Every leaf is a mite metropolis

Insects begin to attack tree leaves almost as soon as they emerge from buds in spring. Some eat foliage but others turn leaves into secure homes.......... 

......... like these little red galls on sycamore leaves produced by a microscopic mite called Eriophyes macrorhynchus aceribus.

These little eruptions on the surface of an alder leaf are caused by another eriophyid mite, Eriophyes laevis inangularis.

Eriophyid mites are not insects but are related to spiders. 

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. 

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.