Why Weren't the Insects Moving?

My herbaceous borders contain several stands of plants that grow to about five feet in height and have lots of big yellow flowers. The insects love them. They’re obviously good for feeding, collecting nectar, or whatever insects do on flowers.

I went out this morning and saw that lots of the flowers had bees on them. But they weren’t moving – at all. And then I noticed that other flowers had other insects on them, and a couple had butterflies. All completely still; they might have been made of plastic.

I’ve never seen that before. It felt as though I had walked into a scene in one of those sci-fi films in which the protagonist finds himself in a world where time has stood still. It seemed unreal.

I wondered whether the insects were all dead, and touched a couple of the bees. They moved, just slightly, and then stood still again. When I went out again a couple of hours later, everything was back to normal.

So was it because the air was cool and damp? Were the insects in some sort of semi-hibernation, or something? If anybody knows about these things, do tell.