Ciliary hair cells and cuticular photoreceptor of the hornet Vespa orientalis as components of a gravity detecting system: an SEM/TEM investigation

Abstract

This paper describes three types of hair cell configurations with stereo- and kinocilia in the head of the hornet; these were encountered at the vertex and frons regions adjacent to the three ocelli and are assumed to be part of the hornet's gravity detecting system together with cuticular photoreceptors. The first and most common type of hair cell configuration (type A) was a cell surrounded by a septum, having a diameter of 30–50 μm. Aggregates of over 20 such hair cell groups together formed a larger unit, 130–300 μm in diameter, which was also enclosed by a septum. Many of these larger round units were, in turn, arranged in either angular or leaf-like clusters. The hair cells bore numerous cilia of 4.5–6.0 μm long, and were themselves composed of smaller sub-units of about 7–8 μm in diameter, which were not enclosed by a septum. The second type of hair cell configuration (type B) was of discrete cells with a diameter of ∼12.5–14 μm, located in the vicinity of the pore canal outlet of the peripheral photoreceptor. These single hair cells were either devoid of or only partially enclosed by a septum. Their cilia were 4.5–6.0 (xm long as well, but with a diameter of only 150–160 nm. On the exterior of each cilium a tubular system could be detected. Furthermore, the tips of adjacent cilia were interconnected by a kind of fibre bearing a spherical body in its middle. The third type of hair cell (type C), present in the neighbourhood of the second type of hair cell (type B), was chalice-shaped and had interconnecting fibrils comparable to those found at type B. We believe that these three types of hair cell configuration along with the ganglion cells interconnecting their bases are all components of the gravity organ of the hornet (the Ishay Organ) and together with the cuticular photoreceptors function in the navigation system of the hornet. We further conjecture that the described structures are engulfed by endolymph; produced signals by each unit are conducted by neural fibrils to the hornet's central nervous system.