Basic Fly Biology

Images by Stoy Hedges except where noted.

Editor’s Note: The following article is excerpted from the recently published PCT Field Guide for the Management of Structure-Infesting Flies, 2^nd Edition.

Flies are ubiquitous insects being found in virtually every ecological niche, even some with extreme cold or heat. They develop in decomposing matter, manure, fungi, on plants, in water, and a few as parasites of mammals and other animals.

Some prefer to eat sweets (Syrphidae), others like to consume things that are decomposing (Phoridae, Muscidae, Calliphoridae), and some are predators of other insects (Asilidae, Tabanidae), but the most globally impactful are those that feed on blood, including mosquitoes (Culicidae), blackflies (Simuliidae) and sandflies (Psychodidae). Millions of people are afflicted by the diseases carried by these flies, and sadly, many thousands die annually.

Around homes and businesses, the species of flies that become pests are relatively few compared to the 160,000 known species worldwide. Most of these pest species have high moisture requirements in common regarding breeding sites, while others like mosquitoes (Culicidae), crane flies (Tipulidae), midges (Chironomidae, Ceratopogonidae) and horse/deer flies (Tabanidae) are specialized for breeding in aquatic environments. A nuisance pest, the cluster fly (Pollenia spp.), is a parasite of earthworms. Fungus gnats (Sciaridae, Mycetophilidae) breed on molds and in fungi. Hover flies and flower flies (Syrphidae) vary regarding larval developmental behaviors as many species are predaceous on aphids while others breed in decaying organic matter or stagnant aquatic environments.

DEVELOPMENT. All flies are holometabolous, meaning they develop by complete metamorphosis and have the life stages of egg, larva, pupa and adult. Some flies, such as many flesh flies (Sarcophagidae), bypass an exposed egg stage altogether, instead depositing their larvae (hatched internally) directly from the abdomen to the breeding media (i.e., animal carcasses).

Eggs. Upon completion of mating, female flies begin a quest to find the best places to deposit their eggs. The numbers of eggs produced by one female will vary by species but can number in the hundreds during a female fly’s lifetime. House flies and face flies (Muscidae), for example, target animal manure primarily but also will lay eggs in decaying organic matter. Blow flies (Calliphoridae) and flesh flies (Sarcophagidae) prefer dead animals, but blow flies will breed in garbage and decaying organic matter.

Most mosquitoes (Culicidae) deposit their eggs in still or stagnant waters though some floodwater species deposit eggs on dry land in low areas likely to flood. When floods occur — sometimes even years later — the eggs give rise to larvae, which then pupate, and soon produce a mass emergence of adult mosquitoes within a week or so.

Some bot fly species (Oestridae) will deposit eggs on a house fly or mosquito, and once those flies land on a suitable warm-blooded mammal host, the larvae, sensing the body heat, will drop off the mosquito or house fly and burrow into the skin of that animal. Another obligatory parasite of mammals related to blow flies (Calliphoridae), screwworm flies (Cochliomyia spp.), deposit their larvae directly along the edges of open wounds on animals.

Larvae. Fly larvae come in many forms, and most all lack any legs at all, although some have vestigial (partial or undeveloped) legs that may add in some locomotion. Fly larvae can be exceedingly difficult to identify to species or type without training in immature insect morphology and taxonomy. Pest professionals may be able to identify a few larval types using tips in the fly field guide but, in general, should seek the help of an entomologist when encountering fly larvae that are too difficult to identify.

The soft, legless, often white or cream-colored immatures of house flies (Muscidae), blow flies (Calliphoridae), and flesh flies (Sarcophagidae) are commonly known as maggots (Figure 1). Outdoor trash cans and dumpsters during the warm months often contain a few to hundreds of fly maggots. An experienced service technician may be able to identify the type or species of maggot based on examination of the tail end where the spiracles (breathing valves) are found (see Figure 2). The spiracles are diagnostic in their appearance.

