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Cockroach stomping a no-brainer

Everyone has had to take on a kitchen-corner cockroach, and chances are you failed to flatten your quarry. Dr Karl has been down on all fours to work out the cockie's elusive ways.

Closeup of a dead, broken cockroach

(Source: iStockphoto)

It is summertime, and the living is easy. But the cockroaches are very difficult to stamp on. Why is this so?

One reason is that cockroaches can sense the air currents created by your moving foot or, if you're not particularly squeamish, your moving hand.

The other reason is that the cockroach's brain is not involved in its escape.

Cockroaches have been around for around 250 million years. Today, there are about 4000 different species around the world.

About 500 of these species are native to Australia. But the five or so species of cockroaches in your house are not native Australian. They came from overseas.

The so-called German cockroaches are about 10–15 millimetres long. They are probably the most widespread insect pest in the whole world.

They like warm and moist environments, like kitchens and bathrooms. Under ideal conditions, a colony of just five fertile females could produce 45 million cockroaches in just a year.

The other common household cockroach, the American cockroach, is a bit bigger (28–44 millimetres long) and prefers warm climates.

But the largest cockroach in Australia is not an imported household insect; it's a native bush-dwelling one.

The giant rhinoceros roach from tropical Queensland is a wingless burrower, up to 80 millimetres long and weighing 30 grams.

The household cockroaches are superbly adapted to the quick getaway from a threat.

It's important to realise that cockroaches are a lot smaller than we are. So down on their size scale, they sense the world as a windy and turbulent place.

Now they have special sense organs for detecting wind. But these organs are tuned to sense the wind associated with an incoming threat, and to ignore the random winds associated with nature.

They actually have two of these wind-sensing organs on the back end of the cockroach: one on each side of the centre line.

These organs look like little sticks, poking up into the air. They are called "cerci", and each "cercus" has 220 hairs. Each hair is very fragile and is readily moved in its socket by air currents.

Depending on which way the air currents move, the little hairs on each cercus move in different directions, and this gives different electrical signals to the cockroach's nervous system.

To save time, these signals do not go all the way to the brain, and then all the way back to the legs. Instead, these signals go straight to the back legs. These nerves are large and the distance is very short, so the electrical information travels very rapidly.

On average, the time from the arrival of the puff of air, to when the legs start moving, is about 40–50 thousandths of a second. Very quick indeed.

All of this means that to save time, the decision to start running away is not made by the cockie's brain. Instead, it's made by a little mini-brain near the back legs.

In fact, here's a whole series of these mini-brains running along the centre line of the cockie, all the way from its back end right up to the head (where the main brain is).

These little mini-brains are a bit like the spinal cord in us humans.

So, cockroaches respond very quickly to moving objects. That might be why one of the big car companies asked Russian insect scientists to provide them with 100,000 cockroaches.

They wanted to study the cockies, so they could design intelligent avoidance systems for their cars.

What all of this means is if you want to stamp a cockroach you must do it from practically straight above, and along the centre line of the cockroach's body.

If you do this, the wind current does not come from the left or the right, nor from in front or behind. So the cockroach doesn't immediately know which way to go.

Something like this happens with our human nerves. That is, the decision to move is made not by the brain.

When you accidentally touch, with your hand, something hot (or sharp, or dangerous), the electrical signals go to your spinal cord, which then sends back electrical signals to your bicep muscle to pull your hand away.

Only later, as a courtesy or information note, is an electrical signal sent to your brain saying: "Hey Lola, you've touched something hot — again. Do you mind not doing this, and damaging our hand? Best Regards, Your Spinal Cord."

Tags: animals, biology, microbiology, invertebrates-insects-and-arachnids

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Published 02 February 2010

© 2014 Karl S. Kruszelnicki Pty Ltd