Antennal Morphology and the Use of Olfaction in Host and Mate Location in the Magpie Moth, Nyctemera annulata

Abstract

Insects rely on information obtained from the surrounding abiotic and biotic environment when locating biologically important resources, such as food, mates, and oviposition sites. The transmission of such information is achieved through a range of different sensory modalities, including acoustic, visual, and olfactory cues and signals, with the latter being more predominantly exploited in insects. The detection of these airborne chemicals is mediated by the antennae, which are endowed with various sensory structures, called sensilla. Sensilla type, distribution, and density may provide important information regarding the ability of individuals to detect and process such external stimuli. Moreover, sexual dimorphism in these characteristics within a species may reflect a divergence in sensory functions and requirements relating to the procurement of different resources. The link between olfactory stimuli and antennal morphology in host and mate location is investigated here in the endemic New Zealand magpie moth, Nyctemera annulata. This species provides an ideal system in which to study these aspects of resource location; larvae and adults of this species have a particular partiality towards ragwort (Jacobaea vulgaris) as a host plant (which provides a source of food and oviposition sites) and males possess extremely plumose bipectinate antennae indicative of an adaptation for detecting female sex pheromones. The primary aims of this study were threefold: to investigate whether larvae and adults of this species are capable of detecting and responding to the chemical volatiles emitted from their host plant; to test if adult males can detect and respond to sex pheromones released from conspecific females; and to quantitatively investigate the sensory architecture of larval and adult antennae. I carried out hostand mate-choice behavioural trials using glass Y-maze olfactometers to examine the first two aims, and complemented these findings with scanning electron microscopy (SEM) to quantify antennal morphology and sensilla structures. The results obtained from the behavioural trials provide evidence that N. annulata are capable of using olfactory cues and signals in host and mate location. Larvae and mated females demonstrated a strong attraction to their host plant, ragwort, using only olfactory cues, and furthermore, larvae were able to use these cues to distinguish ragwort from a non-host plant. Virgin females and males had a low response in the host-choice trials, although they did display higher attraction to the ragwort compared to the empty control. Males did, however, show a very high response rate and attraction to the olfactory signals of virgin females during the mate-choice trials. From SEM, I was able to identify 17 different types of sensilla on the antennae of adult male and female N. annulata including four subtypes of sensilla trichodea (s. trichodea), three subtypes of s. chaetica, five subtypes of s. basiconica, and one each of s. auricillica, s. squamiformia, s. styloconica, s. coeloconica, and Böhm’s bristles. Sensilla chaetica, s. basiconica, and s. styloconica were also observed on the larval antennae. A degree of sexual dimorphism was apparent in the overall antennal morphology as well as in the distribution, density, and morphology of particular types of sensilla in adult N. annulata, which likely represents a divergence in sensory functions based on the different resource requirements of males and females. Overall, this thesis demonstrates that N. annulata are capable of using olfactory stimuli to locate resources and that they possess a diverse suite of sensory structures enabling them to do so. Furthermore, this thesis is the first to present a synthesis of qualitative and quantitative data concerning the behavioural and morphological characteristics relating to host and mate location in N. annulata.