The sensory pits of the nymphs of the planthopper Issus coleoptratus were investigated using light and electron microscopic techniques. Sensory pits consist of a bowl-shaped depression in the cuticle (25-70 μm in diameter) covered by a transparent cupola of presumably waxy material. Each pit is equipped with a long sensory hair that emerges from the inner wall of the pit and extends horizontally for about two thirds of the pit diameter. The cupola emerges from the rim of the pit opposite to the socket of this hair. Additional small sensory hairs extend into the base of the cupola. While the ultrastructure of these small hairs resembles that of other mechanoreceptive sensory hairs of insects, that of the long hairs shows several peculiarities. Sensory pits are dispersed over the frontal part of the head, the tergites of thoracic and abdominal segments. On the different parts of the exoskeleton, the orientation of long hairs within the pits varies in a systematic fashion with respect to the body axes. Size, location, and orientation of the pits show almost perfect bilateral symmetry. Their number increases with each moult. Comparative data show that the level of structural complexity of these sense organs varies considerably within this group of insects.
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