An acarinarium is a specialized anatomical structure which is evolved to facilitate the retention of mite

Mite

Mites, along with ticks, are small arthropods belonging to the subclass Acari and the class Arachnida. The scientific discipline devoted to the study of ticks and mites is called acarology.-Diversity and systematics:...

s on the body of an organism, typically a bee or a wasp. The term was introduced by Walter Karl Johann Roepke

Walter Karl Johann Roepke

Walter Karl Johann Roepke was a German entomologist who specialised in Lepidoptera and ColeopteraRoepke was educated in the University of Berlin and the University of Zürich where he took his Ph.Dr’s degree in 1907...

.

Evolution

The acarinarium has evolved to enhance the mutualistic relationship between the mites and the host organism. There are numerous cases where mites are phoretic on organisms that benefit from the mites' presence; cases where the host's body has changed over evolutionary time to accommodate the mites are far less common. The best-known examples are among the Apocrita

Apocrita

Apocrita is a suborder of insects in the order Hymenoptera.Apocrita includes wasps, bees and ants, and consists of many families. It includes the most advanced hymenopterans and is distinguished from Symphyta by the narrow "waist" formed between the first two segments of the actual abdomen; the...

n Hymenoptera

Hymenoptera

Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...

, in which the hosts are typically nest-making species, and it appears that the mites feed on fungi in the host nests (thus keeping away the fungi from host's offspring or their provisions), or possibly other parasites or mites whose presence in the nest is detrimental to the hosts. It is especially telling that nearly all the examples involve only the females of the host species, as it is the females that build and provision the nests. Fossil evidence of halictid

Halictidae

Halictidae is a cosmopolitan family of the order Hymenoptera consisting of small to midsize bees which are usually dark-colored and often metallic in appearance...

 bees with an acarinarium is found in the extinct genus Oligochlora

Oligochlora

Oligochlora is an extinct genus of sweat bee in the Halictidae subfamily Halictinae. The genus currently contains six species, all of which are known from the early Miocene Burdigalian stage Dominican amber deposits on the island of Hispaniola....

from Dominican amber

Dominican amber

Dominican amber is amber from the Dominican Republic. Resin from the extinct species Hymenaea protera is the source of Dominican amber and probably of most amber found in the tropics....

 deposits on Hispaniola

Hispaniola

Hispaniola is a major island in the Caribbean, containing the two sovereign states of the Dominican Republic and Haiti. The island is located between the islands of Cuba to the west and Puerto Rico to the east, within the hurricane belt...

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The presence or absence of this structure has been used as a taxonomic character.

Variations

Several examples can be found among the bee

Bee

Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s, and in most such cases, only the females possess acarinaria:

• Various forms of acarinaria have evolved within different lineages of carpenter bee
  Carpenter bee
  Carpenter bees are large, hairy bees distributed worldwide. There are some 500 species of carpenter bee in 31 subgenera...
  
  s, in the Old World subgenera Koptortosoma and Mesotrichia - in some cases, the entire anterior portion of the metasoma
  Metasoma
  The metasoma is the posterior part of the body, or tagma, of arthropods whose body is composed of three parts, the other two being the prosoma and the mesosoma. In insects, it contains most of the digestive tract, respiratory system, and circulatory system, and the apical segments are typically...
  
   is hollowed out into an enormous internal chamber, with a small opening on the face of the first metasomal tergite through which the mites can travel. Some species in these groups also have supplementary acarinaria on the mesosoma in addition to the metasomal chamber.
• The augochlorine halictid
  Halictidae
  Halictidae is a cosmopolitan family of the order Hymenoptera consisting of small to midsize bees which are usually dark-colored and often metallic in appearance...
  
   genus Thectochlora is characterized by a dense brush of hairs just in front of the anterior face of the first metasomal tergite (the same effective location as in carpenter bees), which sets off a "pouchlike" space in which the mites are carried.
• In the nominate subgenus of the halictine genus Lasioglossum
  Lasioglossum
  The sweat bee genus Lasioglossum is the largest of all bee genera, containing over seventeen hundred species in numerous subgenera worldwide. They are highly variable in size, coloration, and sculpture; among the more unusual variants, some are cleptoparasites, some are nocturnal, and some are...
  
  , many species have the anterior face of the first metasomal tergite modified by the elimination of the central seta
  Seta
  Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....
  
  e, forming a central glabrous area, around which there is a fringe of appressed hairs, and mites cling to this area. Species without this modification rarely if ever have mites in this region, while those with the glabrous area almost never lack mites, so despite its simple nature it does indeed appear to function as an acarinarium. In some cases, sister species may differ in this feature, one having an acarinarium, and one lacking it, suggesting that the trait can be lost and possibly regained.

Other examples include:

• In the potter wasp
  Potter wasp
  Potter wasps are a cosmopolitan wasp group presently treated as a subfamily of Vespidae, but sometimes recognized in the past as a separate family, Eumenidae.-Recognition:...
  
   genus Parancistrocerus
  Parancistrocerus
  Parancistrocerus is a rather large genus of potter wasps whose distribution spans over the Nearctic, Palearctic, Oriental and Neotropical regions. Their species are notable for having a smopth depression acting as an acarinarium at the base of the second metasomal tergum and hidden under the first...
  
  , the base of the second metasomal tergite is concave, and covered by the posterior lip of the first metasomal tergite, and mites are carried in the concavity. This structure appears the same in both males and females, though it is difficult to assess whether both sexes carry mites with equal frequency.