By the time that summer arrives the foliage of most trees shows signs of insect attack, but these little eruptions on the surface of an alder leaf are caused by eriophyid mites, which are not insects but are related to spiders. I think the mite species that has produced these is Eriophyes laevis inangularis.
Each of these little domes is a chamber that's formed when the mites feed on cells on the undersurface of the leaf, leading to uneven growth that results in the formation of a pouch where the mites can feed and breed.
This is the underside of the leaf, with the little yellow, sausage-shaped mites crawling around the entrances to the chambers, which are lined with nutritive cells that provide sustenance for the mites.
Here they are at higher magnification .........
............ and at still higher magnification, when the elongated body with four legs at the head end is visible in the mite in the top, left-hand corner. Each chamber is home to a brood of mites and a tree with a severe infestation could be covered with hundreds of thousands of them. Eriophyid mites also commonly infest sycamore and field maple leaves, producing large numbers of red pouches on the leaf surface.
These are three of the mites, each being about one fifth of a millimetre long, with only four legs.
The outer cuticle of the animal has a distinct pattern that differs between species, although the easiest way to identify species is via the symptoms that they cause on the host plant.
Here is the head, legs and cuticle patterning at higher magnification.
In addition to infesting sycamore, field maple and alder leaves eriophyid mites also attack many other plants, including goosegrass (aka cleavers) Galium aparine, whose growth is distorted by Eriophyes galii.
Typically, infested leaves curve inwards at the edges and become spoon-shaped, like the bottom, second-from-the-left leaf in this picture.
Here's the goosegrass eriophyid - the dark, globular structure top left is an air bubble on the microscope slide.
In this view you can see some of the surface patterning and an internal structure - perhaps an egg?-
... and in this plane of focus the surface pattern of the cuticle is apparent.
