It isn’t fair. A few rogue species and all Cockroaches get tarred with the same brush. Most cockroaches are harmless creatures, living their lives out in quiet anonymity on forest floors around the world, especially the tropics and subtropics. They are important members of their ecosystems, contributing to the recycling of forest nutrients and providing a food source for insectivores. They are tough and adaptable little creatures and inhabit many different habitats besides human dwellings. There are pale, blind cave dwellers, arid-adapted desert species and cold-resistant alpine species as well as forest and grassland inhabitants. There are even amphibious species in Southeast Asia and Central America. There are over 4000 named species with perhaps another 3000 undiscovered and only about 15 that actually cause problems for humans.
Cockroaches are easily recognizable. They are dorso-ventrally flattened and they have a shield-like pronotum covering the head and thorax. The spiny legs, six of course, jut out from the sides and carry the cockroach along at great speed. There are two long antennae and an abdomen with ten segments with two short cerci at the end. Adult have four wings, with the front pair being strongly sclerotized for protection and the hind wings membranous.
Sexes are usually separate in cockroaches, with males having complex genitalia that are concealed by the ninth abdominal sternum. Females have a small ovipositor hidden by sternum seven. Males of some species have dorsal glands that secrete a substance that is attractive to females. Fertilisation is internal with the animals locking end to end. A few species are also known to reproduce parthenogenetically, without males involved.
The eggs travel down the common oviduct where they are fertilised and then extruded into an egg case called an ootheca. The complete ootheca has a tough outer case and pockets inside that contain the eggs, which can number from about 10 to 40. Sometimes the ootheca remains attached to the back of the female, who protects it that way. In other species, the ootheca remains soft and is gradually withdrawn into a brood pouch, so the young are actually born alive (ovoviviparity). The young nymphs resemble the adults, only smaller and wingless. They moult seven to nine times before achieving adulthood. Most species mature in a few months and the adults can live a year or two. Larger species tend to mature more slowly and live longer.
Most cockroaches are nocturnal and hide under logs, stones or bark during the day. These are usually brown or black in color but there are a few brightly-colored, diurnal species too. Most live on the ground but a few are arboreal. Cockroaches have strong chewing mandibles and are omnivorous, eating whatever they can find. Many eat rotten wood and, like termites, have special bacteria living in their guts to help with the digestion of woody material.
Cockroaches suffer from a number of parasites as well as predators. A number of insect parasitoids, mostly wasps, will lay their eggs inside the eggs of cockroaches, These hatch first and eat the eggs out from the inside. Protozoan and helminth worms are known to parasitise adult cockroaches which are also preyed upon by other arthropods, frogs, lizards, birds and mammals. Speed and camouflage work well for cockroaches but many can also exude a stinky predator-repelling substance, especially the brightly colored, diurnal species.
The economic significance of cockroach pests cannot be underestimated. There are nine introduced species in Australia alone that cause considerable damage. Two of the worst are the American Cockroach (Periplaneta americana) and the smaller German Cockroach (Blattella germanica), which are pests world-wide. These are the two most likely suspects when a light is turned on in a city flat and cockies are seen scurrying off in all directions. Not only do they destroy foodstuffs but they defecate everywhere, causing much contamination and spread of diseases. Both polio viruses and Salmonella have been isolated from cockroaches and their excreta. They are also voracious, not only eating food but also chewing the labels off containers and the bindings off books. They are destructive and potentially disease-carrying so there is good reason for home-owners to reduce their numbers, although complete extermination is seemingly impossible. Acknowledging their toughness, it has been said that cockroaches are one of the most probable survivors of a nuclear holocaust.
Like mosquitoes, cockroaches have been subjected to a great deal of chemical warfare by humans. Some of these chemicals have been too deadly, killing pets and beneficial insects as much as the intended targets. Luckily we seem to be taking a more specific approach these days with special baits aimed only at the cockies. Cleanliness is still the best defense. Keeping food in cockroach-proof containers, taking out the garbage regularly, keeping shelves and cupboards clean and only putting baits in hard to reach places is still the best way to reduce cockroach numbers and prevent contamination.
It is important to remember that wild cockroaches in their natural environments are an important part of their ecosystems and should be left to continue in that role. They should not be killed just because they are cockroaches. If you roll over a log in almost any forest anywhere, you will probably see cockroaches and millipedes, among other, less common cryptic arthropods. Do them a favor and roll the log back over when you are finished so they can get on with their little lives in peace.
Reference: CSIRO 1979. The Insects of Australia. Melbourne University Press.
