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THE BUTTERFLY BODY - Its structure and function
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Paul Smart - The illustrated encyclopedia of the Butterfly world |
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Adult butterflies are built on the same general plan as their other insect relatives such as the wasps, bees and beetles. The body is protected by an armour of chitin (forming the exoskeleton) and this is arranged in a series of rings or segments separated by flexible membranous zones which allow movement to take place. The body consists of three main regions, the head, the thorax and the abdomen, all with a specialized structure to equip them for different functions in the life of the insect. Each part is covered in a layer of minute scales which are responsible for the soft, downy appearance of the body as well as the frequently vivid coloration so characteristic of butterflies.
The head
The head is a small spherical capsule which bears the feeding apparatus and sensory structures.' Adult butterflies feed mainly on nectar from flowers, although honey dew (the sweet secretion produced by aphids), decaying fruit, the sap exuding from damaged trees, excrement or the juices of carrion are examples of other butterfly foods.
Butterflies have no jaws and must always take the food in liquid form using a specially modified 'tongue' or proboscis. This is a long, hollow tube which is coiled like a watch-spring and tucked under the head when not in use. It can be quickly unrolled to probe deep into flowers; this extension being brought about by an increase in blood pressure. This feeding tube is composed of two parts which are grooved on their inner surface and joined along their length by tiny interlocking spines. The liquid food is sucked up the central channel between the two parts and there is a special type of pump in the head to assist in this. The continuous passage of sticky fluid tends to clog the channel and so the proboscis has to be cleaned periodically.
Since butterflies never bite or chew their food, jaws or mandibles are absent. A pair of sensory palps or feelers (labial palps) are present instead, one on each side of the proboscis. These palps are densely covered with scales and sensory hairs and serve to test the suitability of the food source. Some species of butterflies actively drink water, by using their proboscis, and the enormous congregations on muddy river banks are a notable feature of the tropics.
The antennae are typically dub-ended in butterflies. Each antenna consists of a series of rings or segments and both the number of, segments and size of the club varies in different families, the skippers (Hesperiidae) for example, often have very pronounced 'hooks' . The antennae are sense organs which are responsible for balance and smell. The base of the antenna houses a specialized organ, Johnston's organ, and it is this which is of value in sensing the insect's orientation, particularly during flight. The smell receptors are scattered over the entire surface of the antenna.
The eyes of butterflies are conspicuous hemispherical swellings on the top of the head. They are called 'compound' eyes because each eye is composed of a large number of optical units or ommatidia. The individual ommatidium resembles a simple eye with a lens and a light receptive region, and each one is capable of forming its own visual image. Each ommatidium is sheathed by a layer of pigment which serves to separate it from its neighbours. A butterfly therefore sees its surroundings as a complex mosaic of tiny pictures, each picture being eated by a single ommatidium. Although butterflies can readily detect the movement of objects, the acuity of their vision is much inferior to that of man. They are able to detect a limited number of different colours by discriminating between light sources
of different wavelengths. This simple colour vision may be illustrated by the fact that certain types of butterfly will frequent flowers of a particular colour ; for example, swallowtails (Papilionidae) regularly visit red flowers. Butterflies are also capable of detecting ultra-violet light, which is invisible to humans, and this suggests they may well see the colour of flowers in quite a different way to us. Other light sensitive structures known as the dorsal ocelli occur on the top of the head between the large compound eyes. These ocelli are very small and play little part in the vision of adult butterflies.
The thorax
The middle zone or thorax of the body is the locomotory region, where both the legs and wings are located. The head is joined to the thorax by a flexible neck or cervix. The thorax is composed of three segments and each carries a pair of legs adapted both for walking and clinging. Each leg consists of several regions, the basal joint (coxa), the thigh (femur), the shank (tibia) and the foot (tarsus). The coxa and femur are joined by a small triangular segment, the trochanter. The foot commonly has five joints and ends in a pair of claws. In the Nymphalidae the front legs are very short and held close to the body, giving the odd appearance of an insect having only four legs.
The shank or tibia of the fore leg in some species has a mobile spur or epiphysis which is armed with a brush of hairs and is used to clean the antennae. Surprisingly butterflies also use their feet to taste their food, so there are numerous sense organs on the tarsi.
