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Résumé :
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Phloem depth in Vicia faba (L.) (Leguminosae) and stylet length in apterous Megoura viciae Buckt. (Homoptera, Aphididae) were both found to increase with age, so that phloem in the late mature stem and corner ridges of the whole stem were not accessible to young nymphs. The aphids of all ages used 75% of their total stylet length to reach the phloem by routes which were shown, by scale drawings and graphs, to be circuitous, curved, extracellular and sometimes intramural. After prolonged infestation, a dense network of stylet tracks was found in the stem cortex. Phloem accessibility determined the distribution of aphids, which preferred the mature stem and the narrow channels either side of the corner ridges. The results are discussed as the aphids' response to a dynamic resource system in the maturing stem.
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