Transformations of head and thorax structures during the metamorphosis of Chrysomela populi (Coleoptera, Chrysomelidae)

External and internal head and thorax structures of last instar (3rd) larvae, the fourth day of pupae and adults of Chrysomela populi were examined using a combination of traditional and modern morphological techniques, especially µ-computed tomography and computer-based 3D reconstruction. Morphological differences and similarities between the stages were assessed. The most profound changes in the adult include the reorientation of the head capsule, the elongation of the antennae and palps, and the presence of compound eyes, a gula, anterior and dorsal tentorial arms and wings. Additionally several changes in the muscular system occur. Most of these transformations are related to different tasks playing a predominant role in the developmental stages: feeding in the larvae, and dispersal and mating in the adults, the latter requiring improved sensory perception. The pupal head and thorax show a combination of adult and larval characters. Almost all adult elements are already present, but the internal anatomy shows several larval traits, especially in the musculature. The central nervous system is intermediate with enlarged optic lobes but still identifiable individual nerves cords. A short historical review on the application of non-invasive methods to study morphological details of metamorphosis is provided and the advantages and limitations of these approaches are discussed.