Eriophyidae

The Eriophyidae or gall mites are a family of mites. They are plant parasites and produce galls.Storch, Volker & Welsch, Ulrich 1997. Systematische Zoologie. Fischer. 

The mites can cause severe damage to the host plant. They have only two pairs of legs and are very small, 0.08 mm - 0.3 mm.Füller, Horst et al 1994. Urania Tierreich - Wirbellose Tiere 2. Leipzig & Jena: Urania-Verlag, 70 & 88.  The gut is straight, there are no outgrowths of the gut. They have no heart or circulatory system.

 Some genera 
There about 270 genera and over 3000 species.Zhang, Z.-Q. (ed) 2013. Animal biodiversity: an outline of higher-level classification and survey of taxonomic richness. Addenda,  p129. Link

Some examples:
 Abacarus hystrix, Cereal rust mite
 Epitrimerus
 Aceria
 Eriophyes tiliae, lime nail gall

Other websites
 Cereal Rust Mite in Timothy 
 Eriophyes tiliae

 References 

Category:Arachnids
Category:Parasites