[eli5] Why do insects have lots of legs while larger animals have 4 or 2? Also, why aren’t there any animals (that I know of) with 3 legs?
In: 381
Warm blooded animals have a better sense of balance than cold blooded animals. Balance works better if reflexes react at a predictable speed. A constant body temperature makes reflex speed somewhat constant. Gravity is constant so how fast you fall over won’t change with temperature. Some cold blooded animals like insects and spiders use extra legs for balance instead of relying on their brains for balance. They also tend to have a wide stance. Many reptiles also have a side stance.
A kangaroo can use its tail as an extra leg, but not while traveling fast (hopping).
Technically, whales and dolphins have 3 legs. But they started as four-legged animals and their 2 back legs fused together for swimming.
And what about the Wild Haggis. 3 legs, 2 of them shorter than the other for stability on the highland hills.
My conclusion on the amount of legs spiders and insects have, after seeing many spiders with missing legs, is that its great to have some legs you can afford to lose and keep functioning, since they are so fragile.
There used to be a three legged dog that we often saw at a nearby park. Our dog used to casually stroll up to it and push it over. Every single time.
The first newt I found had 5 fully functioning legs, no idea why or how. (my dad can vouch for this story lol).
I remember thinking all the ones I later found with 4 legs must have had a bad accident to lose a leg.
Fun fact: kangaroos have 3 legs, in that their tail bares weight and contributes to balance in the same way a leg does (unless you count their front legs, in which case they have 5 legs)
Fun aside for you OP, the anime “A Centaur’s Life” is set in a universe where all mammals evolved from a common ancestor with 6 limbs instead of 4. So people are centaurs, winged humans and such.
In the show they actually ponder how the world would be different if their mammal ancestor had 4 legs, like ours. Their conclusions are entertainingly wrong.
Great question! We don’t know why vertebrates got a second pair of legs, since the front limbs evolved from fins, which evolved from gills, but a mutation happened and a second pair of lateral fins showed up near the tail for no reason. More fins might be better, but fish can’t just get more fins.
Insects are the opposite: they, like miriapods and crustaceans, evolved from animals with multiple segments and a pair of legs on each segment who could add segments and legs to their body easily.
And insects don’t always have 6 legs, they have many! As caterpillars insects have many legs, which they lose as the segments fuse together during the final stage of metamorphosis. The head segments get antennae and mouth parts, the thorax segments keep their legs and the abdominal segments lose them all.
ELI 5 on biology in general: it’s not entirely possible to answer the deep down why of many questions, sometimes there is no real reason that things happen other than: that body plan or evolutionary step happened to work out for that species and they didn’t die off before spawning.
We can talk about things we’ve found in nature, but we can’t really speak to why things worked out that way other than if there was any possibility for trilateral symmetry it was killed off so early that it wasn’t able to evolve.
The coolest bits of evolution are probably cephalopods where they have 9 brains enabling them to be very clever with their tentacles. Just imagine what we would have looked like if they’d evolved bones and made it to land before our ancestors.
Keep in mind “Survival of the fittest” doesn’t mean “in the best shape” it’s more: whatever fits their niche the best. Plankton, for example can’t do much of anything, but they’re still around because they breed in such big numbers that they can’t all get eaten. Insects tend to be incredibly stupid by our definitions, yet they survive with such diversity because of how they breed. Cicadas, for instance are massive and don’t even act like they want to live, but because they all pop out once every couple decades and lay eggs like crazy, they continue to exist. Evolution occurs through breeding and random mutations along with trait combinations over hundreds of generations…it can also happen quicker through artificial selection, like how people have trained dogs from wolves (and even then they aren’t separate species, the general definition of which being two groups of animals that can’t breed…and even for that rule there are exceptions like horses and donkeys and zebras).
Latest Answers