Caterpillar polarisation vision: Histological methods and Rcodes for behavioural analyses

Processionary caterpillars of Thaumetopoea pityocampa (in Europe) and Ochrogaster lunifer (in Australia) (Lepidoptera: Notodontidae) form single files of larvae crawling head-to-tail when moving to feeding and pupation sites. We investigated if the processions are guided by polarisation vision. The heading orientation of processions could be manipulated with linear polarising filters held above the leading caterpillar. Exposure to changes in the angle of polarisation around the caterpillar resulted in orthogonal changes in heading angles. Anatomical analysis indicated specialisations for polarisation vision of stemma I in both species. Stemma I has a rhabdom with orthogonal and aligned microvilli, and an opaque and rugged surface; which are optimisations for skylight polarisation vision, similar to the dorsal rim of adult insects. Stemmata II-VI have a smooth and shiny surface and lobed rhabdoms with non-orthogonal and non-aligned microvilli; thus, optimised for general vision with minimal polarisation sensitivity. Behavioural and anatomical evidence reveal that polarised light cues are important for larval orientation and can be robustly detected with a simple visual system.