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Hello!

I think the long legs are simply an adaption to the grassy habitat of many Tipulidae. Long legs are very useful to grip somewhat distant halms. Also other insects and arachnids, which live in such habitats, have very elongated legs (harvestmen, plume moths, some true bugs, locusts etc.). That seems plausible to me. I often see Tipulids or harvestmen "hanging" on grass halms with their legs.