The two pairs of wings belong to the second and third thoracic segments (meso- and meta-thorax). The delicate wings consist of an upper and lower membrane with a framework of hollow tubes between the layers. These supporting tubes are called veins and. they are arranged in a very precise way. The overall pattern or venation is often a diagnostic feature of a group of butterflies and consequently wing venation is an important tool in classification. The principal veins are given names according to their position on the wing.
At the base of the wings are some small structures known as sclerites which form a flexible articulation between the wings and the thorax. These permit the beating 'of the wings in flight and also allow them to be folded away when at rest, in the upright position characteristic of butterflies. During flight the wings are regularly moved up and down and the movements of the two wings on each side are coupled or linked together in a special way. Usually the hind wing has a lobe which presses against the fore wing, thus ensuring that wing movements are synchronized to give maximum efficiency. The wing movements are created in two ways : firstly by muscles acting at the base of the wing and secondly by distortion of the thorax brought about by the thoracic musculature which is particularly well developed in the wing-bearing segments. During flight the orientation and balance of the insect is controlled by special sense organs. Butterflies are typically active during the day (diurnal) and most species only fly in bright sunshine. The height at which they fly varies some merely skim across the surface of low vegetation whilst others fly very much higher.
The colour patterns on the wings are due to the covering of scales (see Coloration). The scales overlap each other in a regular fashion resembling tiles on a roof. Each scale is more or less racquet shaped and has a small projection or stalk at its base which fits into a minute socket on the wing membrane. The handling of specimens damages this delicate articulation and the scales easily rub off. To the naked eye the scales look like coloured dust. Pigments contained within the scales give rise to the colour of some butterflies, whilst in others, microscopic ridges or striae on the surface break up the light falling on them and so produce the metallic colours of the blues and coppers.
Scattered among the scales are specialized scent scales known as androconia (vảy cách ở bướm đực) which are peculiar to the males. At the base of these scales is a small gland which produces an aphrodisiac to excite the female during courtship. The volatile secretion passes up the hollow stalk of the scale and is disseminated by the fine hair-like processes or plumes at its tip. These scent scales may be scattered over the upper surface of the wing or collected into special patches called scent brands, as in the fritillaries, where they are visible as thickenings on the veins. During courtship the male often flutters around the female waving his wings and attempting to stimulate her by his scent. In some butterflies, although the scent producing area is in the wings, the dissemination takes place by means of the hair pencils which are associated with the terminal abdominal segments. These hair brushes consist of a small sac which is pushed out by blood pressure during courtship. At the end of each sac is a tuft of hairs. During scent dissemination the wings are outspread and the hair brushes brought into contact with the scent producing areas on them. The hair brushes are then expanded fully to disperse the scent.
The abdomen
The abdomen is much softer than the head and thorax and consists of ten rings or segments of which only seven or eight can be easily seen. The end segments are specialized for reproductive purposes and are generally known as the genitalia. In the male there is a pair of claspers which grip the female during mating and surround the central ejaculatory organ. The presence of a pair of claspers at the hind end of the body is a simple and reliable way to distinguish the sexes . In the female some fusion of the terminal segments occurs to give rise to an egg laying tube or ovipositor. The ovipositor is normally telescoped inside the body of the female. There is usually a special opening which receives the sperm from the male.
The female also produces a scent and this is attractive to the males of the species. The glands responsible for this are at the tip of the abdomen. The scent producing area is exposed by the extension of her abdomen. These substances are extremely effective in attracting the males and one female is able to lure vast numbers of males to her vicinity from great distances. These substances are known as pheromones and play an important communication role in the social behaviour of many groups of animals. Once both sexes have been stimulated by the respective scents, courtship is complete and the couple is ready for pairing. The female settles and the male grasps the end of her abdomen with his claspers. Pairing takes some time and the two insects may remain together for an hour or so. The male passes his sperm to the female in a package called a spermatophore which she retains inside her body until egg laying begins. If the couple are disturbed during pairing they will take to flight with one partner (usually the male) being dominant and dragging the other after it. A male is capable of mating with several females.
