Plasticity of pigmentation of the harlequin bug, Murgantia histrionica, in response to developmental temperature and photoperiod
Sustaining homeostasis in a changing environment relies on an organism’s ability to maintain an internal temperature within a specific range through thermoregulation. As poikilothermic ectotherms, insects must rely on mechanisms other than metabolic heat production to protect themselves from extreme temperatures. The harlequin bug (Murgantia histrionica) demonstrates plasticity in the ratio of black to yellow pigmentation on their dorsal surface, possibly taking advantage of solar radiation to warm their bodies during cool ambient temperatures. In order to investigate the impact of temperature and photoperiod on adult pigmentation, we reared harlequin nymphs in multiple thermal environments that mimicked the seasonal temperatures and photoperiods observed in their natural geographic range. We also transferred subsets of developing nymphs between environments in an attempt to ascertain when during development adult pigmentation is fixed. Once the nymphs reached the adult stage, the black to color ratio was quantified using digital imagery. Finally, we monitored the temperature of adult harlequin bugs in dark and basking conditions in order to assess the thermoregulation capabilities across a range of pigmentation phenotypes.
Nymphs that completed their development in chambers mimicking a fall temperature and/or photoperiod had higher levels of melanization. In addition, bugs that completed their entire development under fall conditions showed the same degree of melanization as bugs transferred after 28 days of development, suggesting that M. histrionica, similar to some Lepidopterans, shows melanization plasticity in response to exposure at a late critical period of development, rather than total time spent in a fall environment. Bugs reared in a fall environment were smaller than both bugs in a summer environment and those transferred after 28 days of development. Smaller organisms have a larger surface area to volume ratio. It is possible that bugs developing in the fall, which will likely overwinter as adults, take advantage of their higher surface area to volume ratio to increase their body temperatures more efficiently than larger adults when exposed to solar radiation. In our assessment of thermoregulation, individuals with a darker pigmentation were able to raise their body temperature to greater extent. These results suggest that colder temperatures experienced late in the season may induce the development of a darker phenotype, potentially improving the insect’s immediate and over-winter survival.