Long ago, we noticed that webworms fed on artificial diet were rather "pale" colored compared to larvae grown on plant material.  The yellow coloration of plant-fed larvae is attributable to the preferential sequestration of the carotenoid lutein. 

One benefit of sequestration of this carotenoid is that larvae appear to be less sensitive to UV radiation.

Larvae lacking carotenoids tend to avoid UV. In this experiment, larvae fed artificial diet (no carotenoids), plant material, or an artificial diet containing lutein had the option of locating under UV or under a UV filter.  The larvae without carotenoids began avoiding UV almost immediately, while the larvae fed either host plant carotenoids or dietary lutein tended to favor UV.   In a similar experiment allowing a choice between visible light only and dark, the larvae showed no differences as a function of carotenoid treatment.  These results confirm that UV wavelengths are the ones important in the behavioral response.  UV is an important concern for these animals because it activates toxic properties of the furanocoumarins, chemicals found in large quantities in the webworm's host plants.  Carotenoids may prevent UV from damaging the insect either by absorbing the UV or by lessening the toxic effects of furanocoumarins.