Invertebrates
Cockroach
Blattodea · also called roach
The cockroach has an image problem it mostly does not deserve. Of roughly 4,600 species, only about thirty have anything to do with people; the rest are quiet recyclers of the forest floor. What the whole group does share is toughness — a flat flexible body, a startling turn of speed, tail sensors that trigger an escape before a foot lands, and cells resilient enough to shrug off doses of radiation that would kill a person.
Overview
Cockroaches are the order Blattodea, around 4,600 species, on every continent except Antarctica and most diverse in the tropics. Most are flattened, fast-running scavengers of leaf litter, rotting wood, caves, and burrows, and are important decomposers and prey. Only about 30 species live around humans. Genetics places the termites inside this order — a termite is a highly social, wood-eating cockroach.
Built to escape
A cockroach can run about 50 body lengths a second, senses the air pushed ahead of an approaching foot through tail appendages and starts running within ~15 milliseconds, and can flatten to slip through a few-millimetre gap. It breathes through body spiracles and has a distributed nervous system, so it survives injuries — including, briefly, decapitation — that would kill most animals.
Eggs and milk
Most females produce a hard, purse-shaped egg case (ootheca) holding a dozen or more eggs. The Pacific beetle cockroach is unusual in feeding its embryos internally with a milk-like secretion and giving live birth.
Useful, not just unwanted
Cockroaches are standard laboratory animals for nerve and locomotion research, inspire search-and-rescue robots, are farmed at industrial scale in China for medicine and feed, and are eaten by people in several regions.
Status
Cockroaches as an order are Not Evaluated. Household species are abundant worldwide, but many wild cockroaches are narrow-range specialists, and some, such as the Lord Howe Island wood-feeding cockroach, were driven off their main island by introduced rats and survive only on rat-free islets.
Cockroach: common questions
Would cockroaches survive a nuclear war?
They would tolerate the radiation better than we would, but the popular claim overstates it. Cockroaches can withstand roughly 6 to 15 times the radiation dose that is lethal to a human, because their cells divide less often, chiefly around each moult, and radiation damage does the most harm to dividing cells. That is a real advantage, but other insects, such as fruit flies and some wasps, are even more resistant, and a dose high enough would still kill cockroaches. In an actual nuclear event, blast, heat, fire, and the loss of food and shelter would be at least as deadly to roaches as the radiation. So they are tough, not indestructible.
Are all cockroaches pests?
No, and this is the biggest misconception about them. Fewer than one per cent of cockroach species have anything to do with human dwellings. The rest live in forests, deserts, caves, and grasslands around the world, where they are important decomposers that break down leaf litter, dead wood, and dung, recycling nutrients back into the soil, and where they are a major food source for birds, lizards, frogs, small mammals, and other insects. Some are brightly coloured, some are wingless and burrow, some tend their young, and some are found only on a single island. The handful of pale brown indoor species that give the group its reputation are the exception.
Why is a cockroach so hard to kill or catch?
Several features stack up. Its body is low and flexible, so it can squeeze into cracks and under objects and is hard to hit. Two tail appendages called cerci detect the tiny air movement of an approaching foot or hand and trigger an escape run within about 15 to 20 milliseconds, faster than a human can react. It can run very fast, change direction sharply, and climb sheer surfaces. It breathes through spiracles along the body rather than a single mouth and nose, and its nervous system is distributed, so it is resilient to injury. And the common pest species breed quickly and have developed resistance to many insecticides.
What is the connection between cockroaches and termites?
Termites are a specialised branch of cockroaches. For a long time they were classified as their own order, Isoptera, but detailed anatomical and genetic work showed that termites arose from within the cockroach family tree, from a wood-eating ancestor similar to today's Cryptocercus cockroaches, which live in family groups in rotting logs, share gut microbes to digest wood, and care for their young. Termites took that lifestyle further into full eusociality, with kings, queens, workers, and soldiers. Modern classification therefore places termites inside the order Blattodea alongside the other cockroaches, so it is accurate to say that a termite is a highly social, wood-eating cockroach.