In this article we will discuss about Musca Nebulo:- 1. External Features of Musca Nebulo 2. Life History of Musca Nebulo.
External Features of Musca Nebulo:
Musca nebulo, the common Indian housefly is a heavily built insect and is about 8 mm in length. The colour of the body is gray. The body is distinctly divided into head, thorax and abdomen.
1. Head:
The head is large and freely mobile. It is as broad as the thorax. It bears two large reddish-brown compound eyes, each having about 4,000 ommatidia.
On the dorsal side of the head there are three ocelli or simple eyes on a triangular ocellar plate. Between the compound eyes the dorsal region of the head has a vertex, below which is a frons. In front of the head is a depression bounded by a Q-shaped frontal suture or ptilinal suture. In the depression is a pair of small three-jointed antennae, the last joint being the largest.
The antennae can be raised in front of the head or can be withdrawn into the depression. On the last segment of the antenna is a bristle which is plumose to the tip, it is an arista. Above the bases of antennae is a small crescentic sclerite known as frontal lunule. Below each compound eye is a lateral gena. Below the depression is a membranous ridge called the epistome.
Mouth Parts:
The mouth parts are of sponging type, i.e., they are adapted to suck the liquid food. The mouth parts comprise a fleshy and retractile proboscis which lies under the head. The proboscis is formed of three parts, a basal rostrum, a middle haustellum, and a distal pair of labella.
The rostrum is cone-shaped and has a clypeus in front. Morphologically the rostrum is a part of the head and it bears a pair of one jointed maxillary palps. Inside the rostrum is a chitinous fulcrum which encloses the pharynx. Under the lower end of the fulcrum is a small chitinous hyoid sclerite which serves to keep the lumen of the pharynx distended.
Hinged to the rostrum is a haustellum formed by a highly modified labium, the posterior part of the haustellum has a weakly chitinised theca or mentum. The front side of the haustellum has a deep oral groove enclosing a labrum-epipharynx and a hypo pharynx. The hypo pharynx has a salivary duct.
The labrum- epipharynx is grooved, the goove is closed below by the hypo pharynx to form a tube or food canal. The mandibles and maxillae are absent. The distal labella are large lobes fused in the middle, their outer surface has a series of channels called pseudo tracheae which are kept open by a series of incomplete chitinous rings which make them look like tracheae.
The pseudo tracheae open externally by a double row of tiny holes through which liquid food is taken in. The pseudo tracheae converge into a mouth lying between the two labella. Near the mouth are very small prestomal teeth which are used for rasping solid food. The proboscis can be retracted below the head and it folds between the rostrum and haustellum.
Feeding:
The housefly feeds on any organic fluid, its mouth parts are modified for lapping up liquid food.
The proboscis is extended and the labella are placed on the fluid, the labella can smell and taste the food. By the suctorial action of the pharynx fluid food and very fine solid particles are sucked up into the pseudo tracheae from where they go into the mouth, and then into the food canal formed by labrum-epipharynx and hypo pharynx, then the food enters the pharynx and goes to the alimentary canal.
Houseflies also feed on solid substances, especially sugar and sweets, the fly then regurgitates a drop of liquid from the alimentary canal and saliva from salivary glands onto the solid food through the pseudo tracheae. The alimentary canal fluid and saliva liquefy the solid particles of food which the fly sucks up.
2. Thorax:
On the dorsal side of a grey-coloured thorax are four black-coloured longitudinal stripes. The thorax is formed mostly by an enlarged mesothorax, the prothorax and metathorax are greatly reduced and largely hidden on the dorsal side. The notum of mesothorax is formed of three large sclerites, a prescutum, a scutum and a scutellum with transverse sutures between them.
The large mesothorax bears a pair of wings. The wings are almost transparent, and when folded at rest they cover the abdomen and project beyond it posteriorly. On the lower inner side of the wing is a free lobe, the alula, and beyond this toward the thorax there are two other lobes called squamae which are opaque.
These three lobes fold below when wings are closed. The metathoracic wings are much reduced and modified to form halteres which are balancing organs, they vibrate rapidly during flight. A haltere has a broad basal scabellum, a narrow stem or pedicel and a terminal knob, the capitellum. In the scabellum are several sensillae which are sound receptors.
Below the thorax arise three pairs of legs which have normal structure with five-jointed tarsi. Each tarsus ends in two claws below which are two pad-like pulvilli, the pulvilli secrete a sticky fluid by means of which the fly can rest and walk on ceilings and smooth surface, like glass panes, without falling. The entire leg bears a large number of bristles.
3. Abdomen:
The attachment of abdomen to thorax is narrow, and the abdomen is broad in the middle and narrow towards the apex. It is yellowish basally and dark yellow above with a black longitudinal stripe mid-dorsally.
There are ten segments in the abdomen, but the first has atrophied and the second is reduced, the third to the sixth are well developed and visible, but segments seven to ten are reduced and lie telescoped inside the anterior segments. In the visible segments the terga are large and extend ventrally.
A pair of spiracles are present in the ventral edges of the terga on segments two to six. In the female the hidden segments seven to ten form a tubular ovipositor which protrudes and can be seen when the fly is depositing eggs. The tenth segment bears a pair of cerci.
In male the terminal segments are curved below and form a hypopygium or external genital organ. The ninth segment has a pair of claspers with an aedeagus between them. The tenth segment is fused with the ninth and bears a pair of cerci.
Life History of Musca Nebulo:
(i) 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.
(ii) Egg-laying:
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.
(iii) 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.
(iv) 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.
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.
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.
(v) 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.
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.