I don’t know much about crane flies or flies in general, but why do crane flies have such long legs? They seem like such a hindrance, and I doubt they’re much help when trying to land on walls or walking. They seem much too thin to draw the attention of predators away from the main body, and it seems they slow down the insect in the air. What purpose do their (almost comedic) long legs serve? It seems like they’d work much more effectively if their appendages were shortened…
First is helping with laying eggs, second is of course they’re a first thing predators catch and they’re very easy to loose, so it’s a major point of having them. Also they’re handy for sitting in grass.
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Questions about purpose are always tough. Any suggestions will be hypotheses in the absence of data (but hypothesizing is good and fun! as long as one knows it’s not an actual explanation). And data that get at purpose are very hard to come by.
The evolutionary path that leads to any one particular trait will almost always have involved a bit of natural selection, a bit of a random component, developmental constraints (overall genetic architecture that creates the body during development from single cell to adult), physical constraints (physics limits what’s possible), and historical constraints (what came before limits what can become). For any particular trait, it’s always possible that some other trait might be a bit better but not possible due to these constraints. Sometimes, what was beneficial in the past is less than ideal today (due to environmental changes) and evolution just hasn’t caught up yet. Generally speaking, an organism’s set of traits was adequate enough for their ancestors to produce enough offspring to replace themselves (taking into consideration that some offspring will die before reproducing). But each individual trait has to function in combination with all their other traits, so there’s this huge tradeoff among all the traits. The most adequate leg length (there is no ideal) would be the length that allows the organism to collect enough food, avoid predators long enough, to hold onto a mate, and to maneuver about their environment such that they reproduce ever so lightly more than members of the same species with a different leg length. All of the above are why they have such long legs (in combination with all those constraints).
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specifically to freak me out!! (one of the few bugs ill run away from, lol.)
I bet the legs being easy to live without is a good benefit, as melodi_96 said. interesting to think about the limits of adaptation and evolution, though.
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Perhaps the legs act like trip wires when the cranefly is at rest. A predator running over the surface is likely to touch a leg before it gets close enough to bite the body, and that gives the fly time to lift off.
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A very good and succinct summation. I would only add one thing. The trait is not a disadvantage. Human knees could be better engineered, but for the reasons you outline above, and because they are not a significant disadvantage to reproduction, they persist.
My standard response to questions phrased this way… Crane flies have long legs because their parents had long legs. It’s a bit snarky but a good reminder that organisms aren’t built from a vast collection of traits with some plan to take on the next generation’s challenges.
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I understand all answers in line with “no aim in evolution”, but it relly doesn’t help and ordinary (or any) person just knowing things are because they are, we can answer everything with one easy answer.
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I have not been able to find any research papers on the topic (tipulid ecology is very very understudied, so I bet no one knows) but I strongly suspect their legs are antenniform and serve some chemosensory function (in slow motion videos I have captured they constantly palpate objects with their legs while flying).
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I think it does help or I wouldn’t answer this way. Often, I get a strange look or a followup question to my snark. That’s when I can explore how various parts are used by the organism. And how that may or may not be a factor affecting reproductive success. I try to be very careful when explaining evolution topics because it is so easily misunderstood. “Why” questions are probably the worst/hardest. We rarely have enough evidence to say why something is like it is. It’s a difficult thing to test. We can talk about how it’s used. And we can speculate about adaptation. Other biologists understand shorthand language but general public may not.
There’s a quote I heard once, can’t find the source, that I think about often. Simple answers to complex problems are elegant, understandable, memorable, and often wrong.
