Not only do cockroaches creep most of us out, they can be responsible for allergies or aggravating asthma. ­See more insect pictures.

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.



­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.

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.

­ 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.

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