Why Insects Can't Grow Big?

Insects are class of invertebrates which present the greatest number of species on the world. They are relatively small compared to other invertebrates. But why they can’t grow as big as they’re shown in some movies, for instance? There are few reasons for that.

First of all, insects don’t have endoskeletons. Their defensive method for the body is one type of skin named chitin. Also, when they grow they change the old chitin with new bigger parts of chitin. During this process they are very vulnerable. The bigger the chitin is, the longer it takes to be replaced. Let’s say there is a crab with the size of an elephant. The crab will need a month to replace its chitin.

Other than that, they have a type of bloodstream that comes to organs only, not cells. If they were bigger their organs would be also bigger and they wouldn’t be able to feed themselves.

However, the biggest reason why they can’t grow big is their respiratory system. They don’t have lungs, but they breathe with a special system made of tubes called tracheae. For this, insects have openings on their bodies called spiracles, which are directed toward the inner body, where they continue into a network of tracheae. The air moves passively from the outside to the organs.

If they were as big as elephants, for example, their tracheae would be so long that the air would need too much time to get from the outside to organs.