June, 2006.
^ ^a ^b "Cockroaches". Alamance County Department of Environmental Health. Retrieved on 11 May, 2008.
^ Li J. and Ho S.H. Pandan leaves (Pandanus amaryllifolius Roxb.) As A Natural Cockroach Repellent. Proceedings of the 9th National Undergraduate Research Opportunites Programme (2003-09-13).

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