An electron microscopical study of aphid antennal sensilla has revealed two types of trichoid sensilla. Type I, innervated by a single neuron is mechanoreceptive; type II, innervated by three to five neurons is both mechanoreceptive and chemoreceptive with possibly a third function. Johnston's organ in the pedicel comprises a peripheral ring of scolopidia inserted into the joint with the flagellum; two non-peripheral groups of scolopidia lie in the lumen with attachment points in the wall of the third segment. The fine structure of a campaniform sensillum on the pedicel is described together with two homologous and previously unknown sense organs at the joint between the fifth and sixth antennal segments. An unusually placed scolopidium in the lumen of the sixth segment has also been found. The function of this scolopidium is unknown but Johnston's organ, the campaniform sensillum and joint receptors are suggested to act as antennal proprioceptors.

An electron microscopical study was made of the coeloconic and placoid sensilla on the antennae of the aphids Aphis pomi, Macrosiphum euphorbiae, Nasonovia ribis-nigri, and Pemphigus bursarius. Scanning electron microscopy revealed some variation in morphology which may be functionally important but is more likely to reflect the evolution of these species. The placoid sensilla were shown by tranwo or three neurons is surrounded by two ensheathing cells. The ciliary regions of the dendrites pass through a vacuole into a cavity between an outer and an inner cuticle where they may be connected to the dendritic branches although such connections were no seen. Small pores (8 nm diameter) partially penetrate the cuticle implying that these sensilla have an olfactory function. They are suggested to be important in host selection by alate aphids. The coeloconic sensilla are poreless pegs with nonsensory cuticular projections at their tips. The distal portions of their dendrites contain densely packed microtubules and the cellular arrangement of the sensila is similar to that of the placoid sensilla. It is suggest that they may function as thermoreceptors.

Electrophysiological recordings have been made from the primary rhinaria of the lettuce aphid Nasonovia ribis‐nigri Mosley (Homoptera: Aphididae) and over 70 volatile chemicals screened for neurophysiological activity. Although the receptors appear to lack specificity, it is suggested that they may be more important in detecting terpenes. The two rhinaria respond antagonistically to terpenes and a mechanism for perceiving concentration gradients is proposed. The importance of terpenes to aphids is further discussed with reference to the role of olfaction in host location. RÉSUMÉ Étude électrophysiologique de l'olfaction chez le puceron Nasonovia ribis‐nigri Les enregistrements électrophysologiques ont été faits à partir des rhinaria primaires de Nasonovia ribis‐nigri Mosley (Homoptera: Aphididae); plus de 70 substances volatiles ont été testées pour leur activité neurophysiologique. Bien que les récepteurs manquent de spécificité, il est possible qu'ils soient plus importants pour détecter les terpènes. Les deux rhinaria répondent de façons opposées aux terpènes et un méchanisme pour percevoir les gradients de concentration est suggéré. En outre, l'importance des terpènes pour les pucerons est discutée en relation avec le rôle de l'olfaction dans la localisation de l'hôte.