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 Raoiella 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, but have a “hollow” axis represented by a protoplasmatic core containing dendritic branches. This combination of ultrastructural characters indicates that the setae might be multimodal receptors: acting as both mechanoreceptors and contact chemoreceptors. The epidermal cells that underlie 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 seta, 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, water insoluble, lipoid fluid through the cuticle, the function of which is still obscure.

螨类的刚毛不被视为分泌结构，但在扁平螨属Raoiella 中，每个发育阶段都会出现与其背侧刚毛尖端相关的液滴。为了了解这种液体的来源，我们使用扫描和透射电子显微镜技术研究了Raoiella bauchani Beard & Ochoa 和入侵害虫物种Raoiella indica Hirst 的雌性背毛的超微结构。

背刚毛沿其整个长度有倒刺，并具有加宽的羽状或扁平的铲状尖端。在超微结构上，它们呈现机械感受器的典型特征，但具有由包含树突分支的原生质核代表的“空心”轴。这种超微结构特征的组合表明刚毛可能是多模式受体：既充当机械感受器又充当接触化学感受器。位于刚毛窝下方的表皮细胞呈柱状，并具有超微结构，表明它们具有腺体功能。此外，这些细胞在顶端呈现规则的微绒毛并形成细胞外角质管，包含表皮细丝，这些细丝在近端与微绒毛相连，并通过孔向远端打开到刚毛基部的表面上。这种排列表明来自微绒毛的分泌物进入管内，然后被引导到刚毛基部的孔中，然后在那里积聚并沿着倒刺的纵向凹槽向上移动。基于一些昆虫类群的类似排列，这里观察到的结构组织Raoiella建议非极性、水不溶性、类脂流体通过角质层，其功能仍不清楚。