Larvae with olfactory input restricted to the Or42a-expressing olfactory receptor neurons (expressing Orco^Scer\UAS.cLa under the control of Scer\GAL4^Or42a.PF in a Orco¹/Orco¹ background) follow a concentration path distant from the odour source in an assay to study the response to a stable gradient of the odour ethyl butyrate (30mM odour source). To orbit around the source in this way, the mutant larvae implement small-amplitude turns, which maintain their bearing close to 90[o]. Turns are triggered in the mutant larvae at odour concentrations three times lower than those that trigger turns in the wild type. The targeted concentration does not coincide with the region where the amplitude of the concentration gradient is maximal. In contrast to wild type, turns in the mutant larvae are preceded by episodes where the bearing remains constant. For a low-concentration odour source (7.8mM), the mutant larvae show attraction throughout the odour gradient's range and accumulate near the source. For a high-concentration odour source (125mM), the mutant larvae navigate up-gradient, before they bifurcate to avoid regions of high odour intensities. In contrast, wild-type larvae display invariant attraction to low and high concentrations. The mutant larvae turn towards the gradient for concentrations lower than 0.5μM, but turn away from the gradient at higher concentrations.

Larvae with olfactory input restricted to the Or42a-expressing olfactory sensory neurons (expressing Orco^Scer\UAS.cLa under the control of Scer\GAL4^Or42a.PF in a Orco¹/Orco² background) are more sensitive to isoamyl acetate than wild-type animals in an assay to study the response to a radially symmetrical odour gradient, showing significant responses to the odour at source concentrations as low as 0.12M. Mutant and wild-type larvae navigate efficiently along the odorant line (from low to high odorant concentration) in an assay to study the response to an exponential odorant gradient, in response to either isoamyl acetate, isopropyl acetate, 2-hexenal or anisole.