Cockroaches

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.

Origins

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), Diptera (flies), Hemiptera (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.

1. False ovoviviparity. The ootheca is produced inside the female but instead of being laid, it is retained in a uterus or brood sac where the eggs develop. This is the main form of reproduction in the family Blaberidae but has also been recorded rarely in the Blatellidae.

2. True ovoviviparity.  This form of reproduction is different from false ovoviviparity in that an ootheca is not formed. Instead, eggs pass from the oviducts into the uterus where they lie in no particular order and undergo embryonic development.

3. Viviparity. Viviparity is only known in the genus Diploptera. The eggs are small and have insufficient water and yolk to complete development. They are kept inside the uterus within an incomplete oothecal membrane and their embryos absorb water and disolved proteins and carbohydrates that are produced by the uterus.

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.  

Natural enemies

• Evaniid wasps parasitise oothecae.

• Beetles in the subfamily Rhipidiinae of the family Rhipiphoridae parasitise cockroach nymphs.

• Predators of cockroaches are many ranging from invertebrates such as ants through to vertebrates such insectivorous frogs, reptiles, birds and mammals. Cockroaches protect themselves from predators  mainly by hiding away, but produce defensive secretions if attacked. These secretions can also render them distasteful.

• Internal parasites include amoebae, ciliates, nematodes and Nematomorpha.

Economic importance

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.