To be or not to be… a vampire: a matter of sensillum numbers in Calyptra thalictri?

The mechanisms by which blood feeding in insects has evolved are unclear, primarily because there has been no access to species in which there is a mixture of same-sex blood feeding and non-blood feeding individuals. The discovery of a subset of male Calyptra thalictri (Lepidoptera: Noctuidae: Calpini) that blood feed under constrained experimental conditions, while the majority of these males do not, provides a unique opportunity to investigate members of the same species for potential root mechanisms leading to the ability to blood feed. Previously, C. thalictri populations revealed no morphological differences in the classical structures used for species identification in individuals that took a blood meal compared with those that did not. We report a description of the antennal sensilla and their distribution in male C. thalictri and describe an antennal sensillum distribution dimorphism between individuals that took a blood meal under constrained experimental conditions and those that did not. The number of olfactory sensilla, primarily sensilla coeloconica but also sensilla auricillica, is reduced in C. thalictri males that took a blood meal compared with those that did not. The selectivity of sensilla coeloconica olfactory sensory neurons was investigated. The sensilla coeloconica demonstrated sensitivity to fifteen vertebrate-related volatiles, including ammonia. We propose that the reduction in olfactory sensilla sensitive to vertebrate-related compounds may be correlated to an increase in the likelihood of a male C. thalictri to take a blood meal.