It is apparent that the protocerebral positional units of Arenivaga have only four inputs, i.e., the VNC connectives RCPI, RIPI, LCPI, and LIPI. These VNC inter-neurons show broad, cosine-like directional sensitivities. Willey (1981) has shown that certain PP units have very sharp directional responses, at . values not restricted to the major sensitivity directions of the VNC connectives: e.g., 45°, 135°, 225°, and 315°. Furthermore, it appears that at any ., there will be at most two VNC units carrying tilt information to the protocerebrum, and only one unit if . = 45°, 135°, 225°, or 315°. Thus, the protocerebral interneurons work with a paucity of inputs to advise the animal that it is being tilted at a very speci c . with respect to the gravity vector. Why does the animal need sharp PPU directional responses? Are they involved in the motor control of burrowing, as Hartman et al. (1979) have suggested, and can they help stabilize the animal in ight?
Both the VNC interneurons and the PP units habituate to zero rate. Thus, if the animal burrows deep in ne sandy soil and remains still for several minutes, its protocerebrum is theoretically not being sent any positional information from the tricholiths. However, if the animal is still, it does not need positional information! As soon as the animal moves, because of the dynamic nature of its gravity vector sensing system, it is advised of its orientation and can, for example, sense roll and pitch up as it burrows toward the surface.
It is reasonable to assume that the PP units outputs are used to make adjustments to the leg motor system so that the animal can minimize roll and control pitch when it is burrowing in ne sandy soil. Because of the phasic nature of the tricholith sensillae response to changes in animal position in the gravity eld, it is also possible that the system could be used to stabilize ight, albeit ight is only by males, and is done infrequently.
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