Bug free

日期:2017-06-18 04:35:19 作者:濮捆箅 阅读:

By Philip Cohen INSECTS could have ruled the seas as well as the land if they had learnt how to hide in water, a British scientist has concluded. He argues that some of the adaptations that made insects so successful on land made them easy prey in the deep ocean. By some measures, insects are the most successful group of animals on the planet. Entomologists reckon there are at least 5 million different insect species. Some thrive around ponds and rivers. Others survive extreme heat and desiccation, or tolerate freezing temperatures. Yet others flourish in salt lakes. “Compared to all that, you would think life in the open ocean would seem pretty cushy,” says Simon Maddrell, an insect physiologist at the University of Cambridge. Yet not a single species is known to spend its life on the ocean wave. A solution to this mystery occurred to Maddrell when he realised that salty lakes lack something oceans have in abundance: fish. In the sea, crustaceans—distant relatives of insects—escape being eaten by fish by diving during daylight hours to hide in the dark. But that option isn’t open to insects, Maddrell argues in the current issue of The Journal of Experimental Biology (vol 201, p 2461). So while they can survive in a fish-free salt lake, they would not stand a chance in the open ocean. The respiratory system that serves insects so well on land makes them easy prey there, he says. Insect cells exchange oxygen directly with the air using a network of tubes called tracheoles that spread throughout their bodies. This highly efficient system allows insects to continuously use powerful flight muscles without running short of oxygen. The evolution of insect flight, Maddrell points out, helped insects to spread rapidly. But tracheoles also make them completely dependent on gaseous oxygen. Maddrell says that if insects descended a few dozen metres below the surface of the ocean, any air bubbles that they brought with them would collapse and the whole system of gas-filled tracheoles would fail soon after. If, like some small crustaceans, they stayed near the surface and evaded predators by becoming transparent, the bubbles of air inside them would reflect light, making them as visible to fish as tiny beacons. “They don’t have any way to escape,” he says. Insects could have evolved adaptations for deep-sea diving, says Maddrell, but it would not have been easy. So why can’t an insect simply hover above the sea all day? Many flying insects are grounded during daylight because the air can be just as full of predators as the water,