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Sure we have to say that why is not the best way to phrase questions we have, but it’s still a too easy way to just say they’re long because specimens like that survived, it’s kind of a base of this question and doesn’t really answers e.g. what OP wanted to know and again, it can be an answer to any question and doesn’t provide any greater knowledge and oversimplifies the process as it’s not that chaotic as it’s taught and there’re always some causes to it, even if they were legitimate thousands years ago, in this case we can speculate that first cause of longer legs in all Nematocera was working for all of groups in it and it’s interesting to think about, should be something with their way of laying eggs, especially with many doing it near or in water or in soil, in both cases being higher above the ground has some advantages, but those were more primitive groups and their usage of legs diverged, e.g. mosquitoes clearly using those legs not only for laying eggs in the water, but for biting, or the other way their way of biting became possible because they already had those long legs. So to summarize again I don’t want to sound as if those answers were wrong or bad in any way, but they leave away a lot of stuff that could be said and could be learnt by asker, and spark reasonings and talks about different matters, more global and philosophical while we lack in discussing specific groups.
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Not to get dragged into a big messy debate or anything, but I have to say I agree with melodi_96. It is indeed often difficult to test the evolutionary benefits of a given morphological feature, but saying that long legs exist due to their ancestors having long legs or due to it not being detrimental enough to be weeded out is in my opinion basically a fancy and extremely roundabout way of saying “it exists because it can, but who knows why”, which is true but doesn’t really answer the question.
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Arguably it’s simpler and more concise to say “the evolutionary benefits are too understudied to be known for sure, although it is likely that A, B, or C may be possible functions/reasons”.
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I don’t really think we’re having a big argument here. 
we’re actually agreeing on most things. I also think most of the difference of opinion is how we choose to type a written answer in a forum. What do we emphasize? Do we supplement previous info or will this response stand on its own? Do I type all this thought or boil it down because I type slow?
If we were sitting around talking this would never be a problem. Anyway, I’ve learned a little more about crane flies.
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So you are telling us - your simple answer - is wrong? He has his father’s legs …
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Heh. Maybe. It certainly isn’t a complete answer.
I think someone reading the whole of this thread will have a decent understanding of the complexity of the issue and come away with a few possibilities of evolving really long legs. They will probably understand the limits of our knowledge too.
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I figured it wasn’t an argument but was attempting to avoid it from escalating into one. In any case, fair point about “biologist shorthand may confuse nonbiologists”.
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It is tempting to say “you asked the wrong question,” then explain what the question should have been and answer that. So, the simple answers are because God created them that way, or they evolved that way, or they inherited that trait from their parents. Or one could get into the whole Panglossian explanation about how the long legs may have a function or may just me a mutation that didn’t significantly decrease the likelihood of reproducing.
I think the real question was “how are long legs useful to crane flies,” and the answer is no one here really knows, although Melodi_96 did point out some possible advantages. Based on the number of five-legged crane flies I have seen, I’m inclined to agree with the idea that they are the first thing predators catch, but further research or citation needed.
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The question was a good one, and all the answers I’ve read so far are good responses that, together, start to tackle such a simple yet profound question.
One thing that I learned (or relearned, probably) is that most adult crane flies don’t eat. So they don’t use their legs for obtaining food. Escaping predators (or a bad environment) and reproduction would be the only? other purposes they could serve. I like the “predator escape” hypothesis which would explain why they pull off so easily. But almost all traits serve multiple roles…and almost all traits can be detrimental at times (for example, they may indeed slow them down when flying but their other benefits may serve as an adequate tradeoff. As long as all the pros outweigh the costs, the trait can continue on to the next generation.
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To add another philosophical layer, and with no disrespect to the OP, questions like this are a form of reductionism. In essence, they are an attempt to explain something in simple, concrete terms, to find the reason why something is the way it is. Human beings are notoriously unable to function well with randomness, contingency, in life. We are more comfortable with order, predictability, the sense that there must be a reason why things happen or are the way they are. An answer like ‘that’s just how things are’ leaves us unsatisfied. But the world is not black and white, yes or no. There is a lot of grey and blankness involved as well. Somehow I got used to living with randomness as a concept in life many years ago, so I just accept that crane flies are the way they are without trying to sort out why they are that way. Their design seems stupid if I think about it but so are many things in life. Mayflies are strange things, but it works for them.
I’m not trying to convert anyone - it’s just how I’ve learned to approach life, and thought I’d throw it out there.
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