What’s in a fly? Musca domestica – the greatest traveller of them all | Curator of Diptera

Within the Diptera section we are asked a lot about individual species of flies and so we thought we would put pen to paper (or key to board) and give some species descriptions of the more popular requests.

My co-author for this post, Nigel Wyatt, is the curator of all things bristly (including his own, he adds!) such as some of the most well known of all Diptera – the houseflies. Often seen as the greatest nuisance to humans and animals, this tenacious species has travelled with us all over the planet and enjoys all the creature comforts that we provide for it!

Description

One of the commonest flies on the planet, this species is of fairly typical form for higher flies. They are a medium-sized fly (body length generally between 5 and 8 mm.), predominantly yellowish-grey dusted with four dark longitudinal stripes on the dorsal surface of the thorax and indistinct darker areas on the abdomen.

Males show indistinct yellow areas laterally on the abdomen, though these can sometimes be more marked, and their eyes are more narrowly separated than in females. This is a feature seen in many species of flies where the sexes can be determined by the presence or absence of an eye bridge. Termed holoptic, males eyes are larger and touching each other, as they need greater vision to be able to locate the female.

Taxonomy

The common housefly (Musca domestica) belongs to the family Muscidae. It is one of the first fly species to be properly described by Linnaeus in 1758 and there are now over 4,000 described species of Muscidae.

Linnaeus originally described over 530 species of flies, many of which have now been assigned to other genera. Musca domestica is the type-species of Musca, a genus originally created by Linnaeus for a variety of higher Diptera, many of which are now known to be in other families (including fruit flies, cheese skippers and hoverflies). There are three sub-species, and over 50 species that had previously been described as separate species are now known to just be this species (syntypes).

The collection at the Museum has 11 drawers of this species in the main collection, containing over 5,000 specimens from most parts of the world, including all continents apart from Antarctica. We have one drawer of specimens from the UK.

The oldest specimens include syntype-type material described from the 1840s. Typically, this very common species is under-represented in the collection, specifically because it is so common and so people are less likely to collect it!

The reason for this is that there are so many small variations or similarities which can cause misidentifications. There are several other species which can be confused with this species. In the UK, the species most likely to be confused with Musca domestica is Musca autumnalis, commonly called the face fly. This is somewhat darker grey and the males have a more obvious pattern on the abdomen with lateral orange areas separated by a dark stripe.

Photo of a specimen shown side on and pinned

The face fly, Musca autumnalis

This species is closely associated with cattle, with the adult flies feeding on body secretions and breeding in the dung.

We can separate M. domestica from other species of Musca by the presence of fine dark hairs on the propleural depression. The stable fly, Stomoxys calcitrans, also looks quite similar but this blood sucking species has a much longer, sharp-ended proboscis enabling it to pierce the skin and feed from blood vessels.

Distribution

A truly global species, this species is found throughout the world, except for the polar regions.

Biology

Houseflies are on the wing mainly between April and October, but may occasionally be active at other times of the year, even in winter if conditions are sufficiently mild. They feed on liquids, which they ingest using the spongy tip (called the labellum) of their short proboscis.

They can also feed on solids by liquefying them before ingestion; this is done by regurgitation of digestive juices which liquefy the food thus enabling it to be ingested.

Photo showing the mouthparts under a light microscope

Mouthparts of the house fly, Musca domestica. Image © Brushett (CC BY-NC 4.0)

The female lay eggs in batches of up to around 150 and they breed in a variety of decaying substances. The natural food of their larvae is horse dung and the ideal breeding conditions for them are in stables where the floor is littered with a mixture of dung, urine and straw. They will also breed in other types of dung and in human refuse where this includes waste food.

Photo of a larva with posterior and mouthparts labelled

The larval stage of Musca domestica

The larvae are typical of the higher Diptera in that they are slender, white, headless and taper towards the anterior end, where instead of a head capsule they have a simple pair of sclerotized mouth-hooks which are used to scrape off food and direct it into the mouth cavity. The larvae also have a pair of spiracles at their posterior end; these are dark circular sclerotized plates at their posterior end which have respiratory slits.

The larvae undergo three stages (instars) during their development, reaching a length of about 12 mm when fully grown. The final larval skin hardens and darkens to form a brown case, or puparium, inside which the pupa is formed, and from which an adult fly will eventually emerge.

musca_domestica_-_life_cycle1

Illustration showing the lifecycle of the housefly. Source: Wikimedia

Along with all the other flies within the Schizophora, the flies emerge from the pupal case with the aid of an eversible air sac on their head – the ptilinum. This inflates just as the fly emerges causing the puparium to split at a predetermined place, thus enabling the adult to exit with relative ease.

You can also see this happening in real time in the time-lapse film below:

The life cycle takes around 3-4 weeks at normal (!) UK summer temperatures but may take as little as two weeks in hot weather (30-35o C), while it is slowed down in cooler conditions.

There can be several generations in a year, including overwinter, although larval development temporarily stops during the colder months before resuming once more in spring when the temperatures rise.

Medical Importance

Houseflies are not biters of humans or livestock but can transmit a number of harmful pathogens, especially those causing gastro-intestinal illnesses such as Salmonella and Campylobacter, because they readily settle on human food, often defecating as they feed thus contaminating food in the process. Always as vectors, they are also implicated in transmitting viruses, bacteria, fungi, protozoa, and nematodes.

However, their importance as disease vectors is much diminished in areas with good sanitation, especially where this relates to the treatment of sewage.

Houseflies as food

Due to increased concern about livestock production we are looking at new sources of feed for our domestic animals. Intensive rearing at the moment focusses on using soya or fishmeal but these have environmental and dietary issues.

Already used in rural environments across the globe the use of pupae in chicken and fish diets is now being trailed as a commercial prospect. As well as being a cheap and abundant source of protein (over 50% of the insect is protein) it also helps in the alleviation of manure accumulation. However, a concern over the transmission of harmful pathogens necessitates studies into this food source as a viable prospect.

This blog has been a joint effort from me and my colleague Nigel Wyatt, with photographic assistance from Dawn Painter (volunteer extraordinaire), and some fantastic images used with permission from Alex Wild.