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Cockroaches could be confused with members of the order Orthoptera (crickets and grasshoppers) but they do not have well-developed hindlegs for jumping. Most cockroaches, such as the American Cockroach, lay their eggs in a brown oblong case called an ootheca and in many species these are carried around by the female for some time before being deposited on the ground. The Cape Mountain Cockroach has an interesting biology in that the eggs mature and hatch inside the female so that she 'gives birth' to young. In this species the male is winged and the female wingless (hence the generic name meaning 'without wings'). Cockroaches are mainly nocturnal and by day hide in dark places such as under rocks, dead wood and bark. They are mainly scavengers, feeding on dead organic matter such as plant matter.
Fossil cockroaches have been recorded in deposits dated to
as far back as the Upper Carboniferous, about 305 million years ago. During the
Carboniferous they were one of the most abundant of the insect orders in terms
of number of individuals. Apterygote insect Orders Collembola
and Archaeognatha were already present
as were aquatic orders such as Odonata
(dragonflies) and Ephemeroptera (Mayflies).
There were also a number of insect orders present that are now extinct. The Orthoptera
(crickets, etc) also go back to the Carboniferous but the largest present
day insect orders such as Coleoptera (beetles),
(bugs) and Lepidoptera (moths and
butterflies) had not yet evolved.
Cockroaches are very closely related to termites (Isoptera) and praying mantises (Mantodea).
General life cycle
Adults. Depending on species, adult cockroaches can range in size from 3 mm to at least 65 mm long. Cockroaches are flattened in appearance which enables them to crawl into narrow crevices. In some species males and females look superficially similar but in others the adult females are winged and the adult males wingless.
Eggs. Eggs are usually laid in a packet called an ootheca. The ootheca is formed in the female and as it exits it is stamped into shape by the ovipositor valves and hardens on being exposed to air. The shape of the ootheca is often species specific. The female carries the ootheca round on the end of her abdomen for varying lengths of time before dropping it on the ground or gluing it to something. After depositing it, females of some species cover the ootheca with debris so that it is difficult for it to be located by predators and parasitoids. In some species females carry around the ootheca for the entire embryonic development. Although the majority of cockroach species are oviparous in that they lay their eggs externally (in oothecae), there are some in which egg development is internal. Internal egg development can be divided into three main categories.
Nymphs. Being hemimetabolous, the nymphs are similar in general shape to the adults but are smaller, lack wings and genitalia are undeveloped. They hatch more-or-less simultaneously from the ootheca by swallowing air and inflating themselves, in this way splitting open the two halves. They pass through a series of moults before reaching the adult stage.
The vast majority of cockroach species (more than 99% of them) live in the wild and are of no economic importance. However, there are a few species that thrive in and around human habitations. They are pests because they destroy food and contaminate it with their smelly excreta. They can also eat book labels and bindings. The most common pest cockroach in South Africa is the American Cockroach Periplaneta americana. The smaller German Cockroach Blattella germanica can also be encountered indoors, and on the subtropical coast one can encounter the large Indian Cockroach Blatta orientalis.