More than dead males: reconstructing the ontogenetic series of terrestrial non-biting midges from the Eocene amber forest

Abstract

of the diversity within Holometabola results from only four hyperdiverse lineages, one of which is Diptera. Nonbiting midges (Chironomidae) have been considered to be among the most successful ingroups of Diptera in the history of the group (Marshall 2012). There are at least 6300 formally described extant species of Chironomidae. Their larvae have colonized most of the available freshwater habitats, as well as some marine, sub-terrain and terrestrial habitats (Langton 1995, Ferrington 2008, Andersen et al. 2016). Terrestrial larvae of Chironomidae are important for our understanding of the physiology and evolutionary biology of the group. The reason for that lies in the habitation of the larvae in non-aquatic habitats, such as wet soil, dung, leaf-litter, moss carpets etc., which is generally considered to be a secondary specialisation for the group (Langton 1995, Delettre 2005). Therefore, for long, terrestrial larvae of Chironomidae have been considered as models for desiccation adaptation, osmoregulation and cryptobiosis (Frouz 1997, 2010; Wichard et al. 2002). In extant ecosystems representatives of Chiro nomidae with terrestrial larvae are most common in areas with tropical or moderate climate and high saturation of the air with water vapor (Andersen et al. 2015, 2016; Zelentsov et al. 2012). The fossil record is usually devoid of terrestrial larvae of Chironomidae, as these are not normally found in the areas of the active sedimentation (i.e. lake basins, river deltas; e.g. Wichard et al. 2009). Nevertheless, some amber deposits have a rather high proportion of adult non-biting midges that should possess terrestrial larvae, based on phylogenetic reasoning (Zelentsov et al. 2012). In Rovno amber in particular, the share of such adult morphotypes of Chironomidae reaches 40.1% (Zelentsov et al. 2012). Such morphotypes are also relatively abundant in Baltic and Bitterfeld (Saxonian) ambers (Hoffeins & Hoffeins 2003, Seredszus & Wichard 2007). Giving the abundance of these adult morphotypes, we should expect to find corresponding immature stages, hence terrestrial larval representatives of Chironomidae in Eocene amber deposits (Zelentsov et al. 2012). Such a probability is even increasing due to the relatively high abundance of bryophytes, which serve as a substrate for modern terrestrial larvae of Chironomidae, in amber (Weitschat & Wichard 2010). Indeed, Andersen et al. (2015) reported the exuvia of a pupa of Chironomidae from Rovno amber, more precisely a possible repre sentative