Pleidae belong to the Tripartita which contains the more advanced lineages of true water bugs, and are closely related to the true backswimmers (Notonectidae), but closer still to the Helotrephidae, another family of tiny Nepomorpha, which usually swim upside-down and, like the Pleidae, have a sensory organ in the center of the clypeus.[2] Either the pygmy backswimmers are united with the Helotrephidae in the superfamilyPleoidea, or these two and the true backswimmers are placed in a single superfamily Notonectoidea.[3]
These tiny Hemiptera are only 2–3 mm long and usually of a light brownish color overall.[9] Their body is oval, plump, with a concave back.[9] Their compound eyes are large and the ocelli vestigial, like in other Nepomorpha.[9] Also characteristic for the infraorder are the short and weak antennae which are usually carried tucked to the head.[9] In the case of the Pleidae the antennae are shorter than the head is long and only consist of three segments.[10]
The scutellum is small and triangular.[8] The wings are reduced in some fully flightless species, but normally developed in most; due to their compact, rotund bodies and generally short wings, even those with well-developed wings fly weakly at best.[9] By and large the Pleidae may be considered an effectively flightless group when it comes to biogeography and dispersal into new habitat.[7]
The tarsi consist of two or three segments; two claws are borne on the last tarsal segment of the hindlegs.[11] Though the hindlegs are hairless and appear ill-suited for swimming compared to the stout "flippers" of the water boatmen (Corixidae) or the backswimmers (Notonectidae), the small size of the pygmy backswimmers makes for different physics and allows them to swim well regardless[citation needed].
Both sexes are able to stridulate. The sounds they produce apparently have an intraspecificcommunication function, as the animals are able to perceive and react to them.[12] Possibly they make sounds to maintain contact among the loose swarms in which the Pleidae roam their habitat[citation needed].
Pygmy backswimmers inhabit lacustrineecosystems, where they occur in loose groups.[5] An example of a pygmy backswimmer habitat is a suburban pond with abundant filamentous algae. Like many of their relatives, they are predatory, hunting other tiny invertebrates, from which they suck out the body fluids with their rostrum.[9] They can to some extent biologically controlmosquito larvae[citation needed]. Unlike true backswimmers (Notonectidae), they are completely harmless to humans, as their rostrum is far too small to pierce skin.[9]
Their hindleg claws enable them to clamber through vegetation which apart from swimming is their main form of locomotion. Like Notonectidae, they are in an upside-down position when swimming.[9] This is ultimately because as all true water bugs, Pleidae are air-breathers without gills.[9] Similar to the true backswimmers, pygmy backswimmers carry an air reserve with them which is periodically replenished by a dash to the water's surface.[9] This air is contained in a felt-like cushion on the underside. Thus, the belly has higher buoyancy, resulting in the animal turning upside down as soon as it lets loose from the substrate.[9]
Hebsgaard, Martin B.; Andersen, Nils M. & Darmgaard, Jacob (2004): Phylogeny of the true water bugs (Nepomorpha: Hemiptera–Heteroptera) based on 16S and 28S rDNA and morphology. Systematic Entomology29(4): 488-508. HTML abstractPDF fulltext
Zack, Richard S.; Moore, Aubrey & Miller, Ross H. (2007): First record of a pygmy backswimmer (Hemiptera: Heteroptera: Pleidae) from Micronesia. Zootaxa1617: 67-68. PDF fulltext
