The sensory morphology of insect pollinators : From structure to behaviour and ecology

Sammanfattning: Insect pollinators play an important role in balancing our ecosystems and maintaining plant and food diversity. Due to anthropogenic activities such as global warming, habitat loss and degradation, pesticides, and pathogens, many pollinator populations have been undergoing drastic declines in recent decades. Sudden changes in environmental conditions may lead to selection pressures to alter sensory systems, their structures and functions, and, consequently, behaviour. Despite the multitude of studies on insect pollinators and their population declines, these effects are often overlooked. To address this knowledge gap, I studied the morphology of sensory systems in butterflies and bumblebees – two pollinator groups found in temperate regions – from the aspects of development, behaviour, ecology and adaptation. In Chapter I, I used allometry to study how investment into sensory systems varies within and between different sexes of the butterfly Pieris napi. I measured the size and other parameters of sensory traits including eyes, antennae, proboscis, and wings. I showed that sensory system investment varies between sexes and only antennal length and wing size increase allometrically with body size. These findings suggest that not all sensory organs scale with body size and energetic investment between them can vary among sexes of the same species. In Chapter II, I explored the effect of a sub-optimal temperature on the development and morphology of sensory systems in P. napi, an ectothermic solitary insect. I exposed the pupae of P. napi to 23°C (optimal temperature) and 32°C (sub-optimal temperature) and measured their body and sensory organ size after emergence. I found that the mortality rate was higher at the suboptimal temperature and that the eclosion time decreased. Also, body and proboscis size decreased in both sexes, while antennal length decreased only in males. These results show that global warming can have negative consequences for the survival of butterflies and affect the size of their sensory systems potentially by accelerating the developmental process. In Chapter III, we studied the effect of heatwave-like temperatures on the sensory systems and behaviour of another insect pollinator, Bombus terrestris. Our results revealed that development in suboptimal temperatures had a negative impact on behavioural responses of bumblebee workers. Interestingly, the elevated temperature did not have a significant effect on the size of their antennae, eyes and forewings. These findings indicate that an elevated developmental temperature can impair important behavioural responses to sensory stimuli without causing any visible changes in sensory organ morphology. In Chapter IV, I explored how well the qualitative light micro habitat associated with a distribution range of insects, matches with carefully measured quantitative values. I used three butterfly species (P. napi, Pararge aegeria, Vanessa atalanta) that are associated with different light habitats and have different dispersal ranges. The results showed that only P.napi distribution was affected by light intensity. Eye and brain neuropil investment varied among the three species. P. napi had highest eye size investment while V. atalanta had highest optic neuropils investment. These findings suggest that visual and neural investments could only in part be associated with quantitative and/or qualitative light micro habitat and dispersal in these species.