SENSILLA ON THE MAXILLARY PALPS OF HELICOVERPA-ARMIGERA CATERPILLARS IN SEARCH OF THE CO2-RECEPTOR

The ultrastructure of sensilla on the maxillary palps of Helicoverpa a rmigera caterpillars has been investigated in order to find candidates for CO2-receptors. The following sensilla are found on the palps: a) 8 chemosensory pegs at the tip; b), a large distal pore plate; c), a s maller proximal pore plate; d), a digitiform organ; e), a campaniform sensillum; and f), 3 scolopidia. Each chemosensory peg at the tip is i nnervated by 4-5 sensory neurons. Five of these pegs are most probably contact chemoreceptors, because each has a dendrite with a tubular bo dy. The distal pore plate has a porous cuticle and is innervated by 3 sensory neurons, each of which sends a highly branched dendrite into a large cuticular cavity. The proximal pore plate is made up from two f used organs, has also a porous cuticle, and is innervated by two senso ry neurons which send their dendrites into a narrow cuticular channel. The digitiform organ is innervated by one sensory cell which sends a highly lamellated dendrite into a narrow channel within a chip-shaped protrusion of the porous cuticle. For several reasons, the digitiform organ is the most probable candidate for the CO2-receptor. Another pos sible candidate is the distal pore plate.