A topographic map of sensory cell terminal arborizations in the cricket CNS: Correlation with birthday and position in a sensory array

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Abstract
The development of a group of club-shaped sensilla called clavate hairs, located on the cerci of the cricket (Acheta domesticus), was examined morphologically. The clavate hairs are located on the base of the cercus and are thought to inform the animal of its orientation with respect to gravity. There are two groups of clavate hairs distinguished from one another by the orientation of their sockets: a dorso-medial group whose sockets are oriented perpendicular to the long axis of the cercus and a ventro-medial group inclined 45–60° away from the perpendicular. The ventro-medial group consists of a series of rows of sensilla running parallel to the long axis of the cercus. By examining a cast-off exoskeleton in the scanning electron microscope and comparing it with newly developed cuticle of the subsequent instar (Fig. 3), we showed how receptors were added to the ventro-medial array of clavate hairs. The first ventral hair (#10, Fig.1) appeared in the second instar. Three more hairs were added in the third instar: two (#11 and #12) proximal to hair 10 forming the first row and one (#20) medial to 11 and initiating the second row. After the third instar one hair was usually added proximal to each row each time the specimen molted. Because of the regular positioning of hairs and their orderly addition to the array, it is possible to identify uniquely all of the hairs in the three largest rows of ventral hairs (Fig. 4). We developed a simple method for staining the neuron associated with each hair. A hair was injured by cutting off its tip. A bubble of cobaltous acetate was then placed on the hair for 18–20 hours and only the neuron associated with the injured hair took up the stain. The synaptic terminal arborizations of identified neurons examined in this manner were unique and reproducible from specimen to specimen (Fig. 6). Furthermore, there is a topographic order to the terminal arborizations. Within one row the oldest neurons project furthest into the nervous system and arborize over the greatest area, whereas younger neurons arborize in more restricted areas near the entrance of the cercal nerve. Thus it was concluded that birthday was corretated with the morphology of the synaptic arborization. By staining neurons that were the same age but located in different rows, we determined that birthday was not the only variable influencing the morphology of the terminal arbors. The terminal arbors of neurons 11 and 20, both of which first appear in the third instar, were very different from one another. Thus another variable, presumably position on the body surface, was also correlated with the morphology of a neuron's terminal arborization. We concluded from these results that position on the cercus as well as birthday is encoded in the developmental program of these neurons and that the morphology of their terminal arborizations is a joint function of these two variables.