eys²⁶⁴⁹ mutants do not show defects in walking, feeding or flying at room temperature. However, at the restrictive temperature (37[o]C), the flies gradually lose the ability to fly, to stand upside down and to climb walls, until eventually they can only lie and sporadically move their legs, wings and mouthparts in an uncoordinated manner.

At 21[o]C mutant flies show robust inward currents in the mechanoreceptor neurons (MRNs) in response to bristle deflections (as do control MRNs). However, after 30 minutes exposure to 37[o]C, the amplitude of the response in the mutant MRNs is reduced by over 80%. The electrophysiological response of the Johnston's organ to antennal rotation is almost abolished at 37[o]C in the mutant flies.

Mutant flies show a dramatic deformation of their mechanoreceptor neurons (MRNs) in response to heat treatment (this phenotype is not seen in control flies); the entire neuronal cytoplasm invades the lymph space, such that the region that would normally contain only the cilium and extracellular fluid becomes filled with cellular material from the MRN cell body.

If eys²⁶⁴⁹ mutant flies are subjected to 37[o]C in high humidity (greater than 90%), this prevents the manifestation of the mutant phenotype, both morphologically in the mechanoreceptor neurons and behaviourally.