The olfactory sense is crucial for organisms, facilitating environmental recognition and inter-individual communication. Ithomiini butterflies exemplify this importance not only because they rely strongly on olfactory cues in both inter- and intra-sexual behaviours, but also because they show re-emergence of macro-glomerular complexes (MGCs). These specialized structures within the antennal lobe, lost in butterflies, but present in moths where they enable the integration of information from various types of pheromones, refining responses to specific cues. In this study we present high-quality genome assemblies for four Ithomiini species, investigating chemosensory evolution and associating expression profiles with neuroanatomical differences. We found that antennal transcriptomes across species exhibit profound divergence, indicating distinct species adaptations in environmental sensing. Noteworthy, sexual dimorphism is also characterised both in chemosensory genes and non-chemosensory genes, suggesting their relevance in behaviour. Lipid-related genes exhibit sexual dimorphism, potentially linked to pheromone production or host selection. The study broadens the understanding of antennal chemosensory adaptations in butterflies, highlighting the intricate interplay between genetic diversity, ecological specialization, and sensory perception with neuro-anatomical differences. Insights into chemosensory gene evolution, expression patterns, and potential functional implications enhance our knowledge of sensory adaptations in butterflies, laying the foundation for future investigations into the genetic drivers of behaviour, adaptation, and speciation in insects.

Insect herbivores, such as lepidopteran larvae, often have close evolutionary relationships with their host plants, with which they may be locked in an evolutionary arms race. Larval grouping behaviour may be one behavioural adaptation that improves host plant feeding, but aggregation also comes with costs, such as higher competition and limited resource access. Here, we use the Heliconiini butterfly tribe to explore the impact of host plant traits on the evolution of larval gregariousness. Heliconiini almost exclusively utilise species from the Passifloraceae as larval host plants. Passifloraceae display incredible diversity of form, leaf shape and a range of anti-herbivore defences, suggesting they are locked in an arms race with Heliconiini larvae. By analysing larval social behaviour as both a binary (solitary or gregarious) and categorical (increasing larval group size) trait, we revisit the multiple origins of larval gregariousness across Heliconiini. We investigate whether host habitat, leaf defences and leaf size are important drivers of, or constraints on, larval gregariousness. Whereas our data do not reveal links between larval gregariousness and the host plant traits included in this study, we do find an interaction between larval host specialisation and behaviour, revealing gregarious larvae to be more likely to feed on a narrower range of host plant species than solitary larvae. We also find evidence that this increased specialisation typically precedes the evolutionary transition to gregarious behaviour. The comparatively greater host specialisation of gregarious larvae suggests that there are specific morphological and/or ecological features of their host plants that favour this behaviour.