Gravity Receptors Of The Cockroach Arenivaga Sp

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This section shows that the tricholith gravity sensors of the cockroach Arenivaga sp. are an excellent example of a small receptor array. Tricholiths have been shown to respond to the position of the cockroach with respect to the Earth's gravity vector. Only four large identi able interneurons carry spatial orientation information from the tricholith arrays to the animal's CNS where it is processed by many interneurons. Willey (1981) recorded from single, position-sensitive neurons in the animal's pro-tocerebrum and demonstrated that a de nite central enhancement or sharpening of the spatial orientation response occurs. The Arenivaga spatial position sensing system is a rare example of a divergent sensory system, where a sparse sensory array projects only four bers to the CNS. Most neuro-sensory systems consist of large receptor arrays whose outputs are processed convergently. That is, the sensory output of many receptors is processed in neuropile to give a few afferent outputs that are "operations" or feature extractions on the sensory input data.

Insects as a class are generally thought to lack organs analogous to statoliths, otoliths, or semicircular canals used to sense an animal's orientation in the Earth's gravity eld. Man y insects determine their orientation by vision coupled with mech-anoreceptor information provided by limb loading and joint positions (Walthall and Hartman, 1981). Certain crickets have clavate hair sensors on their cerci, which have been shown neurophysiologically to respond to changes to the cricket's gravity orientation (Bischof, 1975). (The cerci of orthoptera are two roughly cylindrical organs projecting to the rear from the medial anal region of the insects, tapering to points. They are very noticeable in crickets and cockroaches. The cerci project at about 25° to 45° from the animal's midline depending on the species, in the horizontal plane; they are generally covered with various hairlike sensillae; (Hoyle, 1977).

A few insects have evolved arrays of specialized gravity-sensing sensilla on their cerci. Most widely studied has been the gravity response of the desert-burrowing cockroach, Arenivaga sp. (Hartman et al., 1979; Walthall and Hartman, 1981). Similar sensillae have been noted on the cerci of three genera of crickets, Achaeta, Gryllus, and Gryllotalpa, and on the cerci of several species of polyphagid cockroaches (Hartman et al., 1979).

Cockroach Sem

FIGURE 2.6-1 (A) Ventral view of an adult male burrowing cockroach, Arenivaga sp. Note the small, hornlike cerci projecting from the insect's posterior. (B) SEM image of the ventral surface of the right cercus showing the ball-like masses of the tricholith gravity receptors. Other interesting hairlike receptors are also seen. (From Walthal, W.W. and Hartman, H.B., J. Comp. Physiol. A, 142: 359, 1981. With permission.)

FIGURE 2.6-1 (A) Ventral view of an adult male burrowing cockroach, Arenivaga sp. Note the small, hornlike cerci projecting from the insect's posterior. (B) SEM image of the ventral surface of the right cercus showing the ball-like masses of the tricholith gravity receptors. Other interesting hairlike receptors are also seen. (From Walthal, W.W. and Hartman, H.B., J. Comp. Physiol. A, 142: 359, 1981. With permission.)

The cerci of a male Arenivaga project at about 45° from the body axis, and lie in the horizontal plane. A cercus is about 1 to 1.5 mm in length, and is covered on its ventral surface with sensory hairs (trichobothria) and sensory bristles (sensilla chaetica). Figure 2.6-1A shows a ventral view of an adult, male Arenivaga sp; the cerci emerge from the rear of the abdomen. Figure 2.6-1B, a low-power, scanning electron microscope (SEM) micrograph, illustrates the hairs and tricholiths on the ventral center surface of the right cercus. Note that the tricholiths are arranged in a depression on the ventral surface of each cercus in two parallel rows. In an adult Arenivaga, each row has seven or eight tricholiths, giving a total of 14 to 16 tricholiths per cercus. The dense round ball of a tricholith is about 22 ^m in diameter and it is suspended downward on a slender, cylindrical stalk from a socket in the cercus. Gravity pulls down on the ball so the axis of the stalk is aligned with the gravity vector. If the animal experiences roll or pitch, the stalk makes an angle less than 90° with the axis of the cercus. In males, the tricholiths are protected mechanically from above by the wings that overhang them, and ventrally by the trichobothria and the sensilla chaetica. The cerci of larval and female Arenivaga are protected in cavities in the abdomen. Thus, the tricholiths appear to be protected from mechanical perturbation from soil particles when the animal burrows.

A mechanoreceptor neuron associated with the tricholith socket senses the relative position of the tricholith stalk with respect to the cercus, signaling the animal its orientation in the gravity eld. Because Arenivaga burrows in the sandy, quasi-uid soil of its nati ve southwest Texas desert habitat during the day to escape the heat and predators, it is deprived of the normal insect orientation cues of vision, limb loading, and position and appears to rely on its tricholith system to enable it to navigate underground and return to the surface.

Hartman and his graduate students were the rst neuroph ysiologists to characterize the input end of the tricholith system quantitatively (Walthall and Hartman, 1981). Sensory bers (one from each tricholith organelle on each cercus) pass into the sixth abdominal ganglion where they synapse with four large afferent positional interneurons that project up the paired ventral nerve cord to the insect's protocere-brum. There are two large positional interneurons (PIs) in each of the paired ventral nerve cord tracts, facilitating electrical recording of the PI action potentials.

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