Transformations of Head Structures During the Larval Development of the Black Soldier Fly <i>Hermetia illucens</i> (Stratiomyidae, Diptera)

Larvae of the black soldier fly, Hermetia illucens, are currently intensively studied, owing to their potential importance in various fields such as waste bioconversion, forensic entomology, and food supply for humans and life stock. Despite the increased attention, a detailed anatomical documentation of the larvae using modern methods is lacking, and even statements on the number of larval stages are contradictory. Misinterpretations of the ontogeny of this species have led to frequent erroneous identifications of the last larval instar as pupa. Consequently, many studies with a focus on larval morphology have neglected the last larval stage. In this contribution, we describe and document morphological changes of the larval head throughout the postembryonic development, with emphasis on the transition between the last two instars. This is characterized by a crucial behavioral shift from a feeding stage to a stage of increased vagility. We show that different cephalic structures undergo major changes, especially the mandibulo-maxillary complex and the digestive tract, and associated muscles. Our measurements of the body length and the length of the head capsule tentatively confirm that the larval development of H. illucens passes through seven instars.