Larvae expressing Or42a^Scer\UAS.cFa under the control of Scer\GAL4^{Orco.2.642.T:Hsim\VP22} disperse more broadly under the odour source than wild type in an assay to study the response to a stable gradient of the odour ethyl butyrate. Although they do not accumulate at a fixed distance from the odour source, their average bearing before turning is more narrowly distributed around 90[o] than in wild type. Turns in the mutant larvae are triggered for a broader range of odour concentration compared to wild type. In addition, the mutant larvae experience a negative stimulus for a longer period of time before a turn is triggered.