Here’s the only figure in the article. It shows reconstructions of the ‘coxa’ (the first leg joint, where the leg joins the thorax) in green, and the trochanter, the first leg segment, in yellow. The coxa is the thread, the trochanter the screw. There’s also a nice scanning electron microscope image of the trochanter. The system works pretty much like your hip does – except the knobbly bit at the end of your hip has turned into a screw shape. (All the nerves and stuff run down the centre of the trochanter.) Most insect joints work like hinges.
The 3D reconstruction [(A) to (E)] of coxa (green) and trochanter (yellow) of left hind leg of T. oblongus. (A) Depressed position. (B) Elevated position. (C) Coxa cut horizontally along rotation axis; dorsal aspect of trochanter while leg depressed. (D). Isolated trochanter showing external spiral thread and tendon (t). (E) Dorsal portion of coxa corresponding to ventral portion of (C). (F) Scanning electron microscope photograph of the right metatrochanter showing posterior condyle (c) and external spiral thread (e).
So why doesn’t the leg unscrew itself? Most of the weevil’s leg motion will not involve a 360 degree rotation, or the hapless insect would get stuck after a couple of paces. Instead it will simply swing back and forth. Doesn’t the leg get blocked when it’s screwed in to the max? Presumably so, in which case careful observation should show weevils back-pedalling to unwind their legs. And conversely – why doesn’t the leg come unscrewed? The muscles appear to hold it in place. Phew!
Now why does the weevil have this odd arrangement? The obvious advantage is that you can rotate the leg right round. But in that case, why not evolve the axle/wheel combination? The authors speculate that the screw might be better:
We suggest that an advantage of this construction is that the leg comes to a stable resting position, preventing passive straining of leg muscles, which would not be accomplished by an axle construction.
Above all, they think that it might be the weevil’s unique feeding posture, where it shoves its rostrum (its ‘snout’) into its food, that holds the key. Substantial forces will be generated on the weevil’s legs as it tries to grip the substrate; having a screw would effectively block the rotating joint, stopping the weevil from ending up with its head smashed in the food.
If you don’t understand what I mean – try it; put your palms flat on the table, your arms braced, then put your face down onto the table; flex your arm muscles (the equivalent of the screw-lock) and you’ll be fine – let them go (the equivalent of having an axle) and you’ll end up with your head on the table… Most undignified – even for a weevil.
[EDIT – various points clarified after a hasty first draft! h/t Jerry and Adam M. You can sort the rest of it out yourselves in the comments…]