Eristalis tenax

The larval form of the drone-fly, the rat-tailed maggot, are on every continent except Antarctica and range to the highest latitudes in the North.[6] They aren't prevalent in extremely southern latitudes and in arid areas of Europe, Asia, and Africa.[6] In the United States, they can be found as far north as Alaska and as south as California and Florida.[6]

Drone fly on Meadow Salsify (Tragopogon pratensis)

The appearance of the drone fly can vary drastically.[7] Eristalis tenax is a large, stocky bee mimic. Their eyes are marbled in black. Males have hovering displays. Their average wing length is 9.75–13 mm and their average wingspan is 15 mm. They have a stout appearance, like bees.

The abdomen can vary from dark brown to very orange.[7] Pigmentation also has an important role in the control of body temperature; the black areas down the center of the drone-flies abdomen may absorb solar radiation and so warm the dorsal blood vessel that is right underneath.[7]

The males of E. tenax can be heavily territorial in the summer, guarding territories including flowerbeds or bushes for a chance to mate. The males hover still in the air and dart after intruders to chase them out of the territory.[8]

Researcher observations suggest that male drone flies live in the same territory their entire lives.[6] They mate, feed, and groom in this area defending it against other insects.[6] When male E. tenax of the spring and summer generations stop dispersing, they settle within home ranges that provide them with sheltering, resting, basking, grooming, feeding and mating locations.[9] Away from its "home" a male drone fly rarely responds to other insects.[9] On its mating site, the male is very territorial, attacking alien species like bees, wasps, and butterflies.[9] Being on territorial duty is very demanding, so much so that the males take rest periods when they can outside of their territory.[9] When weather conditions don't allow them to leave their territory, they become increasingly aggressive.[9] Males that live in horizontal territories such as flowerbeds are more likely to notice intruders and thusly are more likely to attack them than males living in vertical territories such as shrubs.[9]

Larvae are saprophagous. The larva of E. tenax is a rat-tailed maggot. It lives in drainage ditches, pools around manure piles, sewage, and similar places containing water badly polluted with organic matter.[10] The larvae likely feed on the abundant bacteria living in these places.

When fully grown, the larvae creep out into drier habitats and seek a suitable place to pupate. In doing so, they sometimes enter buildings, especially barns and basements on farms. The pupae are typically 10–12 mm long, grey-brown, oval, and retain the long tail; they look like a tiny mouse.

The adult fly that emerges from the pupa is harmless. It looks somewhat like a drone honey bee, and likely gains some degree of protection from this resemblance to a stinging insect. The adults are called drone flies because of this resemblance. In its natural habitat, E. tenax is more of a curiosity than a problem. Like other hover flies, they are common visitors to flowers,[11] especially in late summer and autumn, and can be significant pollinators. They mainly feed on flowers of carrot and fennel.

Under extremely rare conditions, there have been documented cases of human intestinal myiasis of the rat-tailed maggot (larva of Eristalis tenax). Zumpt proposed a hypothesis called "rectal myiasis". During open defecation in the wilderness, flies attracted to feces may deposit their eggs or larvae near or into the anus, and the larvae then penetrate further into the rectum. They can survive feeding on feces at this site, as long as the breathing tube reaches towards the anus which is quite rare.[10][12]

The drone-fly is reported to have caused accidental myiasis.[6] Myiasis in humans is usually an accident and is caused by the ingestion of contaminated uncooked food or water containing fly larvae.[13] This occurs when fly larvae inhabits a living host by accident, usually from ingestion of contaminated foods. In humans, myiasis can be caused in four ways: intestinal or gastric, nasal, auricular, or anal.[6] Gastric or intestinal is the most common. Most reported myaisis has been reported in countries where nutritional and sanitary conditions are unsatisfactory.[13] There are very few reports of intestinal infestation from India, and Africa.[13] Clinically the symptoms are variable, including cases that are asymptomatic, but usually abdominal pain, nausea and vomiting are some symptoms one with myiasis might face.[6]

Myiasis in a Human

Dronefly larvae are able to survive in the gastric acids due possibly to the fact that they are adapted to living in polluted environments.[6] Myaisis becomes apparent to the host when they notice the larvae in their bowel movements.[6]

Drone-fly eggs are normally laid on surface water and go through three distinct larval stages.[14] The larvae are usually found in still or stagnant water, like water reservoirs or liquid dung.[14] Just before the pupation stage, the larvae leave their aquatic environment.[14]

Drone flies mating on a daisy inflorescence

Mating can happen while a pair of E. tenax is flying, with the male uppermost, or on the ground while resting on foliage.[6] After mating, the adult, female lay clusters of about 10 eggs near dirty, contaminated water, sewage, or decomposing organic substances.[6]

The drone fly's diet consists mostly of honey and the pollen of flowers. During the imago stage, it drinks mostly honey, but will have a little bit of water when it's presented.[15]

Diptera are often neglected but are an important group of pollinators, they play a significant role in agricultural biodiversity and the biodiversity of plants everywhere.[2] Hoverflies are considered to be less specialized pollinators than bees and are more effective in open than tubular flowers.[2] There is a significant difference in pollination efficiency between the two taxa.[17]

Honey bee collecting pollen

There is little doubt that the morphological and behavioral similarities between E. tenax and the honey bee are mainly a result of convergent evolution in response to similar food-gathering requirements.[16] Bees are common models for several Dipteran mimics[7] They are similar in their general form, flight, and coloration.[7] There are reports on the genetics of honeybees that show that the factors controlling the coloration are the same as the ones controlling the coloration in E. tenax.[16] In both species branched hairs and spirally grooved bristles act as collectors and retainers of pollen, and leg-scraping that occurs during hovering allows the transfer of pollen to take place, from hind legs to front legs in E. tenax, and in the reverse direction in honey bees.[16]

Drone fly collecting pollen

E. tenax also has partial mimicry of wasps.[7] To improve the wasp mimicry there seems to be an epistatic influence on the Ap gene on the hair color genes.[7] The lightest phenotype of E. tenax is a light yellow, and seen most clearly as yellow bands along its sides, where a wasp has stripes of yellow pigment.[7] The lighter patterns would be less beneficial from wasp mimicry early in the year, before there are a surplus of wasps.[7]

Drone flies are active during much of the year, from March to December, and sometimes are more numerous than honeybees, especially during autumn in urban areas.[7] Males and females occur in roughly equal numbers in summer and autumn, but males are very rare in the spring when fertilized females come out of hibernation.[7]

• Morphology of Diptera

Wikispecies has information related to Eristalis tenax

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• External images
• Drone fly on the University of Florida / Institute of Food and Agricultural Sciences Featured Creatures website