Diptera: Flies

The Order Diptera have the common name flies. There are >120,000 species world wide, with >6,500 species in Australia. Flies, like cockroaches have a bad reputation as carriers of being dirty and carrying disease, however the members of this order have an enormous range of life strategies, with only a small percentage implicated in human or animal health issues. Flies perform a wide range of ecosystem services including pollination (e.g. Bee flies, Hoverflies, Rhiniidae), biological control (such as larvae of Tachinid flies and adults and larvae of Coenosia sp), waste recycling and decomposition of carrion (Sarcophagidae, Calliphoridae), dung (Musca vetustissima – bush flies), leaf litter (larvae of Bibionidae) and dead plant material (Musca domestica – houseflies).

Many adults feed on flowers, others are predatory (robber flies) and feed on other insects, or on blood (Tsetse flies, mosquitoes). Larvae can be found on plants feeding on pest insects (hoverflies), or as leaf miners (Agromyzidae), or inducing galls (some Tephritidae), in aquatic environments (mosquitoes and midges), in soil (larvae of Coenosia sp) feeding on fungus gnats and other organisms, in caves (some Phoridae and Tipulidae) feeding on insects and guano, on fungi (Sciaridae), and as parasites or parasitoids on animals (Lucilia sp) or other insects (Tachinidae).

Diptera with differing food sources. Cylindromyia sp feeding on nectar and pollen. Drosophila sp on tomato, Asilidae with prey (bush fly), Chromatomyia sygenesiae mining in leaf of a sow thistle, Calliphora dubia after a blood meal and Calliphora dubia larvae in meat.

The features that distinguish a fly from other insects are (for adults):

  • Holometabolous (complete metamorphosis – egg, larva, pupa, adult).
  • Mobile head.
  • Large compound eyes (some species also have simple light receptors called ocelli, they detect movement).
  • Head hypognathous (jaws below head, mouthparts directed down – like in a grasshopper).
  • Mouthparts for sucking (proboscis) , some with a “sponge” (labella) , others a narrow tube for piercing and lacerating (female mosquitoes, robberflies)
  • One pair of wings, The pattern of veins varies with families and species. Always try to photograph the wings if trying to ID from photographs.
  • Second pair of wings is modified to form halteres. These measure movement and are used for balance.
  • Abdomen has 11 segments, although reduced in many species.
  • Ovipositor (for laying eggs or larvae) does not have valves like many other groups, but may have telescoping sections.
  • Antennae are variable.
Diptera anatomy. Photos by Sue Jaggar. Top row: (left) Compound eyes of a Tabanus australicus, ocelli in between compound eyes of Eristalinus punctulatus, (right) hypognathous head and mouthparts of a Calliphora dubia feeding on sugar.
Middle row: Antennae and mouthparts of (left) a mosquito then robberfly (piercing & sucking), Tabanidae (horsefly, biting and sucking), Simosyrphus grandicornis (hoverfly, lapping and sucking), Calliphora albifrontalis (blowfly, lapping and sucking) and antennae of a midge (right).
Bottom row: (left) Two wings of Comptosia tendens, halteres of Therutia sp, (right) telescoping ovipositor of Calliphora albifrontalis.

Larvae are called “maggots” and they:

  • Lack true legs
  • Head varies from a distinct capsule to an internal structure.

.Pupae are:

  • Adecticous (lack mandibles)
  • Are in a hardened shell or skin which encloses the pupa (puparium).

Lifecycle of Diptera

The lifecycle for flies is from egg, to larvae, to pupa to adult. There are variations in species, with some being ovoviviparous (also called larviparous), where the eggs develop inside the female and she oviposits 1st instar larvae (for example Calliphora dubia). The oviparous species lay eggs which hatch after developing; for some predatory Muscidae two instar larvae occur in the egg, so the 3rd instar emerges. The number of instars varies between species. Blowflies have 3 instars, mosquitoes have 4 instars, while some Tabanidae have 11 instars.

Typical life cycle of a blowfly. This diagram shows two types of development, the red arrows represent the most common form where eggs are laid (oviparous), 1st instar emerge, they go through molts to 2nd and 3rd instar, when they wander to a dry location. They then pupate, gradually changing exterior colour during development. The adult emerges from the pupa, inflating their head while the body is still in the pupa, then emerging completely and inflating their wings. They then feed, mate and begin ovipositing. The second type are larviparous (blue arrows) where eggs are inside the female, and the adult lays 1st instar larvae. Photographs and diagram by Sue Jaggar.

It is possible to tell the instar of a maggot by examining the spiracles on the end. The number of spiracles shows the number of instars.

Male and Female

Distinguishing male and female varies for different families (and within families). In some families such as Calliphoridae (Blowflies), Syrphidae (hoverflies) and Tabanidae (horse flies) males can be distinguished as they have larger eyes that meet in the middle (holoptic), while females have a gap between their eyes.

Other species have sexual dimorphism which may be a major difference (eg Bibio imitator) or a smaller difference such at spots on the wings (such as in Pygophora apicalis). For some species such as mosquitoes the antenna have larger hairs for males as they use these to find the females. Other species differ in size (see Pygophora apicalis below), with the female being larger than the male, however many species can vary in size depending on the availability of food in their larval stage, so this can be hard to determine. For other species it is necessary to examine the terminalia.

Families of Diptera

There are >150 families of Diptera. The order is divided into two suborders Nematocera (elongate flies, thin antennae, primarily aquatic larvae) and Brachycera (generally with reduced antennae). The following show a selection of families within Nematocera.

Brachycera have several infraorders including Asilomorpha, Muscomorpha, Stratiomyomorpha, Tabaomorpha and Xylogphomorpha.

Muscomorpha is divided into 2 groups, Aschiza and Schizophora.

Phoridae (grid is 1mm)

Schizophra can be divided into two subsections, Acalyptrate and Calyptrate. The calypter covers the halteres and is not present in acalyptrate, but is in calyptrate flies.

Must read books on flies

McAlister, E. (2017). The secret life of flies. Clayton South, Victoria: CSIRO Publishing.

McAlister, E. (2020). The Inside Out of Flies: Firefly Books.