Data and code for: Why are biting flies attracted to blue objects?

Diurnal biting flies are strongly attracted to blue objects. This
behaviour is widely exploited for fly control, but its functional
significance is debated. It is hypothesised that: blue objects resemble
animal hosts; blue surfaces resemble shaded resting places; and blue
attraction is a by-product of attraction to polarised light. We computed
the fly photoreceptor signals elicited by a large sample of leaf and
animal integument reflectance spectra, viewed under open/cloudy
illumination and under woodland shade. We then trained artificial neural
networks (ANNs) to distinguish animals from leaf backgrounds, and shaded
from unshaded surfaces, in order to find the optimal means of doing so
based upon the sensory information available to a fly. After training, we
challenged ANNs to classify blue objects used in fly control. Trained ANNs
could make both discriminations with high accuracy. They discriminated
animals from leaves based upon blue-green photoreceptor opponency, and
commonly misclassified blue objects as animals. Meanwhile, they
discriminated shaded from unshaded stimuli using achromatic cues and never
misclassified blue objects as shaded. We conclude that blue-green
opponency is the most effective means of discriminating animals from leaf
backgrounds using a fly’s sensory information and that blue objects
resemble animal hosts through such mechanisms.