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ECOLOGY - Butterflies and their environment
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Paul Smart - The illustrated encyclopedia of the Butterfly world |
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In a particular area or habitat there may be several species of butterflies; some present in very large numbers and others represented by only the few individuals. All the individuals of each species within that area may be thought of as a population, the members of with interbreed and are responsible for the survival of the species each adult female may lay several hundred eggs during her life but only a small proportion of these will produce viable adults. Deaths or mortality occur at all stages of the life cycle and it is this which keeps the size of the population stable. Of course variations in number of animals in a population are apparent from time to time, mainly thought changes in the birth or death rates and through the immigration of individuals into or emigration away from the habitat.
There are two main ways in which the size of population or the population density may be regulated. Firstly by factors which have the same impact on the population regardless of the numbers of ANIMALS PRESENT. These are sometime referred to as density independent factors and are mostly climate influence which may have an advantageous or perhaps disastrous effect on a population of butterflies regardless of whether there is an enormous colony or only a few breeding pairs. The other way in which population regulation may occur is dependent on the number of individuals present; density dependent factors. Its these which are of vital importance when an essential resource such as food is in short supply. Overcrowding may occur when space is limited and this can have severe consequences on the success of population. The more butterflies there are present, the more effects of density dependent factors are felt.
Climatic factors
Changes in climate have a considerable effect on different stages in the life cycle. The unfavourable seasons of year, such as the winter in temperate regions and the dry season in the tropics, are usually accompanied by an arrest in growth and development. This period of hibernation (or aestivation in the tropics) acts as a sort of timing device for insect. It ensures that the occurrence of the activities larvae coincides with the availability of the suitable food plant in the correct condition and also that the more vulnerable and delicate stages are not subjected to frosts or excessive dryness.
Daily fluctuations in temperature and moisture are also important to insects and when they are subjected to extremes heavy mortality may result. Butterflies, like reptiles, are cold- blooded animal and have to derive their body heat from external sources. They warm up by basking in sun, usually with the wings out spread and the body orientated so that the maximum area of the wings is exposed to the sun. The colour patterns on the wings may assist in heat absorption; those species with large black patches are particulary efficient heat gatheres. Buttterflies cool themselves by seeking the shade, or it shelter is not available they may close their wings together and face the sun so that the smallest possible surface is exposed to the sun’s ray. It has been suggested that cooling of the body may also take place by the evaporation of water, but this idea is not fully accepted. Excessive dryness is advoided by the larvae or adult seeking out areas og high humidity for resting sites. Adult butterflies shelter from heavy rain by settling on the underside of the leaves with their wings held slightly open so that the rain can run off them. If the wings were held tightly together in teir usual resting posture and then became wet the surface layer of scales might well become damaged. Generally butterflies are not found flying in high winds, although during the migration flights of some species they are able to look after themselves satisfactorily. Other phenomena, such as forest fires, may have a serious effect on a butterfly population; the adults are usually able to fly to another suitable area, but their eggs, larvae and pupae risk total destruction by fire.
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Relationships with other species
The success of a population also depends on its relations with other species and also with other individuals of its own species. The effects of these interrelations are usually correlated with the density of the population in a particular area.
Almost all butterflies are herbivores in their larval stage and would therefore compete with other herbivores for the available plant material in a particular habitat. Butterflies have overcome the problem of competing with their own larvae by having the adults feed on different parts of the plant (eg flowers) and usually on different species to those used by the larvae. For example, the 'large white' larva feeds on species of cabbage (Brassica) whilst the adult will suck nectar from a wide variety of flowering plants. In this way competition for food within the species is kept to a minimum. Even so, when an increase in the size of a population occurs, food and space become in short supply and this may result either in heavy mortality or in the migration of some individuals to a less crowded habitat. In addition, many adult butterflies prevent overcrowding by maintaining territories rather like birds do, and may be seen from time to time driving off other butterflies which attempt to enter their territory. The threatening behavior usually consists of facing the intruder and jerking the wings.
Methods of self-defense
All stages of the life cycle are vulnerable to attack by predators which may include other insects as well as spiders or vertebrates such as birds, reptiles and small mammals. Consequently, methods of protection have been evolved which reduce the chance of attack by these predators. There are two main types of device, those which enable the butterfly to escape attention because of its dose resemblance to the surroundings and those which make the butterfly appear startling and unattractive, an indication that it is unpalatable to the predator.
