Pygmy Backswimmers: these tiny insects are able to breathe underwater by keeping pockets of air tucked against their bellies
Above: a pygmy backswimmer with an air bubble on its belly
Aquatic insects from the families Pleidae, Notonectidae, and Corixidae are commonly known as backswimmers or boatsmen, because they swim backward with their bellies facing up, and they often propel themselves through the water by "rowing" with their legs. The pygmy backswimmers (family Pleidae) are the smallest of these insects, measuring just 1.5-3mm long.
Above: backswimmers of family Pleidae
Pygmy backswimmers have compact little bodies, wide heads, and large, red eyes that give them a very distinctive appearance. The adults are typically gold, beige, and/or yellow with silver markings on their shells.
Above: Plea minutissima, a species that can be found throughout Europe
Like many aquatic bugs, the pygmy backswimmer is an air-breathing insect that can only survive underwater by carrying its own pocket of air around as it swims, like a tiny scuba diver with a tank of oxygen. The bubble is carried on the ventral side of the backswimmer's body, which gives the belly a silvery appearance.
Above: a pygmy backswimmer with a pocket of air
As this article explains:
Pleidae are air-breathers and carry an air reserve with them which is periodically replenished by a dash to the water's surface. This air is contained in a felt-like cushion on the underside, thus the belly has a higher buoyancy.
These backswimmers can also secrete a substance that contains large amounts of hydrogen peroxide and other bactricidal compounds; they routinely use that secretion to disinfect the cuticle that surrounds their belly and other surfaces where the oxygen is stored.
Above: this photo shows a pygmy backswimmer standing on a person's fingertip, demonstating just how tiny these insects really are
Pygmy backswimmers are predators that hunt other small invertebrates, using a beak-like appendage known as a rostrum to pierce the victim's shell and then feeding on the fluids within. Their mouthparts cannot penetrate human skin, however, and they're completely harmless to people.
According to this article:
[Pygmy backswimmers] occur globally except at the Poles and some distant oceanic islands, and almost all dally in clear, still, weedy waters. Look for them, often in small groups, among the leaves of submerged vegetation. Though they may inhabit ephemeral ponds and can dry out for a while when the pond does, they generally live in permanent waters.
Above: Plea minutissima submerged just below the surface of the water
Sources & More Info:
UNH Center for Freshwater Biology: Pleidae
Saukville Field Station: Pygmy Backswimmer
Scientific Reports: The Discovery of an Overseen Pygmy Backswimmer in Europe
Royal Entomological Society: Secretion-Grooming in the Water Bug Plea minutissima
Bugs of the World: Metathoracic Scent Gland Secretions of Adult Pleids
Australian Faunal Directory: Family Pleidae
Guide to Common Plants and Animals of North American Ponds and Lakes: Pygmy Backswimmers
