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The pea aphid (Acyrthosiphon pisum Harris) harbors two morphologically distinct procaryotic intracellular symbionts. The genes for the 16S rRNA from these symbionts have (4,15). In addition, treatment of the aphid with procaryote-specific antibiotics leads to the decrease or elimination of the symbionts, with concomitant reduction in weight, fecundity, and longevity of the aphid (5). Evidence suggesting that the symbionts may provide the aphid with plant cell wall-degrading enzymes (8, 10), essential nutrients (5), or a protein (symbionin) necessary for nymphal development (17) has been presented elsewhere.Since the endosymbionts have not been cultivated outside the host aphid, speculation as to their identity has been based on their morphology and the moles percent guanineplus-cytosine content of their DNAs (16,18). It has been suggested that the symbionts are related to the rickettsias and chlamydias, two groups of obligate intracellular pathogens, or to the mycoplasmas (15, 21). It has also been proposed that the endosymbionts constitute an evolutionary stage which culminated in the formation of mitochondria (14,15

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The probing behavior of biotype C of the greenbug, Schizaphis graminum (Rondani) on susceptible and resistant lines of sorghum, Sorghum bicolor (L.) Moench., was electronically monitored. Waveforms corresponding to salivation, phloem ingestion and nonphloem ingestion are described. The results of a parallel study revealed that the rate of population growth of S. graminum was significantly greater on susceptible lines of sorghum [i.e.. NC + 7OX. SC423 (Purple). SC423 (Tan) I than on resistant lines (i.e.. TAM 2567, IS 809). Aphids probing the resistant lines of sorghum showed a significantly reduced imbibition of phloem sap compared with those aphids which fed on susceptible varieties. Also. increased numbers of separate probes and increased duration of nonprobing were associated with greenbugs feeding on resistant lines. Greenbugs monitored on the nonhost plant, rice, exhibited non-phloem ingestion, but not phloem ingestion.Resistance in sorghum to greenbugs is associated with the phloem. There appear to be no physiai differences between the sorghum varieties in the location of or extent of lignification around the vascular bundles. Hence, resistance probably involves plant natural products associated with the phloem sap.

The ultrastructure of the sensilla, and other structures, within the stylets and precibarium of Macrosteles fascifrons were examined by transmission and scanning electron microscopy. Precibarium is a new term, defined here, for the canal that precedes the cibarium inside the leafhopper head. Within the precibarium are found 20 chemosensilla and a previously undescribed structure, the precibarial valve. Twelve mechanosensilla, three in each stylet, are found within the maxillary and mandibular stylets. The relationship between all of these structures and feeding by the insect is detailed in a feeding mechanism hypothesis. It is concluded that leafhoppers (and probably all homopterans) utilize the precibarial chemosensilla alone for gustatory discrimination, the stylet sensilla for proprioception, and the precibarial valve for regulation of fluid uptake and compartmentalization of the sensilla.