Apparently, there is an advantage to having 6 legs in the insect world. What is that advantage, if anything? Why would such an advantage exist for insects, but not for other, larger land animals?

What do those middle legs do that the front and hind legs cannot, if anything? Are they really essential, or do they just generate more muscle power?

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    It seems like this question presupposes perfect adaptation. Many features of modern life-forms aren't necessarily 'better' than other possible features, but are the way they are as a result of historical chance and the constraints imposed by development. Would you be satisfied with the answer "the last common ancestor of all insects had six legs, and that worked well enough that it has been kept by most of its descendant lineages"? – bshane Oct 2 '15 at 5:41
  • Insects are not the ancestors of higher organisms. And BTW octopus is one organism which will be a counter-example. – WYSIWYG Oct 2 '15 at 6:21
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    This is a little semantically tricky, because not all insects use all of their 'legs' as 'legs', in the sense of 'a thing to walk with'. Consider the raptorial forelegs of mantises - the animal uses them to catch prey, not to walk. They are called 'legs' because of homology, not function. So if your question is 'why do insects have six things that they walk on?' then the answer is that often, they don't. In that case, there is no apparent advantage to explain. If your question is 'why six "legs"?', then the answer is 'by definition, from common descent'. – bshane Oct 2 '15 at 7:04
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    As it stands I agree with @fileunderwater -- this is a question about evolutionary advantages. However, if you were to make this question about the various functions that insects put their "legs" to then it is no longer a "duplicate" of that question. Even if this question is closed as a duplicate, you can edit it and it can be reopened :) – Luke Oct 2 '15 at 10:59
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    @bshane I disagree that this pre-supposes perfect adaptation. Rather I think we should focus on highly-preservedness. Very few members of Insecta have lost a set of legs. Why would such a body plan be highly preserved, when it has such obvious costs? It must offer broad-ranging adaptiveness. What would that be, exactly? – user151841 Oct 14 '17 at 16:44

Short answer
Six legs allow for locomotion, while maintaining a supportive tripod at all times.

Background
There are several million species of insects, all on 6 legs. This implies that any change in this number is promptly selected against. It is generally agreed that insects were derived from many-legged ancestors, e.g. centipedes.

One explanation is the tripod gait that results from having six appendages. This hypothesis, formulated more than 6 decades ago (Lanham, 1951), reasons that a reduction of the number of legs during evolution did not go further than 6, because locomotion of a small animal encased in a rigid exoskeleton is not effective with less than 3 pairs of legs. Insects generally walk by lifting the two outer legs on one side and the middle on the other side, sweeping them forward and placing them down together. Hence, insects support their rigid structures with a tripod at all times. Tripods are among the most stable configurations, and they never wobble (why on earth do tables have 4 legs?). Figure 1 shows an illustration of insect locomotion (Lanham, 1951).

insect tripod walk
Fig. 1. Insects' locomotion resembles a double tripod. Insects have a cyclic gait which consists of two phases, the stance phase and the swing phase. The stance phase is the power stroke, it pushes the body forwards in the direction of travel while the leg remains on the ground. Three legs are used is this phase by forming a tripod with the front leg and the hind leg on one side of the body and the middle leg on the other side. This formation is why this gait is known as the tripod gait. Source: Insect robotics.

Larger animals can afford to have less legs, because their vestibular systems have more time to maintain balance and adjust gait during locomotion. Because insects are so small, their strides are so quick that the nervous system may not be fast enough to deal with gait control. Instead, insects rely on the tripod gait to prevent any imbalance, rather than adjusting it. In addition, the exoskeleton effectively restricts small bodily movements to control balance. Larger animals such as mammals make small adjustments in their gait constantly to maintain balance. An insect has less opportunities to do so, because of their rigid exoskeleton (Lanham, 1951).

Reference
- Lanham, Science (1951); 113(2946): 663

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    Great answer! I'm going to go saw a leg off of all of my tables now, that sweet sweet tripod stability! – C_Z_ Oct 2 '15 at 16:40
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    [...cnd] I can also think of several alternative explanations for why 6 legs is useful for insects, which are not mentioned in this answer. To me, this answer is an interesting hypothesis, but it currently reads as an evolutionary just-so story. "*[R]eduction of the number of legs during evolution did not go further than 6, because locomotion...'" is another really strong statement that is not supported at all. – fileunderwater Oct 28 '15 at 9:45
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    @fileunderwater all the info is obtained from the Science paper, and the figure legend is from the linked robotics web page. There is nothing that I made up and nothing is unsupported. If anything is lacking from my answer, feel free to write a better one. No need to close a question, and especially not due to it being a duplicate of an abstract, general philosophical community question. If you don't like the answer, -1 will do, throw these questions open to allow others to share their views! – AliceD Oct 28 '15 at 9:46
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    @fileunderwater - Thanks for the downvote, but I asked for it didn't I :) Somehow I linked the wrong page to the reference - nice catch. I inserted the correct one (Science paper) and placed some direct references in-text. It's not open source, got it through uni library. Thanks. I'm not an evolutionary biologist, just very interested in it. Downvote away, shoot at interesting questions, close the best ones, whatever - I'll stand my ground 8) – AliceD Oct 28 '15 at 10:12
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    This article says that cockroaches use the tripod movement at low velocities (upto 1m/s) but adopt bipedal/quadripedal movement at higher velocities. – WYSIWYG Oct 29 '15 at 7:07

Apparently, there is an advantage to having 6 legs in the insect world. What is that advantage, if anything? Why would such an advantage exist for insects, but not for other, larger land animals?

"Legs" is a tricky term, with insects. Colloquially, in the terrestrial world, they are things that an organism walks on. But the scientific definition of 'legs', for insects, relies on homology: many insects have six 'legs' because they all descend from a common ancestor, which had six legs. The anatomical structures derived from those legs, regardless of whether they are functionally used as paddles, or as claws, or as feeding appendages or elaborate mate-signalling devices, all get called 'legs'. Referring to all of these as 'legs' makes as much sense, from a functional perspective, as referring to our arms as 'legs'. We only do it to keep the homology clear when talking about evolutionary relationships between species.

The diversity of uses to which insects put their legs makes it clear that there is no single universal advantage of having six - it is just that completely losing (or gaining) appendages is a difficult evolutionary process, so it seldom happens over evolutionary time. Regardless, some groups of insects (members of the Coccidae and Diaspididae, for instance) have lost their legs completely - an apparent adaptation to their obligate parasitic lifestyle.

What do those middle legs do that the front and hind legs cannot, if anything? Are they really essential, or do they just generate more muscle power? If it's an advantage either way, why don't bigger life forms like mammals have 6 limbs?

In the insect groups which use six legs for locomotion, AliceD's answer provides an excellent description of how all six legs are used. However, there really is nothing special which can be achieved with six legs, which could not be achieved with some other number of legs: mantises handle their locomotion perfectly fine on just four of their six legs, and arachnids (not insects, I know - but with many of the same design challenges such as a rigid exoskeleton) manage equally fine on eight.

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