Larva morphology of shore flies <i>Ephydra riparia</i> and <i>Paracoenia fumosa</i> (Diptera: Ephydridae) and adaptation of Diptera to increased salinity

Abstract

Larvae of many shore fly species (family Ephydridae) are adapted to living in water with high or extremely high salinity. Little is known about the morphological and physiological foundations of such adaptation. We described the details of the morphology of third-instar larvae of two shore flies: Ephydra riparia and Paracoenia fumosa and presented the scanning electron microscopy (SEM) images. For the first time, by silver-staining and SEM, we proved that the larvae of both studied species had anal organs (AO) – specialized structures that serve an osmoregulatory function and are responsible for the transport of ions from the environment to the larval hemolymph (but not in the opposite direction). We compared the larvae morphology of the studied species with some other shore fly species from the genera Ephydra, Paracoenia, Hydrellia, and Coenia, as well as with larvae of the model species Drosophila melanogaster (family Drosophilidae). Special attention was paid to the morphology of AO, which contribute to the adaptation of larvae to increased salinity. Extremely halophilic species either do not have AO, or they are poorly developed, while moderately halophilic shore flies have more developed features connected with the permeability of the AO cuticle and active ion transport. These features are most developed in freshwater shore flies. AO activity can vary due to the shape and area of the AO, the smoothness or wrinkling of the cuticle, and the presence of nanoscale pits on it. Described variability of AO structure is probably adaptive since, at high salinity, both the permeability of the AO cuticle and the active transport of ions from the environment to the hemolymph become less beneficial or even harmful.