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I saw an adult caddisfly last week performing some unusual (to me at least) antenna behaviour.

It was resting on the outside side wall of a greenhouse, sitting in the usual position with both antennae fully outstretched in front of it. I noticed it was intermittently rotating its right antenna a full 360 degrees. By that I don't mean keeping the antenna straight ahead and describing a small circle with the tip, it was physically bringing the antenna back behind its head and back round straight in front of it in a circular motion, a bit like a cowboy twirling a lasso above his head. The difference being it would only rotate it once and then stop for a few seconds and then do it again. It did it very quickly, almost like a physical tic.

It was only doing this with its right antenna, but I noticed the left one (which was straight in front of it the whole time) was only about 40% as long as the right. So possibly this was some sort of compensation thing, after losing some of its left one, but its position otherwise was very much of an insect at rest. Maybe it was trying to remove an irritant ?

Is this something you've ever observed on a caddisfly, butterfly or moth ?

I should have taken a bit of film of it doing it.

Just educated guessing here but perhaps it was trying to pick up pheromones from a potential mate? Moving the antennae would probably allow it to get a better idea of what direction to fly in if it did pick any up.

That's a very interesting suggestion Bugboy, thanks. Each full rotation was performed very quickly, in a small fraction of a second, but that might be all the time it needed to pick up pheromones and assess direction. I watched it perform the twirl 5 or 6 times in the space of 30 seconds after which the rate slowed down. It really gave me the impression of having a physical tic, but I'm sure it was doing it for a reason rather than involuntarily.