Transformation of head structures during the metamorphosis of Chrysomela populi (Coleoptera: Chrysomelidae)

External and internal head structures of last instar (3rd) larvae, 4th day 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. In addition to long known transformations such as the appearance of compound eyes and the reorientation of the head capsule, the adult differs from the larva by having elongated antennae and palps, a gula and anterior and dorsal tentorial arms. Additionally, several changes in the muscular system occur. Most of these cephalic 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 4th day pupal head shows a combination of adult and larval characters. Almost all adult cephalic 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