Determination of instar for the window gnat Sylvicola fenestralis (Diptera: Anisopodidae)

The Window Gnat Sylvicola fenestralis is a major nuisance fly inhabiting the biological filters of sewage treatment works. Analysis of measurements of head capsule width, head capsule length and larval body length of 303 laboratory hatched and field collected larvae, established that this species possesses four larval instars. The size range of each instar for the three parameters was determined by plotting cumulative frequency against sample category on probability paper; the inflexion curve method. The growth rate between instars was shown to fit Dyar's rule for all three parameters. The efficacy of many insecticides depends on accurate targeting against the most susceptible stage of the insect life cycle. Hence a more detailed knowledge of the instars of S. fenestralis will allow for the most effective control of this nuisance species.