Why did a lot of common insects evolve such a thin waist?

Question

Something that doesn't quite make sense to me is why lots of insects like ants, bees and wasps have a such a small petiole when it connects many major organs to the rest of their body from a suddenly much wider abdomen. Normally from an engineering standpoint, you don't increase the width of a cantilever from a joint because the torque applied to a joint already increases with the length of an overhanging segment, so increasing the width would only multiply the torque that joint or opposing segment needs to balance. It could have something to do with ants being so small that large torque isn't an issue in proportion to their body size, but if it wasn't, then why do their legs also get thinner as they measure further in length from their body?

Answer 1

Very interesting question! The morphological diversity is one of the reasons that insects are one of the most evolutionary successful animals. From an engineering standpoint, I can't give an exact answer in terms of the physics/aerodynamics of how they move (however I am sure there is a lab that has asked the same question so a search in google scholar might be successful).

However, one thing to consider is the internal anatomy. Insects have an open circulatory system, this means that their hemolymph (the insect equivalent to blood) doesn't require specific vessels for transport. Instead the hemolymph essentially circulates freely to around the organs to transport nutrients and hormones, although winged insects use veins in order to maintain structural rigidity in the wings. Their digestive system is fairly simple as well, so only a single "tube" of the gut needs to go through the waist since most of the organs will be centralized in the abdomen.