Global warming has been identified as one of the main drivers of population decline in insect pollinators. One aspect of the insect life cycle that would be particularly sensitive to elevated temperatures is the developmental transition from larva to adult. Temperature-induced modifications to the development of body parts and sensory organs likely have functional consequences for adult behaviour. To date, we have little knowledge about the effect of sub-optimal temperature on the development and functional morphology of different body parts, particularly sensory organs, in ectothermic solitary pollinators such as butterflies. To address this knowledge gap, we exposed the pupae of the butterfly Pieris napi to either 23 degrees C or 32 degrees C and measured the subsequent effects on eclosion, body size and the development of the wings, proboscis, eyes and antennae. In comparison to individuals that developed at 23 degrees C, we found that exposure to 32 degrees C during the pupal stage increased mortality and decreased time to eclose. Furthermore, both female and male butterflies that developed at 32 degrees C were smaller and had shorter proboscides, while males had shorter antennae. In contrast, we found no significant effect of rearing temperature on wing and eye size or wing deformity. Our findings suggest that increasing global temperatures and its corresponding co-stressors, such as humidity, will impact the survival of butterflies by impairing eclosion and the proper development of body and sensory organs.

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.

Insektpollinatörer spelar en viktig roll för att balansera våra ekosystem och upprätthålla mångfalden av växter och livsmedel. På grund av antropogena aktiviteter såsom global uppvärmning, förlust och försämring av livsmiljöer, bekämpningsmedel och patogener har många pollinatorpopulationer genomgått drastiska minskningar under de senaste decennierna. Plötsliga förändringar i livsmiljöer kan påverka utvecklingen av insekters sensorsystem och leda till förändringar i deras morfologi, funktion och därigenom beteende. Trots mängden av studier om insektpollinatörer och deras minskande populationer, så förbises ofta effekten av förändrade livsmiljöer på deras sensorsystem. För att fylla denna kunskapslucka studerade jag morfologin hos sensorsystemen hos fjärilar och humlor - två pollinatorgrupper som återfinns i tempererade områden - med avseende på utveckling, beteende, ekologi och anpassning. I Kapitel I använde jag allometri för att studera hur investeringen i sensorsystem varierar inom och mellan olika kön hos fjärilen Pieris napi. Jag mätte storlek och andra parametrar för sensorsystem, inklusive ögon, antenner, snabel och vingar. Jag visade att investeringen i sensorsystem varierar mellan könen och att endast antennlängd och vingstorlek ökar allometriskt med kroppsstorlek. Dessa resultat antyder att inte alla sensorsystem skalar med kroppsstorlek och den energetiska investeringen mellan dem kan variera mellan könen av samma art. I Kapitel II utforskade jag effekten av en suboptimal temperatur på utvecklingen och morfologin av sensorsystem hos P. napi, en ektotherm solitär insekt. Jag exponerade puppor av P. napi för 23°C (optimal temperatur) och 32°C (suboptimal temperatur) och mätte deras kroppsstorlek och storlek på sensorsystemen efter uppkomst. Jag fann att dödligheten var högre vid den suboptimala temperaturen och att uppkomsttiden minskade. Dessutom minskade kroppsstorleken och snabelns storlek hos båda könen, medan antennlängden endast minskade hos hanar. Dessa resultat visar att global uppvärmning kan ha negativa konsekvenser för fjärilars överlevnad och påverka storleken på deras sensorsystem, eventuellt genom att accelerera utvecklingsprocessen. I Kapitel III studerade vi effekten av värmeböljeliknande temperaturer på sensorsystem och beteende hos en annan insektpollinatör, Bombus terrestris. Våra resultat visade att utveckling vid suboptimala temperaturer hade en negativ inverkan på beteendesvar hos humlearbetare. Intressant nog hade den höjda temperaturen ingen signifikant effekt på storleken av deras antenner, ögon och framvingar. Dessa resultat indikerar att en ökad utvecklingstemperatur kan försämra viktiga beteendesvar på sensorsignaler utan att orsaka synliga förändringar i morfologin hos sensorsystemen. I Kapitel IV utforskade jag hur väl den kvalitativa ljusmikromiljön som är förknippad med en utbredningsområde för insekter överensstämmer med noggrant mätta kvantitativa värden. Jag använde tre fjärilarter (P. napi, Pararge aegeria, Vanessa atalanta) som är förknippade med olika ljusmiljöer och har olika spridningsområden. Resultaten visade att endast fördelningen av P. napi påverkades av ljusintensiteten. Investeringen i ögon och hjärnnervvävnad varierade mellan de tre arterna, men denna variation hade ingen tydlig relation till deras kvalitativa eller kvantitativa ljusmiljö. Därför kunde jag visa att noggrann bedömning av ljusmiljöer är avgörande för att förstå och tolka visuella och neurala anpassningar hos insekter.

