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The following points highlight the five stages involved in the life history of Musca Nebulo. The stages are: 1. Copulation 2. Egg-Laying 3. Egg 4. Larva 5. Pupa.
Stage # 1. Copulation:
The copulation takes place on earth, not in air.
In breeding season which in India is from March to October in the greater part of the country, the male sits on the back of the female and grasps her firmly with the fore and middle pairs of legs and remains passive during the remaining period of operation. Female inserts its ovipositor into the genital atrium of the male, to receive the spermatozoa. The copulation takes a few minutes.
Stage # 2. Egg-Laying:
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Four days after mating the female housefly lays her eggs. The housefly lays her eggs in stable manure by preference, but failing to find this, it may lay in human faeces, garbage or decomposing animal and vegetable matter. The conditions required for laying eggs are moisture and a favourable temperature, hence, stable manure or human faeces should not be dry.
The female extends her ovipositor and lays about 120 to 160 eggs at one time. In the course of a breeding season a single female may lay eggs 4 to 6 times.
Stage # 3. Egg:
An egg is whitish, cylindrical, and 1 mm long. It has two rib-like longitudinal thickenings on one side. The eggs hatch in 8 to 24 hours depending upon the temperature, and larvae emerge in the dung.
Stage # 4. Larva:
The larvae are called maggots. They are highly modified without a distinct head, no thoracic or abdominal limbs, and with the spiracles greatly reduced in number. They are covered with thin soft chitin. Such a larva is known as an apodous larva.
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The larva when it hatches from the egg is the first instar and is 2 mm long. It is metapneustic having only one pair of posterior abdominal spiracles with two slit-like apertures in each, the spiracles are on the last segment.
The first instar lasts for two to three days, then it moults to become the second instar which is larger, and besides the posterior pair of spiracles which become larger, the second instar acquires an anterior pair of spiracles also, it is, thus, amphipneustic with one pair of posterior abdominal spiracles and one pair of prothoracic spiracles.
The second instar lasts for a day, then it moults to form the third instar.
The full grown larva of the third instar is 12 mm long. It has a small insignificant head which can be withdrawn, it is followed by 12 segments, the anterior end is narrow but becomes broader posterioly. The pointed anterior end has two small oral lobes which are sensory, each oral lobe has a minute sensory papilla, these sensory papillae represent reduced antennae.
There is a mouth between the two oral lobes from which project a pair of hooks. The hooks are a part of secondarily developed chitinous sclerites known as cephalopharyngeal skeleton which is composed of three sclerites, a pair of hooks or mandibular sclerites which articulate posterioly with an H-shaped intermediate or hypostomal sclerite.
The hypostomal sclerite has the opening of a salivary duct. Posteriorly is a large pharyngeal sclerite formed of two lamellae which unite ventrally to form a deep groove.
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The cephalopharyngeal skeleton is used for locomotion and for tearing up food. The third instar is amphipneustic with two pairs of spiracles, the anterior prothoracic spiracles lie in the second segment, and each has six to eight finger-like processes with openings at the tips.
The posterior pair of abdominal spiracles are on the posterior dorsal side of the 12th segment, in the 3rd instar they become large, dark-coloured, and C-shaped with three sinuous slits in each.
The spiracles lead into a well developed tracheal system. Below the posterior spircles is an anus in the 12th segment with anal tubercles. On the ventral side of segments six to twelve are spiny pads or pseudopods, one pair in each segment, they are used for locomotion.
The third instar lasts for about 3 to 5 days. The total larval period is from 6 to 8 days, during this time the larva moults twice, and it feeds and grows larger at each moulting. The larva in feeding moves away from light into moist and dark parts of the dung, it feeds on the substance in which it was hatched, it produces enzymes by which food is liquefied and it takes in liquids and small solid particles as food.
Stage # 5. Pupa:
When the larva is ready to pupate it searches out a dry, dark crevice of the manure, the body contracts and segments are telescoped to form a pupa. Thus, the larva changes into a pupa without moulting, the last larval skin hardens to form an outer covering or puparium which encloses the pupa.
Such a pupa is called coarctate. It has no chitinous covering of its own but only a soft pupal skin, the outer puparium has been formed by the last larval skin.
The puparium is barrel-shaped and it becomes dark brown, externally it is segmented and shows traces of larval spiracles and spiny pads which become non-functional. The pupa takes in air by means of a pair of spine-like pupal spiracles projecting between the fifth and sixth segments of the puparium.
The pupa is absolutely immobile, and the pupal stage lasts from 4 to 5 days. During this time internal changes take place, the larval organs are broken down or histolysis occurs by the phagocytes feeding upon the tissues of organs. The imaginal buds of the larva begin to form the organs of the adult, or histogenesis occurs in the pupa.
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Imaginal buds are dormant cells, they are stimulated by a hormone of prothoracic endocrine glands which become active only during metamorphosis and make the imaginal buds grow. By these processes the adult fly or imago is formed in the pupa. A blood-filled bag called ptilinum is formed on the head of the imago which is eversible.
By forming the ptilinum the fly breaks the puparium which splits transversely and the imago comes out, its wings dry and it flies off to become sexually mature in one week.
In the emergence of the fly two processes are involved, firstly the imago is liberated from the pupal skin, and secondly it emerges from the puparium which is broken by the ptilinum. After emergence of the imago the ptilinum is withdrawn into the head, but it leaves behind a mark, the ptilinal suture.
