Suboral fork: a newly discerned gnathosomal structure from the proboscis of eriophyoid mites (Acari, Eriophyoidea).

The infracapitulum of eriophyoid mites comprises a cone-like basal infracapitulum, containing a pharynx, and a distal infracapitulum, forming a proboscis ensheathing a bunch of stylets. A well-developed basal labral section was observed in all studied specimens. A newly discerned structure, the suboral fork, situated in the ventral part of the proboscis was discovered. It is larger in diptilomiopids and Nalepella and notably smaller in eriophyids and phytoptids. This structure presumably determines the site of piercing and functions in a similar way to the pressure foot of a sewing machine which controls the movements of a needle. In diptilomiopids the suboral fork might have an additional function: it is a stopper which prevents the proboscis from further penetrating into plant tissues. It is possible that the suboral fork is homologous with the labium of early derivative acariform mites. The proboscis might be a fusion product of the infracapitular lateral lips, malapophyses and the labium. The proboscis serves as a feeding structure in eriophyoids; two ways of sucking plant cell sap, depending on shapes of proboscis and labrum, are hypothesized. Further work is needed to draw conclusions on homologies and the function of all gnathosomal structures in eriophyoids.