Abstract
Simple SummaryPollinating insects rely on a range of senses such as vision, olfaction, gustation, and mechanosensation to utilise, locate, and fly between floral resources. The size of different sensory organs determines their sensitivity and provides an indication of their relative importance—larger organs can enhance sensitivity by increasing the number or size of sensing structures. However, increasing the relative size of an organ would require additional energy for developing and maintaining it. This likely leads to a trade-off between the energy invested into different sensory systems within individuals. To explore how the size of the sensory organs vary with body size in insect pollinators and how the energetic investment is divided, we performed a series of morphological measurements on the eyes, antennae, proboscis, and wings of male and female Pieris napi butterflies with a range of body sizes. We found that only antenna (in females) and wing size (in males and females) increased with body size. Males also had larger eyes and antennae compared to females regardless of body size. Our results provide insights into how the sensory morphology of these butterflies varies with body size and sex, and highlights unusual instances of organs that do not scale with body size.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.
Highlights
To efficiently utilise, locate, and fly between floral food rewards, pollinating insects rely on a range of senses, including vision, olfaction, gustation, and mechanosensation
We focused our analyses on the allometric scaling relationship of sensory traits mainly related to foraging activity and reproduction—eyes, antennae, proboscis, and wings
We found that the surface area of the compound eyes did not increase significantly with body size in Pieris napi (t41 = 1.7, p = 0.1, Figure 1a, Table 1), which means that the eyes of the larger butterflies were proportionally smaller than the eyes of smaller butterflies
Summary
Locate, and fly between floral food rewards, pollinating insects rely on a range of senses, including vision, olfaction, gustation, and mechanosensation. In the silk moth Bombyx mori and the giant silk moth Antheraea polyphemus, males have enlarged antennal structures that increase their olfactory sensitivity to successfully track the females’ sex pheromone [22] These sex-specific variations in sensory traits reflect differences in behavioural (mating, oviposition, predator avoidance, conspecific communication, competition, and foraging activity) and physiological (feeding and energy intake, energy expenditure, and fecundity or sperm production) requirements [18]. To begin to address this knowledge gap, we performed a comprehensive allometric study—an analysis of how organ size varies relative to body size—on the sensory systems of males and females of a temperate butterfly species Pieris napi This butterfly is an ecological generalist that is active across multiple habitat types and likely needs to cope with a wide range of sensory cues. As in the many pollinating insects that have been the subject of allometric analyses to date [5,26,30,34], there would be a positive relationship between body size and the studied sensory organs but that the rates at which each trait increases in size with body size vary between sexes to reflect the differences in their behavioural ecology
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