The Acroceridae are a small

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 odd-looking flies. They have a hump-backed appearance with a strikingly small head, generally with a long proboscis for accessing nectar. They are rare and not widely known. The most frequently applied common names are small-headed flies or hunch-back flies. Many are bee or wasp mimics. Because they are

parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...

s of spiders, they also are sometimes known as spider flies.

Description

The Acroceridae vary in size from small to fairly large, about the size of large bees, with a wingspan over 25 mm in some species. As a rule, both sexes have tiny heads and a characteristic hump-backed appearance because of the large, rounded thorax. In appearance, they are compact flies without major bristles, but many species have a bee-like hairiness on their bodies, and some are bee or wasp mimics. In most species, the eyes are

holoptic Holoptic refers to one of the ways in which the arthropod eye develops, particularly the eyes of various species of insects. Unlike dichoptic and cycloptic eyes, holoptic eyes meet along the median dorsal line of the head, in many species near ...

in both sexes, the heads seemingly composed mainly of the large faceted eyes. This is in contrast to many insects in which the males have larger (even holoptic) eyes, whereas the females have normal eyes. The squamae are disproportionately large, completely covering the

halteres ''Halteres'' (; singular ''halter'' or ''haltere'') (from , hand-held weights to give an impetus in leaping) are a pair of small club-shaped organs on the body of two Order (biology), orders of flying insects that provide information about ...

, and the abdomen has an inflated appearance, often practically globular. The tarsi are equipped with large claws with three pulvilli below them.

Taxonomy

The Acroceridae are a small family in the

Brachycera The Brachycera are a suborder of the order Diptera. It is a major suborder consisting of around 120 families. Their most distinguishing characteristic is reduced antenna segmentation. Description A summary of the main physical characteristic ...

. They are members of the infraorder

Muscomorpha The Brachyceran infraorder Muscomorpha is a large and diverse group of flies, containing the bulk of the Brachycera and most of the known fly, flies. It includes a number of the most familiar flies, such as the housefly, the Drosophila, fruit fly ...

, and DNA studies suggest that they are most closely related to the families

Nemestrinidae Nemestrinidae, or tangle-veined flies is a family of flies in the superfamily Nemestrinoidea, closely related to Acroceridae. Larvae are endoparasitoids of either grasshoppers (Trichopsideinae) or scarab beetles (Hirmoneurinae). Some are co ...

and

Bombyliidae The Bombyliidae are a family of flies, commonly known as bee flies. Some are colloquially known as bomber flies. Adults generally feed on nectar and pollen, some being important pollinators. Larvae are mostly parasitoids of other insects. Over ...

. A 2013 analysis of morphological data suggested the Acroceridae were a sister group to the Asiloidea and Eremoneura. Lambkin, Christine L., et al. "The phylogenetic relationships among infraorders and superfamilies of Diptera based on morphological evidence." Systematic Entomology 38.1 (2013): 164-179. The roughly 520 species are placed in 50 genera. In 2019, a revised classification of the family based on phylogenetic studies was published, listing five extant subfamilies and one extinct subfamily containing ''

Archocyrtus ''Archocyrtus'' is an extinct genus of small-headed flies in the family Acroceridae. The genus is known from compression fossils from the Late Jurassic Karabastau Formation of Kazakhstan. The genus is the only member of the subfamily Archocyrtinae ...

'' from the Late Jurassic

Karabastau Formation The Karabastau Formation () is a geological formation and lagerstätte in the Karatau Mountains of southern Kazakhstan whose strata date to the Middle to Late Jurassic. It is an important locality for insect fossils that has been studied since ...

of Kazakhstan. Obsolete synonyms for Acroceridae include Cyrtidae, Oncodidae, and Ogcodidae.

Distribution and habitat

Acroceridae are cosmopolitan in distribution, but nowhere abundant. They appear episodically and in most places are rarely observed; of more than 500 species described, most are known from fewer than 10 specimens. They occur most commonly in semiarid tropical locations. Clintonia uniflora 4915 cropped

Behaviour

As far as is known, all Acroceridae are

s of spiders. They are most commonly collected when a spider from the field is brought into captivity. As in the related families, Bombyliidae and Nemestrinidae, members of the family undergo

hypermetamorphosis Hypermetamorphosis, or heteromorphosis, is a term used mainly in entomology; it refers to a class of variants of holometabolism, that is to say, complete insect metamorphosis. Hypermetamorphosis is exceptional in that some instars, usually larva ...

: the adults do not seek out their hosts; instead, the first-instar larva is a

planidium A planidium is a specialized form of insect larva seen in the first-instar of a few families of insects that have parasitoidal ways of life. They are usually flattened, highly sclerotized (hardened), and quite mobile. The function of the plani ...

. Females lay large number of eggs, up to 5,000, and after hatching, the planidia seek out spiders. They do not resemble the triungulin of most beetles with a hypermetamorphosis, but do resemble the triungulin of ''

Stylops ''Stylops'' is a genus of obligately Parasitism, endoparasitic insects in the family Stylopidae. Hosts are typically members of the order Hymenoptera. The name "stylops", used without a capital "s", refers as a common name to any member of the ...

''. The larva can move with a looping movement like a leech or

inchworm The geometer moths are moths belonging to the family Geometridae of the insect order Lepidoptera, the moths and butterflies. Their scientific name derives from the Ancient Greek (derivative form of or "the earth"), and "measure" in referen ...

, and can leap several millimetres into the air. When a spider contacts an acrocerid planidium, the planidium grabs hold, crawls up the spider's legs to its body, and forces its way through the body wall, usually at an articulation membrane. Often, it lodges near a

book lung A book lung is a type of respiration organ used for atmospheric gas-exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open, ventral-abdominal, air-filled cavity (atrium) and co ...

, where it may remain for years before completing its development. Mature larvae pupate outside the host. Acroceridae Psilodera Small-headed fly IMG 1338

Acroceridae Psilodera Small-headed fly IMG 1338

The adults of most species, like various members of the

Tabanidae Horse flies and deer flies are true flies in the family Tabanidae in the insect order Diptera. The adults are often large and agile in flight. Only females bite land vertebrates, including humans, to obtain blood. They prefer to fly in sunli ...

, Nemestrinidae, and Bombyliidae, are nectar feeders with exceptionally long probosces, sometimes longer than the entire body length of the insect. Unlike the other families, however, when not deploying the proboscis for feeding, the Acroceridae carry it lengthwise medially beneath the body, instead of projecting forward. As a result, the proboscis might escape casual notice, though careful inspection may reveal it projecting slightly behind the abdomen. Flies are usually found in small numbers on plants in July and August in the

Palearctic realm The Palearctic or Palaearctic is a biogeographic realm of the Earth, the largest of eight. Confined almost entirely to the Eastern Hemisphere, it stretches across Europe and Asia, north of the foothills of the Himalayas, and North Africa. The ...

References

External links

Family description, illustrationsImages at Diptera.infoImages and information at BugGuideFamily Acroceridae at EOL
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Acroceridae The Acroceridae are a small family of odd-looking flies. They have a hump-backed appearance with a strikingly small head, generally with a long proboscis for accessing nectar. They are rare and not widely known. The most frequently applied common ...

in Italian
Wing venation
{{Authority control Brachycera families Brachycera

Description

Taxonomy

Distribution and habitat

Behaviour

References

Further reading

Species lists

Identification

External links