The locomotory behaviour of aphidophagous syrphid larvae was studied on 24-hr-old larvae of Syrphus balteatus (Degeer). The unfed larvae moved almost continuously on a moist filter paper arena and were able to travel distances well in excess of 1 m. Their rate of turn per minute was approximately constant in the absence of aphids, even after limited mechanical interference. Aphids (adult, apterous Aphis fabae Scopoli) were presented individually to eighty larvae for periods of time less than those permitting complete ingestion of the contents of the aphid. After loss of contact with the aphid, the larval turning rate increased with the duration of previous aphid presentation up to 5 min, beyond which it progressively declined. It was more than doubled after a 0·5-min aphid presentation period during which no ingestion occurred. The speed of forward movement was lower immediately after the period of aphid presentation than immediately before it, and this slowing was apparently associated with greater lateral 'casting' by the larvae. These three components of the locomotory behaviour after aphid contact-slowing, turning and scanning- probably all help to concentrate prey-searching in areas where aphids are most likely to be encountered. © 1969.
