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.