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Why are the legs of insects not always identical?

Why don't any mammals have more than 4 limbs? Why didn't they evolve to have multiple legs or arms like insects?

  • Answer:

    This applies to most vertebrates, not just mammals. For whatever reason, there was a point in the evolutionary process where 4 limbs became the ideal number of limbs (probably fins at the time). Ancestors of this creature or these creatures carried on the pattern of 4 limbs. Losing a limb that is unneeded is fairly cheap and doesn't take much effort. If you don't need a limb, it doesn't affect you much if it becomes smaller or weaker and it may actually be an advantage. Stretch this over countless generations, and you end up with whales, snakes, and a slew of other animals. Adding a limb, on the other hand (no pun intended), is pretty expensive. You're not just adding a single extra part, but a whole network of additional blood vessels, bone structures, tissues, and what-have-you. These all need additional nutrients and a good set of genes to ensure they work together properly. On top of this, you also need the limb to not be a disadvantage. It needs to be in a useful place anatomically and immediately offer an advantage as soon as the extra limb mutation comes into play. These mutations can occur, but it's vastly more likely that they're a disadvantage. Take frogs, for example. It's not altogether uncommon for frogs to develop extra legs. What usually happens, though, is you end up with a frog that's clumsier and slower and packing more meat. Predators tend to like that sort of thing, so the mutated frogs don't live long enough to produce offspring so that the extra limb(s) could eventually become useful.

Zane Magnuson at Quora Visit the source

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In addition to Zane's excellent answer, it's also worth thinking about whether there's something inherent in the vertebrate structure that makes it advantageous to have 4 limbs. In the invertebrate world, it looks to me that there are three main morphologies - no limbs at all, one pair of limbs per body section (e.g. centipedes and millipedes) or multiple limbs, usually paired, radiating from a relatively fixed central point (e.g. the thorax in insects, the head in cephalopods). The control of these limbs is either very simple, stable and centralised (e.g. insect walking patterns that move the legs as two "tripods" - two left and one right leg move forward in synch, take the weight and then the other legs move in the same way) or there is a big investment in the control functions (e.g. an octopus's arms have their own ganglia and sufficient processing power to operate independently if they are separated from the body, which some species will do as a defence mechanism). In vertebrates, the limbs are attached in pairs to points on a spinal column that is in itself a complex moving entity. The limbs are neither simple nor equipped with their own brainpower. In addition, the limb joints are complex and their integration with the spinal column involves special formations (consider the shoulder - multiple unique bones and a complex net of muscles and ligaments to hold together a contact area that in humans is about the size and shape of a teaspoon). Bipeds take years to learn how to use them and even quadrupeds aren't up and running immediately. A quadrupedal gait is also more dynamic than an insect's and It's not clear what an extra pair of limbs would add. Co-ordinating additional limbs in a way that would materially add to a vertebrate's survival chances is likely to be tricky.

Jason Whyte

as far as we know the fish body plan also has 4 limbs, or 4 fins. so it carried over to land animals. Adding a limb is hard for evolution so it had to get along on what it already had, and 4 limb was pretty good enough for the job

Vincent Maldia

There were no environmental pressures driving mammals to acquire more than 4 limbs. There would have to be a threat that was more disastrous for the offspring of mammals with 4 limbs.

John Harper

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