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The Golden birdwing Troides aeacus - Picture: Phùng Mỹ Trung |
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Internal structures
Within the toughened exoskeleton of an insect the internal organs are bathed in blood. Unlike humans the blood system does not consist of veins and arteries but instead the whole of the body cavity is one large blood filled space (the haemocoel). Circulation of the blood is maintained by a long, tubular heart which lies along the back of the insect (dorsal surface. The heart has muscular walls which con tract rhythmically and push the blood or haemolymph forwards into the blood space. On its return from the body tissues the blood re-enters the heart via small pores or ostia.
The digestive system is specially designed to cope with a liquid diet. The base of the 'tongue' or proboscis opens into a spherical muscular region (pharynx.). This region is often referred to as a sucking pump, since it is responsible for drawing liquids up the long proboscis. The sucking action is brought about quite simply by a change in volume of the pharynx. The enlargement of the pharynx creates within it a partial vacuum and the liquid in the proboscis is drawn upwards and into the pharynx. The muscular walls then contract and this serves to push the meal into the oesophagus, which is the next region of the alimentary canal. During this contraction of the pharynx there would also be a tendency for the fluid to flow back down the proboscis, but this is prevented by a shutter-like valve which closes the entrance to the proboscis. Once inside the digestive tract the food may be stored in a small reservoir (the crop) until it is needed. The actual digestion of the food takes place in the stomach and any unsuitable material is passed into the hind intestine and is voided as faeces, via the anus. The digested food is absorbed into the blood and is stored as fat in a structure known as the fat body until it is needed. The fat body takes the form of sheets of fatty tissue which either underlie the outer integument of the insect or surround the digestive tract. The fat body is usually better developed in the female since it is also needed to provide nourishment for her developing eggs.
Excretion is performed by structures known as malpighian tubules. In their mode of functioning they closely resemble kidneys although in appearance they are very different. The tubules are long filaments attached to the alimentary canal at the beginning of the hind gut. They float freely in the haemocoel and extract waste products from the circulating blood. These substances (collectively referred to as urine) are passed through the tubules and into the hind gut, from here they leave the body with the faeces.
The nervous system is composed of nerve cells or neurones and these are grouped together in nerve centres or ganglia. One of these is situated in the head and is generally referred to as the brain. The brain is connected to a nerve cord which lies under the digestive tract and passes to the hind end of the body. The nerve cord has a number of subsidiary nerve centres or ganglia along its length. Most butterflies have two ganglia in the thorax and four in the abdomen. From these centres smaller nerves pass to all parts of the body. Special nerve cells (visceral (thuộc nội dung) nerves) are associated with the digestive system and reproductive system whilst others (peripheral nerves) innervate the surface of the body.
The internal reproductive organs of insects consist of a pair of gonads and a system of tubes or ducts to carry their products (either sperm or eggs) to the outside of the body. In the female each ovary consists of four egg tubes or ovarioles. Each contains a large number of eggs at various stages of development. There is a special sac-like storage chamber (bursa) which retains the sperms until the eggs are ripe and about to be laid, only then are they fertilized. The female also has accessory glands associated with the reproductive system which secrete a sticky substance used for cementing her eggs to the substrate on which they are laid. The male has a pair of testes which are fused together in some butterflies. Sperm reservoirs are also present which store the sperm prior to pairing.
In many animals, including humans, oxygen is carried around the body by the blood but in butterflies (and other insects) the organs of the body are separately supplied with oxygen by a system of 'air tubes' or tracheae. These open to the atmosphere through special apertures in the exoskeleton called spiracles, of which there are nine pairs in butterflies. The actual exchange of gases takes place by simple diffusion at the ends of the tracheae, where the tubes become very narrow. In active insects the diffusion of oxygen into the body and the passage of carbon dioxide out is speeded up by ventilatory actions of the body comparable with our own breathing movements. Even so the system is relatively inefficient and the amount of oxygen available to the insect is quite small. This crucial point represents one of the main limitations on the size of butterflies, and without a major redesign of their whole system they cannot ever be more than relatively small creatures.
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