THE DISTRIBUTION OF THE VETCH APHID ON BEAN STEMS IN RELATION TO STYLET LENGTH AND PHLOEM DEPTH

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