Fly Life Cycle and Complete Metamorphosis Explained

Flies are a group of large numbers of insects belonging to the order Diptera and class Insecta. The order of Dipterans also includes mosquitoes, gnats, house flies, bees, fruit flies, dragonflies, mayflies and so on. But, the wings of dragonflies, mayflies, and others vary from the wings of true flies. The term ‘fly’ refers to any tiny insect that can fly. There are around 125,000 species of “true” flies that are found across different parts of the world. They are omnipresent and can survive in most of the harsh climates ranging from icy weather conditions to warm humid weather conditions. A fly life cycle is very similar to that of most insects. It proceeds as a basic cycle that begins with an egg, followed by development through a larva phase, a pupa phase, and finally, turning into an adult. 

Fly Structure and Morphology

Their bodies are usually short and streamlined. Some of the main components of its structure are-

• Eyes: Their head is movable, having a pair of large compound eyes on either side. On the top of most species, there are three tiny ocelli. The compound eyes are either widely separated or closely spaced. The eyes may be separated into ventral and dorsal areas in some circumstances; these functions are thought to aid in the swarming behaviour of flies.

• Antenna: The antennae of the flies are variable, but well-developed. Antennae can range in appearance from com-like to feathery to thread-like.

• Mouth: The mouthparts of flies are designed for sucking and lapping. 

• Gut: The gut of the flies is made up of large diverticula. This makes it possible to store modest amounts of liquid after a meal.

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Body Parts of Fly

The flies body structure is divided into three tagmata, which are listed below:

• The head of the fly forms the first tagma. The antennae, eyes, and mouthparts (labium, labrum, mandible, and maxilla) are all found on the head.

• The wings are carried by the thorax, which is the second tagma. The flying muscles are housed in the considerably enlarged second segment, while the first and third segments are reduced to collar-like structures. The halteres, or appendages that assist the fly during flight, are likewise found on the third segment.

• The third tagma is the abdomen, which is made up of 11 parts. It's possible that some of the segments will be merged. For reproduction, the three portions in the back are altered.

Reproduction 

For a female housefly to lay eggs, a male has to chase down and mate with the female. Once the mating occurs, female flies lay their eggs on surfaces. Flies are generally solitary insects, unlike the nesting insects. Thus, in the case of flies, they do not stick together after mating nor do they nest. Neither the females care for the eggs after laying them at least, till the hatching phase. In case of flies, they simply lay eggs at surfaces, where predation could be less and the larvae may have plenty of food on hatching. A fly typically lays eggs on the surfaces of decaying organic matter where the adult flies feed.

Weather

Warm and moist weather is suitable for a fly and they thrive well in such conditions. So, during a warm summer when the conditions are optimal for a fly, their lifecycle from fertilised egg to adult, spans a mere 7 to 10 days. Thus, their lifespan is barely from 7 to 10 days and they lay eggs like every other insect before they die. The eggs then follow the same life cycle.

Stages of a Fly Life Cycle

The life cycle of a fly follows four simple stages namely, egg, larvae, pupa and adult. They are discussed in detail below-

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Life Cycle of Fly

Stage 1- The Egg

The female fly lays eggs in clusters of about 300 eggs in a day. The eggs hatch in 8 to 20 hours. Immediately after hatching,  the young ones enter the first of the three stages of larvae.

Stage 2- The Larvae (Maggots)

When the eggs hatch, the young larvae begin to emerge from the eggs. Fly larvae are also known as maggots. They resemble small wriggly worms. Their body looks little more than fleshy with a section with fewer tubes and hooked mouthparts used for feeding. It undergoes several changes and grows very rapidly. They double their size within two days, which is why they moult several times by shedding layers of their skin, before each instar stage. A maggot moults several times, emerging larger and more developed each time. Moulting is a process in which a growing insect sheds its former exoskeleton and grows a new one. They feed on the fluids from their body.

In the 1st Instar, the larvae are 5 mm in length before shedding their skin. By the second instar, they double their size and measure up to 10 mm. By the time they reach their 3rd instar, they measure from 15 mm to 20 mm in length. In a warmer climate, this process completes in 3 days, while in colder climates, it takes up to 8 weeks. Thus, temperature difference plays a major role in the growth of a fly larva. With their third moult, they burrow deep into the substance they've been feeding on and arrive at the pupa stage.

Stage 3- Pupa Stage

Nearing the pupa stage, the skin of the larvae darkens and hardens. In the Pupa stage, the fly becomes stationary and does not move or feed. The pupa develops a last, hard larval skin, which encloses the pupa and is called the puparium. Inside this protective shell, the larva fully develops the body segments and appendages of an adult housefly. Moreover, The emerging fly has a swollen bump on its head. This bump helps the fly break the shell when it is an adult. When they emerge fully, the bump deflates back into the fly's head. This stage lasts for 4 to 6 days under warm conditions.

Stage 4- The Adult Stage

Finally, at the adult stage, the pupa emerges from the puparium and is ready to mate. The newly emerged adult flies are sexually mature and begin the process of mating. An adult fly feeds on proteins released from body fluids. In this stage, they fly to different locations in search of food. They feed, then mate and find a suitable place to lay eggs.

Thus, the entire process in the fly life cycle starts again.

Important Questions

1. Why do flies tend to rub their hands?

Ans: The flies tend to rub their hands in order to clean themselves.The flies do this thing before they are ready to fly again in search of food and look out for predators.

2. Why do Flies constantly fly around Human Beings or in Damp Areas?

Ans: Flies are continuously in search of food. They feed on organic matter, dead skin cells in our body, on oil, salt, etc. which are also their favourite food. They thrive the best in warm and moist conditions, where the development of fly larvae also takes place within hours. Therefore, they are constantly flying around human beings or near damp areas.

Interesting Facts

1. Flies do not consist of any mouth part involved in chewing. Infact, they spit out enzymes that turn the solid food into liquid food, which can be consumed easily.

2. Flies have sensory receptors on their feet, which helps them know the taste of food on which they land. 

3. The world’s biggest fly grows upto 8 centimetres and is named ‘timber fly’. It is found in Central and South America. 

Key Features

• Flies are a group of insects that belong to the order Diptera and class Insecta.

• Their life cycle goes through mainly four stages - egg, larvae, pupa and adult. This process takes around 7-10 days to complete. 

• The suitable weather for flies to complete their life cycle is warm and moist weather.