Evolution of holometaboly revealed by developmental transformation of internal thoracic structures in a green lacewing Chrysopa pallens (Neuroptera: Chrysopidae).

Despite extensive studies on the morphology of holometabolous insects and their larvae, the morphological transformations of internal structures during metamorphosis have been rarely documented. Here, we used micro-computed tomography to investigate the developmental transformations of thoracic structures in the green lacewing Chrysopa pallens (Rambur, 1838) (Neuroptera: Chrysopidae), with emphasis on the development of the digestive, tracheal, and thoracic skeleto-muscular system. All the adult organs were modified during the prepupal or early pupal stage. The histolysis and remodeling began with the skeletal elements, followed by changes in the digestive system before it concluded with modifications of the musculature. Similar to the tracheal system's development, the digestive system did not disappear completely throughout metamorphosis but underwent a dramatic morphological change, which included the midgut significantly decreasing in size during the pupal stage. Our results provide important evidence for understanding the evolutionary pattern of developmental transformations in different major lineages of Holometabola.

Contribution to understanding the evolution of holometaboly: transformation of internal head structures during the metamorphosis in the green lacewing Chrysopa pallens (Neuroptera: Chrysopidae).

BACKGROUND: Metamorphosis remains one of the most complicated and poorly understood processes in insects. This is particularly so for the very dynamic transformations that take place within the pupal sheath of holometabolous insects. Only few studies address these transformations especially with regard to cranial structures of those holometabolous species where the larval and adult forms have a similar diet. It thus remains unclear to what extent the internal structures undergo histolysis and rebuilding. Here, the development of the brain and skeleto-muscular system of the head of Chrysopa pallens (Rambur, 1838) is studied. This species is a predator of aphids in the larval and adult stage. RESULTS: We used micro-computed-tomography (µ-CT) to study the transformations of the larval, prepupal and pupal head within the cocoon. We first assessed the morphological differences and similarities between the stages. We then determined the point in time when the compound eyes appear and describe the re-orientation of the head capsule which transforms the prognathous larva into a hypognathous adult. The internal head muscles are distinctly more slender in larvae than adults. In addition, the adults have a significantly larger brain which is likely needed for the processing of the signals obtained by the adults vastly expanded sensory organs that are presumably needed for dispersal and mating. Our study shows that the histolysis and modification of the inner muscles and skeletal elements take place within the prepupa. The central nervous system persists throughout metamorphosis but its morphology changes significantly. CONCLUSION: Our study reveals that not only the inner structures, but also the outer morphology continues to change after the final larval moult. The adult cuticle and internal structures form gradually within the cocoon. The histolysis and rebuilding begin with the skeletal elements and is followed by changes in the central nervous system before it concludes with modifications of the musculature. This order of events is likely ancestral for Holometabola because it is also known from Hymenoptera, Diptera, Mecoptera, and Coleoptera.

New short-horned flies (Diptera: Eremochaetidae) from the Early Cretaceous of China.

Eremochaetidae is a rare family found from the Late Jurassic to the Early Cretaceous. So far, only 8 genera with 12 species have been recorded. Herein, we describe a new species, Dissup clausus sp. nov., and the first male of Eremomukha (E.) tsokotukha Mostovski, 1996, from the Yixian Formation, the Early Cretaceous of Northeastern China. Additionally, E. (E.) insidiosa Mostovski, 1996 is considered as a new synonym of E. (E.) tsokotukha. An updated list of all known Eremochaetidae is presented.