Where the Immatures of Triassic Diptera Developed

Abstract

Immature Diptera are more diverse and abundant in fresh water than any other insect order. The question arises whether the earliest dipterans, known from the Upper Buntsandstein of Europe (early Anisian, Middle Triassic), already developed in water and whether such mode of life was inherited from the ancestor of the order or if, alternatively, the dipteran larvae were ancestrally terrestrial. Nematoceran immatures have become common and diverse in the Mesozoic fossil record since the Middle Jurassic, but the vast majority of them represent only two culicomorphan families, Chaoboridae and Chironomidae. Earlier records and records of immatures of other families from the Mesozoic are extremely rare. A total of four larvae and about 30 pupae have been described from Anisian assemblages of France and Spain. Among these, one larva clearly belongs to Culicomorpha, the infraorder most closely associated with fresh waters, and one larva to Bibionomorpha, the infraorder most closely associated with terrestrial habitats, while the rest are hard to classify. Nevertheless, most of the pupae are morphologically similar to the semi-aquatic pupae of extant Limoniidae and Ptychopteridae from wet habitats. The oligopneustic respiratory systems of the Anisian larvae and their apparently allochthonous burial also suggest their semi-aquatic development, which appears to have been the ancestral mode of life of Diptera. The absence of dipteran immatures in lacustrine Konservat-Lagerstätten of the Madygen and the Cow Branch Formations (Ladinian–Carnian and Norian, respectively) is explained by the aquatic conditions being unfavorable for insect immatures and invertebrates in general.