Poreless sensilla with inflexible sockets. A comparative study of a fundamental type of insect sensilla probably comprising thermo- and hygroreceptors

Poreless sensilla (np-sensilla) on the antennae of 18 species from 9 insect orders, and on the maxillary palps of Periplaneta americana were investigated using chemo- and cryofixation. The number of np-sensilla is low. Common features of these sensilla are: (a) the presence of a peg, (b) the lack of pores that lead to the dendritic outer segments (DOS), (c) the lack of socket structures, which would indicate flexibility, and (d) the presence of three types of sensory cells. The type-1 sensory cells are characterized by large DOS, which proceed into the peg where they fit tightly to its wall. As a rule, two DOS are present, being arranged in bilateral symmetry. Within them, in two species, a pattern of microtubules similar to that of a tubular body was observed. In the type-2 sensory cells, the DOS end beneath the peg and mostly display membrane invaginations. In the type-3 sensory cell, the DOS is an unmodified 9 X 2 + 0 cilium. Electrophysiological investigation of the np-sensillum in the maxillary palp of Periplaneta showed the presence of a hygro- and/or thermoreceptor. In a comparison of np-sensilla with an inflexible socket in different insect species, it is shown that these sensilla represent one fundamental sensillum type and that their structural features can be regarded as adaptations to hygro- and thermoreception.