Fine structure of a putative pedipalpal thermo/hygrosensillum in Tarsonemus nodosus Schaarschmidt, 1959 (Acari: Tarsonemidae)

A putative thermo/hygroreceptor has been detected in Tarsonemus nodosus Schaarschmidt, 1959 by means of scanning and transmission electron microscopy. It is dome-shaped and located on the distal-lateral part of each pedipalp. The receptor shows a thick and electrondense cuticular wall, moreover no articulation or specialized socket has been observed. As no wall pores have been found, it is inferred that it belongs to the type NP-sensilla with inflexible socket. The dendritic sheath is thick and no outer receptor lymph sinus has been detected. According to their morphology, it was possible to distinguish at least two receptor cells. One presents extensive lamellations of the dendrite whose outer segment fills the dome completely with stacks of parallel lamellae. Moreover in the dendritic outer segment, microtubules are present. The other dendrite terminates at the base of the dome. According to the morphological and structural organization of the described structure and the homology with those described in other arthropods, and particularly in ticks, its thermo and hygroreceptor function is hypothesized.