Pleidae, the pygmy backswimmers, is a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

of aquatic

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s in the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Hemiptera Hemiptera (; ) is an order of insects, commonly called true bugs, comprising more than 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, assassin bugs, bed bugs, and shield bugs. They range in size from ...

(

infraorder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between Family_(biology), family and Class_(biology), class. In biological classification, the order is a taxonomic rank used in the classific ...

Nepomorpha Nepomorpha is an infraorder of insects in the "true bug" order (biology), order (Hemiptera). They belong to the "typical" bugs of the suborder Heteroptera. Due to their Aquatic insects, aquatic habits, these animals are known as true Heteroptera# ...

, or "true water bugs"). There are 37

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

in three

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

, distributed across most of the world, except the

polar region The polar regions, also called the frigid zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitu ...

s and remote

ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...

ic islands.Zack ''et al.'' (2007) 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

, which usually swim upside-down and, like the Pleidae, have a sensory organ in the center of the clypeus. Either the pygmy backswimmers are united with the Helotrephidae in the superfamily

Pleoidea Nepomorpha is an infraorder of insects in the "true bug" order (Hemiptera). They belong to the "typical" bugs of the suborder Heteroptera. Due to their aquatic habits, these animals are known as true water bugs. They occur all over the world ...

, or these two and the true backswimmers are placed in a single superfamily Notonectoidea.

Distribution

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

there is a single species, '' Plea minutissima'', widely found in that continent. ''

Plea In law, a plea is a defendant's response to a criminal charge. A defendant may plead guilty or not guilty. Depending on jurisdiction, additional pleas may be available, including '' nolo contendere'' (no contest), no case to answer (in the ...

'' species occur in the

Old World The "Old World" () is a term for Afro-Eurasia coined by Europeans after 1493, when they became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia in the Eastern Hemisphere, previously ...

, while ''

Neoplea ''Neoplea'' is a genus of pygmy backswimmers in the family Pleidae. There are at least three described species in ''Neoplea''. This species was also identified in the city of Naqadeh (Soldoz), West Azerbaijan Province, Iran. Species These three ...

'' is found in the

Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...

. '' Paraplea'' can be found in all warmer parts of Earth, including

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, where three species occur. Given their inability to fly well – if they can fly at all – it is not surprising that the Pleidae do not have as many

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

island

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

as some other

Heteroptera The Heteroptera are a group of about 40,000 species of insects in the order Hemiptera. They are sometimes called "true bugs", though that name more commonly refers to the Hemiptera as a whole. "Typical bugs" might be used as a more unequivocal al ...

(true bugs). Only five species are known from the entire

Malesia Malesia is a biogeographical region straddling the Equator and the boundaries of the Indomalayan and Australasian realms. It is a phytogeographical floristic region in the Paleotropical kingdom. It was first recognized as a distinct region ...

archipelago An archipelago ( ), sometimes called an island group or island chain, is a chain, cluster, or collection of islands. An archipelago may be in an ocean, a sea, or a smaller body of water. Example archipelagos include the Aegean Islands (the o ...

for example. ''

Neoplea apopkana ''Neoplea'' is a genus of pygmy backswimmers in the family Pleidae. There are at least three described species in ''Neoplea''. This species was also identified in the city of Naqadeh (Soldoz), West Azerbaijan Province, Iran. Species These three ...

'' has been introduced to the

Hawaiian Islands The Hawaiian Islands () are an archipelago of eight major volcanic islands, several atolls, and numerous smaller islets in the Pacific Ocean, North Pacific Ocean, extending some from the Hawaii (island), island of Hawaii in the south to nort ...

Polynesia Polynesia ( , ) is a subregion of Oceania, made up of more than 1,000 islands scattered over the central and southern Pacific Ocean. The indigenous people who inhabit the islands of Polynesia are called Polynesians. They have many things in ...

. One species, originally native to

South Asia South Asia is the southern Subregion#Asia, subregion of Asia that is defined in both geographical and Ethnicity, ethnic-Culture, cultural terms. South Asia, with a population of 2.04 billion, contains a quarter (25%) of the world's populatio ...

and

Southeast Asia Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...

