Soldierfly adults are typical true flies in shape, and are often brightly coloured, including some with wasp mimic colouration and somewhat resembling hoverflies, but with a broader flattened abdomen..
They legs the eggs on plants near water and the larvae hatch and craw to the water. The larvae’s long clearly segmented body looks armour plated, and is somewhat laterally flattened with a small head. The skin is dull coloured, usually brown or dull green and is hardened and leathery, limiting their flexibility, though their segmentation allows some movement. The species belonging to the large sized Stratiomys can reach over 25mm long.
They have a set of hydrophobic (water repellent hairs) at the tip of the abdomen which allows them to pierce the waters surface and breathe the air above it. When the larva submerges these hairs trap a bubble of air so to act as an air supply and physical gill.
The larvae usually feed on algae and detritus though some species are predators. They are found in ponds and ditches, typically in shallow water and among vegetation, with some species being tolerant of saline conditions and other able to survive their water bodies drying out by living in the remaining mud. Some species are found in running water, like springs a streams.
Once fully grown the larvae pupate, with the pupa forming within the larval skin, which hardens as this takes place. Pupation is reported to happen on land., but I have found these pupa a few times found floating and not moving on the surface of pond, presumably having been dislodged from the marginal vegetation.
Adults emerge from the pupa and hardened larval skin and depending on species can be found around bogs, meadows and marshland, where adults feed on plant sap and nectar and pollen from flowers.