Sensilla of the cricket palp. Fine structure and spatial organization

On the distal segment of the maxillary palp of the cricket Gryllus bimaculatus there is an extensive multimodal sensory field comprising approximately 5000 sensilla of nine morphologically distinguishable types: three types containing pores in their walls (5% of the total), three types with a pore at the tip (65%) and three types lacking pores (30%). The approximately 27,000 axons of their sense cells join in the distal palp segment to form two nerves. On the slightly smaller sensillum field on the tip of the labial palp, the same sensillum types are found in comparable density and proportion. By analogy with sensilla of which both structure and function are known, the structure of these three groups of sensillum types suggests that they function as olfactory, contact-chemoreceptive, and/or mechanoreceptive sensors. Moreover, the positioning of the sensilla on the tip of the palp is appropriate to these function. The cuticle of the pulp tip is elastically deformable; all sensory hairs are fixed via a special, presumably flexible socket suspension. These structural features could be an adaptation to stresses encountered during tactile activity of the palp.