Fine structure of the urnulae of Balaustium mites (Actinotrichida: Erythraeidae) representing peculiar defense organs

•Ultrastructure of prodorsal urnulae of Balaustium spp. is studied with TEM and SEM.•Primary function of urnulae should be attributed to their role as defense organs.•Secretion is expelled through the slit formed between the roof and wall of urnula.•Columnar cells with lipids play a role in restoration of the secretion layer.•Evolutionary origin of urnulae is hypothesized.

The urnulae, until now the enigmatic paired dorsal protrusions on idiosoma dorsum in active postlarval forms of Balaustium mites, were studied using electron microscopy. They consist of walls made of unmodified integument, which form a cylinder covered by a roof of thin cuticle. At the posterior border of the urnula, the roof has a crescent slit. On its inner surface, a rather large muscle inserts with several tendons. The roof forms a flap under which the modified columnar epidermal cells containing numerous lipid inclusions are located. These lipids are probably secreted through pore canals of the overlying cuticle. Materials mainly originating from an extensive vesicular tissue situated underneath the columnar cells of the urnula and under the adjacent unmodified epidermis are extruded through the mentioned slit. Our results support previous studies that have suggested a function of the urnulae as defensive organs. Our study further suggests that the agent that provides the repellent effect comes mainly from the vesicular tissue, whereas the columnar cells with their lipid secretions are likely to restore the external secretion layer of the epicuticle after its destruction during the repellent release. Further structural and functional details are discussed and compared with other putative defensive secretory organs.

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