Heatwaves are increasingly common globally and are known to have detrimental impacts on animal morphology and behaviour. These impacts can be severe, especially if heatwaves occur during development, even on animals that can regulate the temperature of their developing young. The onset and duration of heatwaves are stochastic and therefore may affect all or only part of development. In the heterothermic bumblebee Bombus terrestris, elevated temperatures over the entirety of development cause morphological changes in adults, despite their capability to regulate brood temperature. However, the effects of heatwaves that occur during a short period of development are unclear. We test the impact of elevated developmental temperature during the latter fraction of development on the behaviour and morphology of adult worker B. terrestris. We show that exposure to elevated temperature over a portion of late development is sufficient to impair the initial behavioural responses of workers to various sensory stimuli. Despite this, exposure to elevated temperatures during a period of development did not have any significant impact on body or organ size. The negative effect of elevated developmental temperatures was independent of the exposure time, which lasted from 11–20 days at the end of the workers’ developmental period. Thus, heat stress in bumblebees can manifest without morphological indicators and impair critical behavioural responses to relevant sensory stimuli, even if only present for a short period of time at the end of development. This has important implications for our understanding of deleterious climactic events and how we measure indicators of stress in pollinators.

Differences in organ scaling among individuals may play an important role in determining behavioural variation. In social insects, there are well-documented intraspecific differences in colony behaviour, but the extent that organ scaling differs within and between colonies remains unclear. Using 12 different colonies of the bumblebee Bombus terrestris, we aim to address this knowledge gap by measuring the scaling relationships between three different organs (compound eyes, wings and antennae) and body size in workers. Though colonies were exposed to different rearing temperatures, this environmental variability did not explain the differences of the scaling relationships. Two colonies had differences in wing versus antenna slopes, three colonies showed differences in wing versus eye slopes and a single colony has differences between eye versus antenna slopes. There are also differences in antennae scaling slopes between three different colonies, and we present evidence for putative trade-offs in morphological investment. We discuss the utility of having variable scaling among colonies and the implication for understanding variability in colony fitness and behaviour.

In solitary insect pollinators such as butterflies, sensory systems must be adapted for multiple tasks, including nectar foraging, mate-finding, and locating host-plants. As a result, the energetic investments between sensory organs can vary at the intraspecific level and even among sexes. To date, little is known about how these investments are distributed between sensory systems and how it varies among individuals of different sex. We performed a comprehensive allometric study on males and females of the butterfly Pieris napi where we measured the sizes and other parameters of sensory traits including eyes, antennae, proboscis, and wings. Our findings show that among all the sensory traits measured, only antenna and wing size have an allometric relationship with body size and that the energetic investment in different sensory systems varies between males and females. Moreover, males had absolutely larger antennae and eyes, indicating that they invest more energy in these organs than females of the same body size. Overall, the findings of this study reveal that the size of sensory traits in P. napi are not necessarily related to body size and raises questions about other factors that drive sensory trait investment in this species and in other insect pollinators in general.