ABSTRACT.When presented with dishes containing a range of ‘foliar’ shapes, the onion fly, Delia antiqua (Meigen), laid the most eggs around narrow (4 mm) vertical cylinders. Response to cylinders diminished when their diameter was increased or decreased, when cylinder height was reduced to less than 2 cm, and when cylinder/substrate angle deviated from 90d̀. Differences in egg numbers on stimulatory and non‐stimulatory forms reflected primarily differences in post‐alighting pre‐ovipositional behaviours. Females alighting on narrow vertical cylinders initiated and completed stem runs rapidly and without interruptions, and frequently went on to probe and oviposit. Females alighting on other shapes, or larger, smaller, or non‐vertical cylinders, either did not initiate or did not complete stem runs, or did not go on to probe after completing a stem run. Such females rarely oviposited. Possible causes of such examining behaviours a
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