By George Beccaloni (Curator of cockroaches etc, The Natural History Museum, London)

Cockroaches (order Blattodea) are a group of largely omnivorous insects with incomplete metamorphosis which together with the carnivorous praying mantids (order Mantodea) form the superorder Dictyoptera. It is thought that cockroaches split from the common ancestor they shared with the mantids during the Lower Cretaceous about 140 million years ago. The cockroach-like species ("roachoids") with external ovipositors (egg laying tubes) which existed before this split, originated as far back as the Upper Carboniferous period some 315 million years ago. Recent studies (e.g., Inward, Beccaloni & Eggleton, 2007; Eggleton, Beccaloni & Inward, 2007) have shown that the termites are actually a lineage of cockroaches (epifamily Termitoidae) and not a separate order of insects as previously thought. Termites are most closely related to the wood-feeding cockroaches of the genus Cryptocercus, with which they share many structural characters and behavioural traits. Although termites are cockroaches we are only concerned here with the six families of 'true' cockroaches: Nocticolidae, Polyphagidae, Blattidae, Cryptocercidae, Blattellidae and Blaberidae.

Evolutionary Relationships of Cockroaches from Eggleton, Beccaloni & Inward, 2007

Cockroach diversity

To date approximately 4,500 cockroach species have been named and there are probably at least twice this number still to be discovered worldwide. Although most species are found in the tropics a few occur in temperate regions. There are about 130 native European cockroaches and, perhaps surprisingly, new species are still being discovered in this well studied region.

Regrettably most people seem to regard all cockroaches as offensive and destructive vermin. However, this reputation is deserved by less than 30 species (< 1% of the total) - the vast majority being secretive insects which never associate with man. As a group cockroaches exhibit a remarkable diversity of size, form, coloration and behaviour and occupy a very wide range of habitats from caves to mountains, from rainforests to deserts.

Sand-burrowing desert cockroach of family Polyphagidae, from dunes of the western USA. Copyright Marshal Hedin.

Some tropical cockroaches are thought to live only in the nests of social insects and there are even amphibious species which dive under water when threatened. Although most cockroaches are probably omnivorous, the ability to feed exclusively on rotting wood has evolved at least three separate times: in the ancestor of Cryptocercus and the termites; in the blaberid subfamily Panesthiinae; and in the blaberid Parasphaeria boleiriana.

Adult male Gromphadorhina grandidieri from southern Madagascar.

Many cockroach species are wingless or have reduced wings and some (e.g. the Cuban burrowing cockroach Byrsotria fumigata) have fully winged males and females with greatly reduced wings. Asian and Australasian Panesthia species, which burrow in decaying wood, have well developed wings when they first become adults, but these soon break off about one third of the way down their length, presumably once the insects have dispersed. Many cockroaches are sexually dimorphic, for example the Madagascan hissing cockroaches (Gromphadorhina and their relatives) which are often kept as pets. The males of these cockroaches have well developed 'horns' on their pronota (the plate covering the head) which they use to fight rival males - the largest individual usually emerging as the victor.

Adult male Lucihormetica fenestrata from Brazil.

The males of one South American cockroach, Lucihormetica fenestrata, have raised yellowish tubercles on their pronota which are bioluminescent and may play a role in courtship. It is currently unknown how this bioluminescence is produced, but one suggestion is that the spongy material inside the tubercles harbours bioluminescent fungi or bacteria which the cockroach may acquire from the rotting wood in which it lives. Related species have similar tubercles which may also emit light.

The world's heaviest cockroach is the wingless Australian rhinoceros cockroach (Macropanesthia rhinoceros), which weighs up to 33.5 grams and has a body length of up to 80 mm. It has one of the most complex life-histories of all cockroaches and, with a lifespan of more than 10 years, it is among the longest lived of all insects. The species with the greatest wingspan is the Central and South American Megaloblatta blaberoides, which has a spread of up to 185 mm. The smallest is the North American Attaphila fungicola, which measures less than 3 mm long and lives in the nests of leafcutter ants.

Some of the largest & smallest cockroaches. Megaloblatta longipennis (top), Attaphila bergi (left) and Macropanesthia rhinoceros (right).

