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Question: Why do flies only live for about 24 hours?
Different kinds of flies live for different lengths of time. The difference is tied to their size. There is a rule known as the 'rate of living theory,' which gives us an understanding of these differences. It was found that, when comparing animals of a given kind, bigger ones live longer on average than smaller ones. One way to understand this is to consider the rate of chemical activity (metabolism) going on within an animal. Bigger animals tend to have a slower metabolism when they are resting. This measurement is taken as the rate of production of a metabolic waste product, carbon dioxide, or as the overall rate of heat energy produced during metabolism. Size difference also affects the heartbeat rate. All animals of a given kind produce about the same total number of heartbeats per lifetime, and smaller animals have a faster heart rate, using up their allotment of beats faster than do bigger animals, which have a slower rate. So, smaller things are faster, and get used up sooner.
This rule has exceptions, and people are one of them. We live much longer than would be predicted for a mammal of our size, compared to, for example, chimpanzees or gorillas. We don't really understand why this is, but we do know why for another exception, the kangaroo rat. Throughout its day this rodent periodically stops in a kind of trance, when its metabolic rate and heartbeat rate drop very low. So it isn't really using up its heartbeats on average as fast as you would think given its small size. Getting back to the flies; if there is one that lives for only one day, we would predict that it must be a fairly small one. A fruitfly, at only about an eighth of an inch long, lives about 40 days at ordinary temperatures, but has a shorter life in warm temperatures, which speed up its metabolic rate. So, the fly you mention must either be really very small or else live in a very warm place, or both. Maybe it's a no-see-um.
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