Plant galls are everywhere. Zillions of hungry insect grubs are ready to use leaves first as a home and shelter, then a place to have a feast. In doing so, the poor host responds by making a gall in response to the chemically-induced "hacking protocol" between the attacker and the host. The first tries to persuade the host of its innocence and tells it only wants a friendly place to live, it won't start late-night parties, and it won't be a bother. The latter, being suspicious by its very nature of a stationary existence, raise its hackles and starts to build protective walls around the point of intrusion. Some gall-inducing insects only hatch in the gall then exit to the outside and fly away, while others devour the leaf to a large extent, making the plant exude more chemicals in an attempt to make the intruder go away. It is a micro-scale war surging back and forth.
An example, the galls of the insect Euura proxima on leaves of the Crack Willow Salix x fragilis. I collected the galled foliage on my current trip, as I recalled Ian Watson's earlier post, and made a quick snapshot of the galls. The tiny grubs inside looked pretty and blue under my 14X hand loupe, but are too small for my 105mm MC to make anything sensible portrait, so I'll bring them back to the studio when I return from my trip. If they survive and haven't shrivelled up.