Reproductive biology

Cockroaches are more diverse in their reproductive biology than probably any other order of insects. One species, Pycnoscelus surinamensis, is parthenogenic (the ability to reproduce without mating), but males are found in all other species which have been studied.

Species in the families Nocticolidae, Polyphagidae, Blattidae, Cryptocercidae and most Blattellidae, produce hardened oothecae (egg cases) which are dropped on the ground, buried, or attached to substrate using a salivary cement. In contrast, species in the family Blaberidae plus a few Blattellidae, have membranous oothecae which are incubated in a brood sac within the female's body until the eggs hatch. One blaberid, Diploptera punctata, has a greatly reduced oothecal membrane which does not cover the eggs. Remarkably this species produces a nutritious 'milk' from the wall of the brood sac on which the developing embryos feed. Species in the blaberid subfamily Geoscapheinae have lost the oothecal membrane altogether and the eggs are deposited straight into the brood sac. These cockroaches exhibit a high level of parental care. The nymphs live with the mother in her burrow until they are about half grown and the female provides food for them by pulling dead leaves and other vegetation into the burrow.

Female epilamprine cockroach from India carrying young.

The most unusual parental care is exhibited by species in the blaberid subfamilies Epilamprinae and Perisphaerinae. Thorax porcellana, a species of the former subfamily from India, carries its young (i.e. nymphs) under its domed forewings for their first two instars. The nymphs obtain liquid food from specialized pores on the upper surface of their mother's abdomen and they also pierce her cuticle with their specialized sharp mandibles and feed on her blood (haemolymph)! Species of the genus Perisphaerus (Perisphaerinae) from South-East Asia and Australasia have a somewhat similar mother-offspring relationship. When the nymphs are born they are white, blind and have strange tube-like mouthparts, which are unique amongst cockroaches. The mother has four apertures at the bases of her mid- and hind legs into which the mouthparts of the nymphs exactly fit and the nymphs are believed to suck up a nutrient 'milk' from these pores. The nymphs cling beneath the mother's body for the first two instars and only in the third instar do they develop normal eyes and body pigmentation.

Perisphaerus sp. from Malaysia with well developed young, which have recently left the underside of the mother.

Predator defences

Female Perisphaerus sp. from Malaysia rolled up into a defensive ball.

Cockroaches have evolved a wide range of strategies to avoid being eaten by predators. Most, like the leaf green Panchlora species, rely on camouflage, whilst a few have warning coloration or mimic distasteful insects (e.g. Prosoplecta from South-East Asia, which mimic ladybird beetles, Coccinellidae). Cockroaches of the genus Perisphaerus roll up into a ball like pill millipedes or woodlice when molested, but most depend on speed and agility to escape. One currently unnamed South African species of Blattellidae has greatly enlarged hind legs which enable it to jump like a grasshopper. This species hops between grass stalks and apparently specialises in eating bird droppings.

A few cockroach species possess active defence mechanisms. These include spraying repellent fluid from abdominal glands like Diploptera punctata or Archiblatta hoeveni and producing startling noises by expelling air rapidly through abdominal spiracles, as in the Madagascan hissing cockroaches (Gromphadorhina and their relatives).

Adult female Archiblatta hoeveni from Malaysia.

Studying cockroaches

Apart from extensive studies of the major pest species very little work has been done on cockroaches as a whole and they remain relatively neglected by both amateur and professional entomologists alike. The majority of species are easy to keep and breed in captivity and they don't require a constant supply of fresh leaves like stick-insects or live-foods like praying mantids. Why don't you start keeping and studying them? There is still a lot to discover!

References

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  Inward, D., Beccaloni, G. & Eggleton, P. 2007. Death of an order: a comprehensive molecular phylogenetic study confirms that termites are eusocial cockroaches. Biology Letters 3(3): 331-335 [Published online 5 April 2007. doi: 10.1098/rsbl.2007.0102]


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  Eggleton, P., Beccaloni, G. & Inward, D. 2007. Invited reply: Response to Lo et al. Biology Letters 3(5): 564-565 [Published online 14 August 2007. doi: 10.1098/rsbl.2007.0367]