Species interactions represent one of the central topics in biological research. Due to their high ecological and economical significance a lot of effort has especially been put into furthering our understanding of associations and adaptations between insects and their corresponding host plants. These adaptations are crucial for a species evolutionary fate as they can affect the exploitation of a specific resource and, thus, the fitness of multiple generations. The concept of modularity has become an increasingly important tool to understand and explain the emergence and development of host specific adaptations. In the context of this project, adaptations are considered as modules of co-expressed genes and their associated phenotypes. Plants consist of a diverse and complex composition of various chemical compounds with only a portion being involved in defensive mechanisms against herbivores. This chemical variation can now represent a challenge for polyphagous insects since it requires a flexible set of adaptations to cope with different plant properties. The comma butterfly, Polygonia c-album, covers a wide range of different host species. Expressing all genes necessary to feed and survive on all potential hosts would probably be detrimental and costly. As opposed to this, previous studies showed that different modules of genes are activated in respect to a particular host. To further investigate these patterns, host switch experiments were performed, in which larvae were switched to different host plant species (Urtica, Salix and Ribes) during the course of their development. Gene expression data from larval gut genes were combined with data (RNA and performance) from a previous project to compare and further investigate the exclusiveness of expression patterns and the degree of modularity in respect to a particular host. In an additional experiment (“selection experiment”), gene expression will also be monitored over multiple generations of individuals that were selected for increased performance on a particular host. In combination with further host switch experiments, this data will give information about the stability of host specific modules over time and also allows testing for the role of trade-offs for the development of host-specific adaptations. Moreover, two ecotypes of P. c-album, that differ in the degree of specialization to a particular host, will be compared and crossed (“cross experiment”). This will not only allow to test how conserved transcriptional modules are across populations but also provide new insights into their role in specialization to different hosts. In the scope of a pilot project RNA from single cells will be collected from midguts of larvae that were reared on different hosts ("single cell experiment"). This will help to understand the involvement of specific cell types in the metabolization of plant material, and will, thus, allow us to study the modularity aspect of host specific adaptations at a cellular level. This project will significantly contribute to a further understanding of the mechanisms and genetic basis underlying adaptations to specific environments (i.e. hosts) and, thus, to an increase in the knowledge and predictability of species associations and interaction dynamics.