Why insects are so small?

Have you ever wondered why there are no giant insects ?

Today’s scientist wonder why insects are not as big as they were in the Prehistoric age. Yes, rightly read, about 300 million years ago there were insects about the size of birds or animals. Ants were the size of hummingbirds and dragonflies were about the size of Hawk.

Oxygen is the biggest reasons behind the small size of today’s insects. Today our atmosphere has changed. Millions of years ago, the air surrounded our planet was warmer, moister and oxygen rich. During the Carboniferous and Permian periods, Earth’s air contained 31-35 percent oxygen, as compared to just 21 percent oxygen in the air today.

Scientist have found that today insects don’t have enormous breathing pipes which can actually fill their bodies with the abundant amount of oxygen which is required for their growth.

Today insects have simple breathing system which are perfect for their sizes. Their breathing apparatus works fine for their small sized cells. They inhale oxygen and get rid of carbon dioxide slowly by simple gas diffusion across the cell membrane.

In insects oxygen is not transported to their bodies via blood .They take in oxygen and expel carbon dioxide through holes in their bodies called spiracles. These holes connect to branching and interconnecting tubes, called tracheae.

Whereas humans have one trachea, insects have a whole tracheal system that transports oxygen to all areas of their bodies and removes carbon dioxide. As the insect grows, tracheal tubes get longer to reach central tissue, and get wider or more numerous to meet the additional oxygen demands of a larger body.

Insects can limit oxygen flow by closing their spiracles. In fact, one reason insects are so hardy is that they can close their spiracles and live off the oxygen they already have in their tracheae.

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