Ultrastructure and functional morphology of the mouthparts in Raoiella mites (Tetranychoidea: Tenuipalpidae): how they use the cheliceral stylets during feeding

The flat mite genus Raoiella Hirst (Tenuipalpidae) has attracted considerable research interest due to the rapid spread of Raoiella indica Hirst which feeds most commonly on species of palms (Arecaceae). Raoiella are frequently observed with their stylets inserted into the leaf stomatal openings, suggesting they feed on this site. Herein, we use transmission electron microscopy (TEM), scanning electron microscopy (SEM), low temperature scanning electron microscopy (LTSEM) and light microscopy (LM) to describe the mouthpart organization and the feeding behavior found in R. indica and R. bauchani Beard & Ochoa. The feeding mechanism is also discussed based on their distinctive morphological aspects. The structure of the gnathosoma of Raoiella generally agrees with the organization previously observed in Brevipalpus and Cenopalpus. As in other tetranychoid mites, there is no anatomical connection between the inter-stylet tube and the food channel, corroborating the theory that the stylets are used to pierce plant tissue (and assumedly inject saliva) but play no role in sucking up fluid from damaged host cells. As in Brevipalpus, the adoral setae are mechanoreceptors that may help guide the mite to the site for feeding. The ultrastructure of supracoxal setae e remains enigmatic. We also present observations of possible epidermal feeding on Phoenix roebelenii O'Brien (Arecaceae) for R. indica. Raoiella indica were shown to penetrate both the guard cells of the stomata and the epidermis of P. roebelenii, suggesting that feeding via the epidermis could be possible on this host plant.