The family of the Eriophyidae is the largest of the Eriophyoidea, gall mites. Gall mites are without exception phytophagous and live on the outside of the above-ground parts of plants. They are distinguished from all other mites by having but two pairs of feet. They are fusiform to worm-like and very small, 0.2 mm on average. This minute size enables them to live within the length grooves on the upper-side is grass leaves, below the stellate hairs at the underside pf plants like olive or Eleagnus, within the leaf sheaths of grasses and sedges or the needles of conifers, or, for the hibernation, behind bud scales. In a few cases, like in Eriophyes pyri, the doy not live on the plant surface but internal, in the sponge parenchyma.
Like in many mites the mouthparts are arranged for piercing and sucking. Gall mites pierce the cell walls of individual epidermis cells and drain them. When the epidermis cells have thick walls, these remain upright and the cells are filled with air, which give the plant’s surface a silvery appearance. In other cases the epidermis cells collapse, losing their protective function for the tissues below. There lose moisture, get discoloured turn brown or die off. Leaves may change shape, their margins curling in.
In preparation of the piercing of the cells, salive is secreted to weaken the walls. In some cases the plant reacts to substances in the saliva by abnormal growth, causing galls. Often erinea are formed in this way: fields with an abnormal hair cover, between which the mites are hiding. But also more or less pouch-like structures may be formed, housing the mites.
Gall mites that live free on the plant (generally at the underside of the leaves) that in high densities may cause russeting, but not the development of typical galls, are called “vagrants” in the English literature.
In a number of species, next to males and normal females, a second type of females does occur, deuterogynes, that are specifically adapted to dispersal and hibernation. Deuterogynes are morphologically different from the protogynes.
Amrine & Stasny (1994a), Farkas (1965a, 1966a), Michalska, Skoracka, Navia & Amrine (2010a), Nuzzaci & de Lillo (1996a), Pećinar, Stevanović, Rector & Petanović (2011a), Petanović (2008a), Rančić & Petanović (2002a), Rancic, Stevanovic, Petanović ao (2006a), Skoracka (2006a), Skoracka, Lewandowski & Boczek (2005a), Thomsen (1976a, 1988a).