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Methods used for sample storage, specimen clearing, slide mounting, species illustration and morphometric description in alpha-taxonomic studies are essential for the Eriophyoidea. Eriophyoid mites are very tiny and delicate, for which truly permanent specimen slides currently cannot be prepared, resulting in eventual loss of material, including type specimens. Often, published descriptions and drawings have not achieved the required level of quality, and thus many relevant taxonomic details have been permanently lost or neglected. These shortcomings can make certain identifications impossible and cause significant confusion. Consequently, there is a considerable need for accurate and uniform descriptive and illustrative data for the Eriophyoidea. Based on their expertise on this topic, the authors provide guidelines and advices, assisted also by illustrations, of the main critical aspects in managing eriophyoid mites in order to supplement and improve techniques for handling and preparation of specimens, and for improving their taxonomic study. The effects of the short- and long-term preservation methods (i.e., fresh, dried and liquid preservative choices) on digesting the internal tissues of the mites are discussed. Clearing and mounting procedures are analyzed, and special tips are suggested for handling mites and designing tools needed during these steps. Methods for recovering specimens from unsuitable slides (i.e., undercleared and overcleared specimens) are proposed and described. Techniques and tricks to produce descriptive line drawings of good quality are highlighted, and the content to include in plates is stressed. Finally, detailed instructions for standardization of measurements are given.

Eriophyoid mites are excellent candidates for ethological research using the approaches of behavioural ecology and sociobiology. These tiny haplodiploid mites are highly specialized plant parasites, producing galls, forming nests, inhabiting refuges or living freely on plants. They reproduce via spermatophores deposited on a substrate and without pairing, which is a fascinating, though still poorly understood, mode of reproduction widespread in some groups of arthropods. Eriophyoid males can be involved in external sperm competition. In some species they also guard pre-emergent females and deposit spermatophores beside them. Although slow-walking, the minute eriophyoid mites can disperse for long distances on air currents or specific animal carriers. After landing on a plant they can distinguish between suitable and unsuitable hosts. Biological observations on a deuterogynous species indicate that parasociality could occur among eriophyoid mites. Many eriophyoids are of economic importance. Knowledge of their behaviour may promote understanding their ecology, may resolve problems in their phylogeny and may help developing methods for their control. In this paper, attention is directed to dispersal modes of eriophyoid mites, their feeding and host acceptance, spermatophore deposition and mating, defence against predators, and social behaviour.

Eriophyoid mites, which are among the smallest plant feeders, are characterized by the intimate relationships they have with their hosts and the restricted range of plants upon which they can reproduce. The knowledge of their true host ranges and mechanisms causing host specificity is fundamental to understanding mite-host interactions, potential mite-host coevolution, and diversity of this group, as well as to apply effective control strategies or to use them as effective biological control agents. The aim of this paper is to review current knowledge on host specificity and specialization in eriophyoid mites, and to point out knowledge gaps and doubts. Using available data on described species and recorded hosts we showed that: (1) 80% of eriophyoids have been reported on only one host species, 95% on one host genus, and 99% on one host family; (2) Diptilomiopidae has the highest proportion of monophagous species and Phytoptidae has the fewest; (3) non-monophagous eriophyoids show the tendency to infest closely related hosts; 4) vagrant eriophyoids have a higher proportion of monophagous species than refuge-seeking and refuge-inducing species; (5) the proportions of monophagous species infesting annual and perennial hosts are similar; however, many species infesting annual hosts have wider host ranges than those infesting perennial hosts; (6) the proportions of species that are monophagous infesting evergreen and deciduous plants are similar; (7) non-monophagous eriophyoid species have wider geographic distribution than monophagous species. Field and laboratory host-specificity tests for several eriophyoid species and their importance for biological control of weeds are described. Testing the actual host range of a given eriophyoid species, searching for ecological data, genetic differentiation analysis, and recognizing factors and mechanisms that contribute to host specificity of eriophyoid mites are suggested as future directions for research.