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.