Stoy Hedges

The shape of fly larvae can vary greatly. Mosquito larvae (Culicidae) seen in water are typically adorned with many spines and possess a tube-like appendage (or siphon) at the tip with which the larvae use to breathe at the water’s surface. Crane fly larvae (Tipulidae) are also aquatic but are long and worm-like in appearance with several short tubercules extending from the head end. Moth fly larvae (Psychodidae) found in aqueous areas are worm-like and have a siphon tube for breathing, but the larva’s body lacks the spines and hairs seen on mosquito larvae.

The larvae of some flies (non-Brachycera/old suborder Nematocera) have chewing mouthparts, but most fly larvae have the mandibles modified into mouth hooks for use in tearing and rasping soft food. Mosquito larvae have mouth brushes used for filter feeding — guiding floating particles of food, such as algae, into their digestive systems.

Stoy Hedges

Development of a fly larva from egg to pupa varies greatly, usually dependent on fly species, quality of breeding media and temperature. A general rule around structures is that if an area or item holds water for at least seven days, it can breed mosquitoes, at least those species (e.g. Aedes spp.) known as “tree hole” mosquitoes. Fruit flies and house flies can complete the larval stage within a matter of days. Cluster flies may take several weeks for the larvae to develop within the bodies of earthworms.

By contrast, the human bot fly, Dermatobia hominis, found in tropical areas, takes up to six weeks to complete its development within a human host.

Pupa. The pupal stage is where all the action occurs, turning a soft larva into the adult fly. The length of time for the pupal stage varies by species and temperature, but flies with short larval development periods typically have short pupal stages also. Fly larvae of many families (e.g., Muscidae, Calliphoridae, Sarcophagidae, Stratiomyidae, Drosophilidae, Phoridae) form pupae within the final larval skins (exuviae). Pupae encased by larval exuviae are called puparia.

Pupae and puparia can vary by type of fly and in some cases, you may be able to identify the family, and sometimes species by shape. For example, soldier fly (Stratiomyidae) puparia bear the shape of the surrounding larval exuvia (Figure 3). Phorid (Phoridae) puparia have two protrusions at one end (Figure 4).

Mike Corbitt

Indoors, one may encounter red-brown to brown puparia along baseboards, in a ceiling or piled in a corner (see Figure 5). The sizes of the puparia may vary, and these typically will be those of blow/bottle flies or flesh flies. The source will be some dead animal within a wall, ceiling or chimney flue. The pupae of house flies and other related flies will be similar in appearance but are seldom found indoors.

Adult. Adult flies are characterized by having only two wings. Flies are found in all different sizes, shapes and colors. Most are dully colored with grays, blacks and browns but may have splashes of color here and there, particularly the eyes. Some species of horse fly eyes, for example, may have brightly colored stripes over the compound eyes when the light hits a certain way (see Figure 6).

Some flies are noted for the size of the eyes (big-headed flies, Pipunculidae) or the long, thin legs (stilt legged flies, Micropezidae), while feather legged flies (Trichopoda spp., Tachinidae) are known for the feather-like fringe of flattened hairs on the rear legs (see Figure 7). Some flies are hairy (robber flies, Asilidae), while others have far fewer prominent hairs or spines (most hover flies, Syrphidae).

Most adult flies have sucking mouthparts, although certain crane flies (Tipulidae) have no functional mouthparts as adults. Flies that feed on the hemolymph (blood equivalent) of other insects, or the blood of host vertebrates, have piercing-sucking mouthparts (e.g., mosquitoes, biting midges, stable flies, horse and deer flies, robber flies). Flies that feed on nectar have siphoning-sucking mouthparts (e.g., some crane flies, bee flies). The majority of adult flies (e.g., house flies, blow flies, flesh flies, soldier flies, fruit flies, phorid flies, hover/flower flies) have sponging-sucking mouthparts for feeding on available liquids, often including liquified solids on which the fly has regurgitated digestive enzymes and saliva.

Many flies are mimics of bees and wasps. Some hover flies (Syrphidae) are mistaken for wasps (see Figure 8), while one genus of robber fly (Asilidae) is easily mistaken for bumble bees (see Figure 9).

The fact that the specimen has only two wings is the primary clue that it is a fly and not a bee or wasp. A closer look at robber fly’s tiny antennae and piercing-sucking mouthparts also help to distinguish it from a wasp, which has longer segmented antennae and mandibles for chewing.