Dorsal setae in Raoiella (Acari: Tenuipalpidae) their functional morphology and implication in fluid secretion

Authors: DI PALMA, ANTONELLA - Crea-Research Centre For Cereal And Industrial Crops; BEARD, J.J. - University Of Maryland; Bauchan, Gary; Ochoa, Ronald - Ron; SEEMAN, OWEN - Queensland Museum; KITAJIMA, ELLIOT - Universidad De Sao Paulo

Submitted to: Arthropod Structure and Development
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2020
Publication Date: 1/2/2020
Citation: Di Palma, A., Beard, J., Bauchan, G.R., Ochoa, R., Seeman, O., Kitajima, E.W. 2020. Dorsal setae in Raoiella (Acari: Tenuipalpidae) their functional morphology and implication in fluid secretion. Arthropod Structure and Development. 60(1):1-10. https://doi.org/10.1016/j.asd.2020.101023.
DOI: https://doi.org/10.1016/j.asd.2020.101023

Interpretive Summary: Flat mites in the genus Raoiella are major pest of palms and trees in the myrtle family. We describe the functional morphology of the fluid associated with the tips of their dorsal setae using transmission scanning microscopy (TEM) and low-temperature scanning electron microscopy (LT-SEM). Research scientists, Pest Control Programs, IPM, USDA-APHIS will use this information for the evaluation of predator mites against this flat mite pest.

Technical Abstract: The setae of mites are not regarded as secretory structures, yet in the flat mite genus Raoiella, each developmental stage presents droplets of fluid associated with the tips of their dorsal setae. To understand the origin of this fluid, the ultrastructure of the dorsal setae is investigated in females of R. bauchani Beard & Ochoa and the invasive pest species Raoiella indica Hirst using scanning and transmission electron microscopy techniques The dorsal setae are barbed along their entire length and have either a broadened plumose or a flat spatulate tip. Ultrastructurally, they present the typical features of mechanoreceptors. Moreover, the setae have a “hollow” axis represented by a protoplasmatic core containing dendritic branches, and although there are no pores along the shaft of the setae, it is suspected that pores are present apically. This combination of ultrastructural characters indicates that the setae are multimodal receptors: acting as both mechanoreceptors and contact chemoreceptors. The epidermal cells that underly the setal sockets are columnar and have an ultrastructure that suggests they have a glandular function. Moreover, these cells present regular microvilli apically and form extracellular cuticular canals, containing epicuticular filaments, that are connected with the microvilli proximally and which open via pores onto the surface of the setal base distally. This arrangement indicates that the secretion from the microvilli passes into the canals and is then conducted to pores at the base of the setal where it then accumulates and moves up the setal shaft, along the longitudinal grooves of the barbs. Based on similar arrangements in some insect taxa, the organization of the structures here observed in Raoiella suggests the passage of a non-polar, waterproofing, lipoid fluid through the cuticle, the function of which is still obscure.