Why do crane-flies lose legs so easily?

I've often wondered why crane-flies (like the individual that you can see if you click here) have a tendency to shed limbs so easily. This one, that found its way into our conservatory, has a full set of six but I've often seen individuals with half that number, still busy laying eggs in autumn. Maybe, for an insect that skims low over the grass to lay its eggs amongst grass roots, there's a benefit in being able to lose a limb because at this time of year there are spiders' webs everywhere. Perhaps being able to escape at the expense of a snared leg and maybe finish laying a full complement of eggs has some natural selective advantage in these insects. One way to find evidence, I suppose, would be to inspect spiders' webs for detached crane-fly legs - left by the ones that got away.

This individual is a female, identifiable by its pointed abdomen, and I think it's Tipula paludosa .... which, before it found its way into our conservatory, was probably laying eggs in my neighbour's lawn where its larvae (leatherjackets) will feed on the grass roots - if the starlings don't find them.

In this photo you can clearly see the paired appendages called halteres behind the wings. They move up and down in opposition to the beating of the wings, damping out the vertical oscillation that would otherwise result from the wing beats and blur the insect's vision, and also providing a way of sensing changes in direction that the wings can respond to. 

More on crane-flies here