Variation in cuticular colour in the mealworm beetle, Tenebrio molitor.
The exoskeleton (cuticle) is an insects’ first physical barrier to infections. The mealworm beetle, Tenebrio molitor, displays cuticular colour variation, from black to light brown, and it was previously found that cuticular colour of T. molitor is related to their ability to resist a fungal entomopathogen. One approach we took was to examine whether selection for tan or black cuticular colour also resulted in correlated selection in haemolymph-mediated innate immune traits. Selection on cuticle colour did indeed result in a correlated increase in phenoloxidase, a key enzyme involved in cuticular hardening and darkening, but there was no effect on antibacterial activity or haemocyte number (Armitage & Siva-Jothy 2005). We also tested whether colour variation in this trait could relate to its physical nature, in terms of cuticular thickness and porosity (Evison et al. 2017), and the time course of cuticular tanning (Thompson, Armitage & Siva-Jothy 2002). We also found that lifespan and fecundity were not affected by cuticular colour, but that activating the immune system with a non-pathogenic immune challenge resulted in a shorter lifespan (Armitage et al. 2003).