Entomologists estimate that there are between 5 and 10 million species of insects on Earth. But if asked which insect they hate the most, many people would have no trouble choosing just one -- the cockroach.

There are plenty of reasons to dislike cockroaches. Their flattened bodies, leathery wings, skittering legs and long, waving antennae give some people the creeps. Because roaches eat garbage and waste, they can spread bacteria like Salmonella and Shigella from place to place. As they walk, they leave trails of fecal matter, which they use to find their way around. On top of being gross, these trails can cause stain­s and odors. The proteins in cockroach saliva and waste can also cause allergies and aggravate asthma.

standard household cockroach
Image courtesy stock.xpert
Not only do cockroaches creep most of us out, they can be responsible for allergies or aggravating asthma.

People also hate roaches because they can be extremely difficult to get rid of. One reason is because of their natural behavior. They reproduce quickly and are hard to kill. Since they're nocturnal, many people don't notice their presence until there are so many that they've run out of places to hide. Roaches are particularly good at dodging and running from shoes, newspapers and other weapons, and several species have become resistant to insecticides.

german, american, brown-banded and oriental cockroaches

But of the 4,000 roach species that exist in the world, only a handful of them plague homes and businesses. These pest species include:

  • Blatella germanica, the German cockroach
  • Periplaneta americana, the American cockroach or palmetto bug
  • Supella longipalpa, the brown-banded cockroach
  • Blatta orientalis, the oriental cockroach
In fact, in many parts of the world, just one species -- the German cockroach -- is responsible for most infestations. Unfortunately, people take much of the blame for this worldwide prevalence. Most cockroach pests have spread across the planet by hitchhiking on boats, airplanes, trucks and even in moving boxes and grocery bags.

Chewing Cousins
Cockroaches are related to other types of insects that are similarly helpful in some situations and harmful in others.
  • Termites, their nearest relative, can destroy people's homes and property. But in the wild, their digestion of wood and cellulose creates nutrients for plant life. Termites are also food for mammals, reptiles and other insects.
  • Mantids eat a range of pest insects and can even be used for pest control. But they also eat helpful insects, including other mantids, as well as small frogs and hummingbirds.
  • Grasshoppers digest plant matter and add nutrients to the soil, much like termites. However, in large numbers, they can destroy food crops and grazing land.

­ While Blatella germanica and a few others make nuisances of themselves, most species of cockroach generally mind their own business. Many cockroaches live in warm, tropical areas and feed on decaying wood and leaves. They help break down this organic debris; in the process, they add nutrients to the soil through their waste. They're also a food source for small reptiles and mammals. In other words, in spite of their bad reputation, cockroaches are ­an important part of many ecosystems.

Whether they're digesting wood pulp in a rainforest or hiding under a refrigerator, cockroaches are fascinating. They're primitive insects -- they existed millions of years before dinosaurs did and have evolved very little since then. In spite of their unchanging nature, they've survived when other species have not. For example, dinosaurs became extinct 65 million years ago, but cockroaches have thrived for 320 million years. We'll look at the physical features behind this uncanny survival next.

Everything Bug
It's easy to think of bugs as annoying, but insects are behind a lot of the planet­'s natural recycling efforts. To learn more, tune in to Animal Planet's Bug Week.