Fly

All of us must have probably seen flies wandering around in the environment, but have you ever wondered what the fly definition is. The fly insect belongs to the family Diptera. However, if we were to define fly, it would be an insect. There are a large number of insects that can fly, which makes flight by using only a single pair of wings. The second pair of wings in these insects is reduced to knobs or halteres and serves as a purpose for balance. 

The commonly used term ‘flies or flys’ usually refers to almost any type of flying insect. In entomology, however, the term fly insect refers to the 125000 dipteran species (approximately), which are known as ‘true flies’. Such flies are widely distributed across the world including the high mountainous regions and subarctic. 

On this page, we will attempt to learn all about fly, their classification, types, and more. 

Flying Insects

All the insects that can fly are usually grouped under the order Diptera. In addition to the various kinds of flies such as the blowfly, horse fly, housefly, fruit fly, robber, bee, and crane flies, the order also includes midges, mosquitoes, gnats, and leaf miners. A number of other insect species, for example, mayflies, caddisflies and dragonflies are also called flies but they are distinguished from the ‘true flies’ on the basis of their wing structures. A number of Dipteran species hold great economic importance. At the same time, certain mosquitoes and the common houseflies are important as they serve as vectors or disease carriers. 

Fly Structure and Morphology

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The body of the flies is usually short and streamlined, as a means for aerial movement. The various parts of the fly structure include:  

Eyes: Their head is mobile with the sides consisting of a pair of big compound eyes. Most of the species also have three small ocelli on the top. The compound eyes are either spaced wide apart or close together. In some cases, the eyes may also be divided into ventral and dorsal regions, this functions most likely assist in the swarming behaviour of flies. 

Antenna: The flies’ antennae are variable but well-developed. The antennae can vary from being com-like or feathery to thread-like.

Mouth: The flies mouthparts are adapted for lapping and sucking while in the case of mosquitoes, robber flies, and black flies they also have mouthparts that allow them to pierce and such, for example, the mandibles and maxillae in flying bugs such as horse flies are knife-like which allows them to make a cross-shaped incision in the skin of the host followed by lapping up of the blood. 

Gut: The gut of the flies comprises large diverticulae. This allows for storing small liquid quantities post-meal. 

The body structure of the files is divided into three tagmata, which are as follows: 

• The head of the fly is included under the first tagma. The head bears the antennae, the eyes, and the mouthparts viz. the labium, labrum, mandible, and maxilla. 

• The thorax is the second tagma and it bears the wings. The greatly enlarged second segment comprises the flight muscles while the first and third segments are reduced to collar-like structures The third segment also bears the halteres, the appendages that support the fly during its flight. 

• The abdomen is the third tagma and it is made up of 11 segments. Some of the segments are likely to be fused. The three segments towards the rear are modified for reproduction 

Life Cycle of Flies

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The life cycle of flies typically comprises of the following stages: 

Eggs

Eggs are laid by a vast majority of flies and the number of eggs that a female can vary anywhere between 1 and 250. Eggs can also be laid by the female in several successive batches. The female flies invest energy and time in scourging suitable places to ay the eggs. The egg-laying sites are chosen by the female flies instinctively and are often closely related to suitable larval habitats. 

Telescopic ovipositors are developed in most female flies and these ovipositors are formed from the three or four hind segments. Using the ovipositor, the female flies press the eggs into a mass of decaying matter. Houseflies and blowflies usually push their eggs into any cavity found in decaying organic material. This type of ovipositor is also seen in Drosophila or fruit flies where the female lays eggs on fermenting materials or rotting fruits. The large fruit flies, such as the  Mediterranean fruit flies possess a stiffer ovipositor as these lay eggs on the rind of developing fruits. Robber flies are known to use elaborate ovipositors to push eggs between the axils of grasses and interstices of flower heads and at times, into plant tissues even so as to protect the eggs from desiccation. 

The hatching period of the eggs may vary from few hours to several days. Upon hatching, the larvae drop onto the ground where they use the soil for burrowing. 

Larva

The larval stage, in the case of many flies, is long with a relatively shorter life for the adults. The larval stage usually develops in protected environments. They are usually found in humid places such as fruit, vegetable matter,  carrion, fungi, etc. Some larvae may be aquatic while others (in parasitic species), larvae develop within the host. The cuticles of the larvae are mostly thin and they desiccate on exposure to air. The heads of most of these larvae retract into their thorax. 

The larvae of all flies have one universal characteristic - they lack true, thoracic, jointed legs. Instead, many of the larvae in flies have false legs or pseudopods (also known as prolegs). Flies can have these pseudopods around any segment of the body. These prolegs help the fly larvae to push through soil or crawl through narrow passages. 

Pupa

The pupae vary widely among the various Dipterian fly species. The pupa in groups such as Nematocera is intermediate between the larva and the adult. Described as "obtect", these pupae have future appendages that are visible structures adhering to the pupal body. A leathery outer surface bearing spines is usually seen in these kinds of pupae. The outer surface also often bears locomotory paddles or respiratory features. On the other hand, the groups described as "coarctate", lack visible appendages. The outer surface of these pupae bears a puparium, a capsule-like structure formed by the last larval skin while concealing the actual pupa within. This puparium is tough and resistant to desiccation and usually inflates like a balloon on its head when the adult insect readies to emerge. 

