PLANET EARTH Caves
And while they're up here the bats produce something very important. This hundred meter high mound is made entirely of bat droppings - **guano **（粪便）. Its surface is covered by a thick carpet of **cockroaches **（蟑螂）, hundreds of thousands of them. Caves are one of the few habitats on Earth not directly powered by sunlight. In the absence of plants this food chain is based on a continuous supply of bat droppings.
The cockroaches feed on the guano and anything that falls into it. The droppings also support other types of cockroaches which spend part of their day resting on cave walls. These in turn become food for giant cave **centipedes **（蜈蚣）, some more than 20 centimeters long. Bizarrely, there are crabs here, too, sifting through the droppings for nutrients.
All these animals spend their entire lives within the cave. They're totally dependent on the digested remains of food that's brought here from outside. Each evening in just two hours three million bats leave the safety of the cave to hunt for insects in the forest outside. But not all will return. As they leave the cave the stream of bats form a **doughnut **（甜甜圈）-shaped ring.
