Eristalis
Common drone fly (Eristalis tenax) on the flowers of a common lantana (Lantana camara)
Global distribution of Eristalis species observations produced from GBIF observations
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Diptera
Family:	Syrphidae
Subfamily:	Eristalinae
Tribe:	Eristalini
Subtribe:	Eristalina
Genus:	Eristalis Latreille, 1804
Type species
Musca tenax Linnaeus, 1758
Subgenera
Eoseristalis Kanervo, 1938 Eristalis Latreille, 1804
Synonyms[1]
Elophilus Meigen, 1803, Eristaloides Rondani, 1845 Eristalomya Rondani, 1857 Eristalomyia Verrall, 1882 Helophilus Leach, 1817 Tubifera Meigen, 1800

Eristalis is a large genus of hoverflies, family Syrphidae, in the order Diptera. Several species are known as drone flies (or droneflies) because they bear a resemblance to honeybee drones.

Drone flies and their relatives are fairly common generalist pollinators,^[2] the larvae of which are aquatic, and breathe through a long, snorkel-like appendage, hence the common name rat-tailed maggots.^[1][3]

Eristalis is a large genus of around 99 species,^[4] and is subdivided into several subgenera and species groups (Eristalomyia, Eristalis, Eoseristalis etc.).

The scientific name was proposed by Pierre André Latreille in 1804. He placed seven species in his new genus, but listed the names as combinations with Syrphus, so it remained unclear what gender he attributed to the name (the gender of the name Syrphus is masculine). In the two centuries following its publication, Eristalis was sometimes considered to be of feminine gender, sometimes to be of masculine gender. George Henry Verrall (1901)^[5] assigned its gender as masculine, a choice followed in British literature, and also in Dutch, Polish, Czech, Spanish and Portuguese literature. In several other European languages and in North America, the tradition was to consider it as a feminine word. In 1993 the International Commission on Zoological Nomenclature placed the name on the Official List, and gave its gender as masculine, without justification for that choice.^[6] In 2004, Peter Chandler, Andrew Wakeham-Dawson and Angus McCullough submitted an application to confirm the gender of Eristalis as feminine.^[7] They referred to ICZN Art. 30.1.1, which states that a name in Latin form takes the gender given for that word in standard Latin dictionaries.^[8] In Composition of Scientific Words by R.W. Brown (1954), "eristalis" is listed as a feminine word that refers to an unknown precious stone. The request of Chandler et al. was granted less than two years after submission.^[9] As of 2006, Eristalis is officially a word of feminine gender.

For terms see Morphology of Diptera.

As a true fly, the species of the genus Eristalis have a single pair of wings and a pair of halteres. As a member of the family Syrphidae, Eristalis have a spurious vein in the wing. Defined by Latreille in 1804, Eristlis was restricted by Meigen in 1882 to those species with a sinuate vein R4+5 and petiolate cell r1.^[10]

Since the origin of the genus Eristalis, many genera have been added that refine the description of Latreille such as Eristalinus, Meromarcrus, Palpada, Helopilus etc. The subtribe Eristalina has been established to contain these genera. Now it is generally accepted as those which have vein R2+3 sinuate, cell r1 usually petiolate and metafemur with basolateral setose patch. With the addition of the following criteria: anepimeron with triangular portion bare, katepimeron pilose, meron bare posteroventrally, without pile anterior or ventral to metathoracic spiracle, eye pilose.^{[citation needed]}

The larvae of Eristalini are aquatic and of the long-tailed type. Those of Eristalis are very commonly found breeding in putrid or stagnant water or in moist excrement, and are called “rat-tailed maggots” or “mousies”.^[11]

The "tail" is actually an extendable breathing tube often used to extend above the waterline. This tube allows the larvae to live in oxygen-depleted water such as sewage and stagnant pools where most other larvae cannot exist. Rat tailed larvae also exploit wet mud, manure and moist rotting vegetation. Many species of Eristalis remain unknown.^[12] Working in areas where larvae are likely to be found (e.g. manure pits, sewage seepage and stagnant pools) is difficult and rearing the larvae to adults is even more so. Basic information on many species of Eristalis remain to be discovered.^{[citation needed]}

Adults of Eristalis species such as the common drone fly (E. tenax) and the orange-legged drone fly (E. flavipes) are pollen and nectar feeders with hairy bodies capable of picking up and transferring pollen, and may act as keystone pollinators in some systems.^[13] Pollinating Eristalis species are often generalists, and are known to act as pollinators for common crops such as Brassica rapa, common onion (Allium cepa), sweet pepper (Capsicum annuum), kiwifruit (Actinidia deliciosa), and apples (Malus domestica).^[14][15][16] Eristalis are tetrachromats, with strong colour differentiation and preferences towards yellow flower colours, compared to trichromatic bees which often prefer blue hues.^[17][18] On islands where bees are absent, their colour preferences may induce selection for higher proportions of yellow in flowers, and they can be seen as frequent and effective pollinators for plants like moon tree foil (Medicago citrina).^[18][19] The mobility of hoverflies may aid in long-distance pollen transfer, facilitating gene flow between unconnected plant populations.^[20]

Some species, particularly E. tenax, have been suggested as candidates for a managed pollination alternative to European honey bees (Apis mellifera) due to their similar pollination efficacies, abundance, wide distribution, flight range, behavioural plasticity, and high reproductive success.^[16][21] Multiple generations may be reared per year, and Eristalis are readily reared in laboratory conditions.^[22] However, there are challenges concerning commercial mass rearing, retaining numbers, and impacts on non-target species.^[21] Due to their ability to fly long distances, dispersal of captive-reared populations may pose a risk to nearby farm equipment and machinery.^[21] Eristalis may also act as mechanical vectors for parasites affecting bees, such as Crithidia bombi.^[23]

The larvae of Eristalis species may be considered pests where they affect livestock, contaminating feed and affecting electrical supplies due their mass congregations seeking dry sites for pupation.^[24] Some species of Eristalis have been known as a rare cause of myiasis, with larvae capable of surviving gastric fluids upon ingestion. Symptoms of myiasis from Eristalis species include diffuse abdominal pain and diarrhoea.^[24] Treatment may include anthelmintics such as ivermectin.^[25][26]