The Structure of Arthropod Chemoreceptors

Abstract

The body surface of most terrestrial arthropods is highly impermeable to water; if it were not, the individuals, nearly all of which are small, would quickly succumb to dehydration. The receptor surfaces of the neural ele­ ments of an olfactory or a gustatory sense organ must be located in a posi­ tion where the stimulating odor or material in solution will reach them promptly. This is accomplished most efficiently when the moist receptor sur­ faces are exposed directly to the air or other media to be tested. These op­ posed necessities, then, that the individual 1. minimize water loss and 2. expose the delicate membranous surfaces of the dendrites to the air is a problem with which insects and other terrestrial arthropods are faced. The problem has been solved in the insects by the exposure of the recep­ tor surfaces at very small openings or pores in the cuticle. Unlike the rela­ tively large apertures of the digestive, respiratory, and reproductive systems and of various glands that are intermittently opened and closed, the pores of the chemoreceptors are continuously open. The openings, however, are ex­ tremely small and their combined surface area presents no serious problem in water conservation (86,87). The openings, indeed, are so small that they could not be seen by earlier workers. It was not until the electron microscope came into use and ade­ quate methods of preparing materials to be examined with it had been devel­ oped that the openings were demonstrated for the first time (86,87). Before this it had been believed generally that the chemoreceptor nerve endings were covered by a specialized cuticle through which the stimulating sub­ stance was able to pass. This view was accepted, at first, by the present writer (61). During the past ten years, special methods for identifying the pores with a solution of crystal violet applied to the external surface of the insect (65), examination of the material in a medium with a suitable refrac­ tive index, and the use of silver-stained sections have permitted the identifi­ cation of the pores in the sense organs of some species with the light micro­ scope. Before the knowledge obtained with the electron microscope became available, the correct interpretation of observations made on such structures with the light microscope was not possible.