Adult

Emerging from the pupa, the adult fly is soft and crumpled and its skin or integuments are colourless. The hairs and bristles in the adult fly are perfectly formed but are not completely pigmented. The adult that newly emerges expands its wings and body and forces blood throughout the body by swallowing air. This process is aided by a membranous sac, ptilinum, which is inflatable and present in the head, in the case of more advanced flies such as Schizophora. Once its function is over, the ptilinum shrivels away but leaving behind a ptilinal suture. This suture is found only seen in Schizophora and resembles a horseshoe-shaped groove running over and beside the sockets of the antenna. In flies, the adult stage is generally short. The primary function of this stage is mating and laying eggs. 

Reproduction in Flies

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For the purpose of reproduction in flies, the male fly genitalia undergoes rotation to varying degrees as compared to the position usually seen in other insects. While in some flies this rotation is temporary and only occurs during mating, in others, the torsion of the organs is more permanent and occurs in the pupal stage itself. As a result of this torsion, the anus in flies is usually present below the genitals and in some cases where the rotation is 3600, the sperm duct wraps around the gut while the external organs remain in their usual position. 

At the time of mating in flies, initially, the male flies atop the female and faces the same direction. It then, however, turns around and faces in the opposite direction forcing the male fly to lie on his back so as to ensure that his genitalia remains engaged with the female genitalia. The torsion of the male genitalia, on the other hand, allows the male to remain upright during mating. All these adaptations allow flies to have quicker and more reproductive abilities as compared to other objects. The ability of the flies to mate quickly and effectively during the mating season is the reason behind their occurrence in large populations. 

Diet of Flies

The diet of flies involves finding the balance between the feeding habits of the adult flies and the larval stage. Flies primarily feed during the larval stages and feeding in adult stages is only meant for compensating the shortcomings in the nourishment of the larval stages.

The fly larva feeds on a wide range of foods which also evidences their adaptability. Besides the parasitic species, the majority of the fly larvae feed on plant tissues. Adult flies, on the other hand, are mostly known to visit flowers and feed on the nectar and pollen grains. 

Flies, like other insects, possess chemoreceptors for the detection of taste and smell and mechanoreceptors responding to touch. The main olfactory receptors are located in the antennae’s third segments and the maxillary palps. The pharynx, labium, wing margins, feet, and female genitalia bear the gustatory receptors that enable flies to taste their food as they walk on it. In females, the taste receptors at the abdomen’s tip help to adjudge the suitability of a particular site for ovipositing. Special sensory structures capable of detecting infrared emissions are present in flies feeding on blood that use them to locate the hosts. 

Flies - Disease Vectors 

Flies are commonly considered a nuisance to humans given their habits, worldwide distribution, and abundance. They often act as vectors of diseases given their constant movement from person to person. 

The housefly or Musca domestica is a common Dipteran of the family Muscidae. Houseflies comprise about 90 percent of the flies that occur in human habitations. They pose a major hazard to public health as they move from person to food, garbage, and even faeces. They are capable of carrying millions of microorganisms on their feet in doses large enough of causing diseases. They act as agents in transmitting dysentery, cholera, typhoid, summer diarrhoea, as well as other viral/bacterial diseases given their ability to transmit organisms from infected people or decomposing material. 

In countries with warm climates, eye gnats are a nuisance with the larvae as plant feeders while adults feeding on physiological secretions, especially around the eyes. Other flies such as black flies, sand flies, horse flies, etc. feed on the blood of the vertebrates by piercing their skin. Horseflies are also responsible for causing Surra disease in camels and horses in parts of Asia and the middle east as they carry the protozoan Trypanosoma evansi. In humans, sleeping sickness and in animals, nagana throughout tropical Africa is caused by the tsetse flies which transport trypanosomes.

Economic Importance and Uses of Flies

Despite being a nuisance, flies also serve a number of uses and are economically important. Blowflies House flies owing to their scavenging properties aid in decomposition. Dagger flies, balloon flies, tachinids, and robber flies serve as parasitoids and predators of other insects aiding in pests’ control. Hoverflies and bee flies also serve as agents of pollination in crop plants. 

The fruit fly or Drosophila melanogaster has long been used in research as a model organism. These flies with their small genome, the counterparts of which can be found in many higher eukaryotes can be easily subjected to breeding and rearing in the laboratory. Several genetic studies on the fruit fly have had far-reaching implications in the study of gene regulatory mechanisms, mutations, and gene expression. 

The fly larvae have also found their use as a biomedical tool employed in the care and treatment of wounds. MDT or  Maggot debridement therapy makes use of the larvae of blowflies for removal of dead tissue from wounds mostly resulting from amputations. This therapy facilitates healthy wound healing and cell growth. The biochemical activities of the larvae such as the antibacterial activity discovered in the secretions they feed upon make them effective and safe for treating chronic wounds. 

Flies - How to Control Them? 

The first step in controlling flies is making their feeding and/or breeding grounds such as wastes, manure, and garbage inaccessible to them. In cases where this is not possible, the wastes can be treated with larvicidal dust or drenches. Insecticidal sprays have proven to be effective against flies for several weeks. Certain houseflies, however, have managed to develop resistance to specific insecticides like DDT.