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People think Crane flies have venom, but no fangs to inject it with.
Those people are idiots, because of the simple fact that evolution isn't that stupid.
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While the urban legend is false, your line of reasoning isn't necessarily true.
You can't easily 'evolve' out of superfluous traits if they're not a hindrance. It's possible that At one point crane flies had long fangs which they used to inject venom with, but the long fangs were evolved out of (For whatever reason it was economical to have shorter fangs) but the venom sacs themselves weren't necessarily harmful to the survival of the species. This never happened, and it's unlikely to have happened, but it is within the realm of possibility.
For the record I've heard the urban legend most commonly said about daddy long legs, which contrary to popular belief, can pierce human skin but carry sub-standard venom for an arachnid. So yeah.