, was found in aquaria in

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

. In 2006, '' Paraplea puella'' was found to be introduced to

Guam Guam ( ; ) is an island that is an Territories of the United States, organized, unincorporated territory of the United States in the Micronesia subregion of the western Pacific Ocean. Guam's capital is Hagåtña, Guam, Hagåtña, and the most ...

Micronesia Micronesia (, ) is a subregion of Oceania, consisting of approximately 2,000 small islands in the Northwestern Pacific Ocean. It has a close shared cultural history with three other island regions: Maritime Southeast Asia to the west, Poly ...

. Like in the preceding case, this originally American species was probably accidentally introduced with aquarium plants.

Description

These tiny

are only 2–3 mm long and usually of a light brownish color overall. Their body is oval, plump, with a concave back. Their

compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...

s are large and the

ocelli A simple eye or ocellus (sometimes called a pigment pit) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most vertebrates. These eyes are called "simple" to distinguish the ...

vestigial, like in other

. Also characteristic for the infraorder are the short and weak antennae which are usually carried tucked to the head. In the case of the Pleidae the antennae are shorter than the head is long and only consist of three segments. The scutellum is small and triangular. 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. By and large the Pleidae may be considered an effectively flightless group when it comes to

biogeography Biogeography is the study of the species distribution, distribution of species and ecosystems in geography, geographic space and through evolutionary history of life, geological time. Organisms and biological community (ecology), communities o ...

and dispersal into new

habitat In ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...

. The tarsi consist of two or three segments; two claws are borne on the last tarsal segment of the hindlegs. 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 Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

and allows them to swim well regardless. Both sexes are able to stridulate. The sounds they produce apparently have an intraspecific

communication Communication is commonly defined as the transmission of information. Its precise definition is disputed and there are disagreements about whether Intention, unintentional or failed transmissions are included and whether communication not onl ...

function, as the animals are able to perceive and react to them. Possibly they make sounds to maintain contact among the loose swarms in which the Pleidae roam their

. The

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e have

gland A gland is a Cell (biology), cell or an Organ (biology), organ in an animal's body that produces and secretes different substances that the organism needs, either into the bloodstream or into a body cavity or outer surface. A gland may also funct ...

s between the third and fourth abdominal tergite. Development is direct, without a

pupa A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...

l stage.

Ecology

Pygmy backswimmers inhabit lacustrine

ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...

s, where they occur in loose groups. An example of a pygmy backswimmer habitat is a suburban pond with abundant filamentous algae. Like many of their relatives, they are

predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

y, hunting other tiny

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s, from which they suck out the body fluids with their

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit ** podium * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ...

. They can to some extent biologically control

mosquito Mosquitoes, the Culicidae, are a Family (biology), family of small Diptera, flies consisting of 3,600 species. The word ''mosquito'' (formed by ''Musca (fly), mosca'' and diminutive ''-ito'') is Spanish and Portuguese for ''little fly''. Mos ...

larvae. Unlike true backswimmers (Notonectidae), they are completely harmless to humans, as their rostrum is far too small to pierce skin. 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. This is ultimately because as all true water bugs, Pleidae are air-breathers without

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s. 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. This air is contained in a felt-like cushion on the underside. Thus, the belly has higher

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

, resulting in the animal turning upside down as soon as it lets loose from the substrate.

Footnotes

References

* Read the

ResearchGate ResearchGate is a European commercial social networking site for scientists and researchers to share papers, ask and answer questions, and find collaborators. According to a 2014 study by ''Nature'' and a 2016 article in ''Times Higher Education' ...

Article
The Pleidae (Hemiptera, Heteroptera) of Thailand, with the descriptions of two new species and a discussion of species from Southeast Asia
* (2004): Phylogeny of the true water bugs (Nepomorpha: Hemiptera–Heteroptera) based on 16S and 28S rDNA and morphology. ''Systematic Entomology'' 29(4): 488-508.