Generally the eggs of butterflies are laid on the undersurface of leaves and are relatively small, so that they are unlikely to be noticed. However, the inactive, soft bodied larvae are very prone to attack, and it is this stage in the life cycle which displays a wide range of these protective devices. Some larvae spin silken webs and live in groups within them. The skippers (Hesperiidae) make tents out of silk and leaves, or may even live in a grass tube made by rolling up a grassblade and securing it with silk. Many larvae are coloured to blend with their surroundings, whilst in the young stages of the white admiral (Limentis camilla) camouf1age is taken a stage further by the larvae adorning (tô điểm) themselves with their own frass (cứt sâu). The later stages do not adopt this habit - and rely solely on protective coloration. Another difference in the devices employed by young and more mature larvae is seen in the swallowtail~ where the very young instars are dark and resemble bird droppings whilst the later instars are brightly coloured.
Several types of warming device are found in the larvae of butterflies. The Nymphalidae are often brightly coloured and have an armour of sharp spines. Other larvae may produce an obnoxious smell or have an unpleasant taste. The Papilionidae have a gland just behind the head called the osmeterium (mấu tuyến mùi) which is everted and produces a strong odour when the larvae are disturbed. In other families similar defensive glands may be present on other parts of the body. Some gregarious larvae may jerk their bodies in unison to deter predators. Occasionally a palatable larva may copy or mimic a distasteful one and avoid predation this way. The phenomenon of mimicry will be discussed in more detail in next Chapter.
The pupae are usually protected by their cryptic coloration patterns and sometimes their shape, for example, the pupa of the orange tip (Anthocharis cardamines) closely resembles a seed pod. The pupae of some Nymphalidae may be spiny rather like the larvae. Larvae normally seek out safe sites for pupation and consequently the pupae are found in inconspicuous positions and some occur within cocoons formed from dried leaves or hidden in the ground.
The adult butterfly is most vulnerable immediately after its emergence from the pupa and before the wings have expanded and dried. After this the insect can take to flight and avoid many predators. Most species visit flowers to suck nectar and are frequently attacked when they alight on the flower.
Protective coloration is also employed by adult butterflies whose wings may have a brilliantly coloured upper surface but are cryptically coloured below so that when they are folded together at rest the insect completely harmonizes with its surroundings. Some families have eye spots on the edges of their wings (Lycaenidae and Satyridae) and the Lycaenidae may also have long tails to their wings. Both the eye spots and the tails are thought to attract the attention of predators and by attacking these parts the actual body of the butterfly is left undamaged. Some species such as the monarch (Danaus plexipus) produce an obnoxious odour.
The role of parasites
Parasites are creatures which live either on or within the body of another animal, the host. Butterflies, particularly their larvae, are especially prone to attack by parasites, many of which are very highly adapted both in their structure and in their own life cycles to take advantage of them. Butterflies are particularly threatened by their insect cousins belonging to the orders Hymenoptera (wasps and bees) and Diptera (flies). Only certain specialized members of these orders live as parasites. The principal parasites of butterflies are the groups Ichneumonidae, Braconidae and Chalcidoidea among the Hymenopterans and the Tachinidae among the Diptera. Often among animals the parasite's way of life is carefully balanced so that both the host and parasite survive. However, in the case of an attack by these insect parasites the host usually dies. In this sense the parasites almost resemble predators and they are then usually described as parasitoids. Many parasitoids are associated with a particular species or group of butterflies while others parasitise a wide range of families. One of the most familiar and widely distributed is the genus Apanteles belonging to the Hymenopteran family Braconidae. Apanteles glomeratus attacks the 'large white' butterfly (Pieris brassicae). The female parasite lays her eggs in the larvae (or very occasionally the eggs or pupae) first piercing the host's skin with her sharp ovipostor. Then a large number of eggs are laid in each host. The eggs hatch and feed on the internal tissues of the host; initially consuming the fat body and later attacking the more vital organs such as the digestive tract and nervous system. This internal destruction results in the death of the host before it has completed pupation. The fully fed parasite larvae then eat their way out of the host and pupate on or near the carcass in bright yellow cocoons from which the adult parasites eventually emerge.
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Sucking minerals in the muddy - Picture: Phùng Mỹ Trung |
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The dipterous parasites in the family Tachinidae are rather variable in their habits, but usually the eggs or in some cases young larvae are deposited by the female fly on the food plant of the butterfly larva. The parasite attaches itself to the larva as it is feeding on the plant and burrows into its tissues.
Other parasites live on the outside of their host where they suck the body fluids. Mites are examples of these ectoparasites and are found associated with butterflies from time to time. Mortality is also caused by bacterial and fungal diseases which are particularly prevalent in conditions of high humidity. Considerable care has to be exercised in rearing butterflies in captivity to prevent diseases caused by these organisms, since they normally result in the death of the entire colony.
An association between butterflies and other insects is not always harmful. An example of one which is mutually beneficial is the relationship between ants and many species of blues and coppers (Lycaenidae). The larvae of these butterflies are provided with a honey gland which exudes small droplets of a sweet fluid. This is extremely attractive to ants, which 'attend' the larvae and stimulate the production of the fluid by stroking movements of the legs and antennae. The ants are pugnacious insects and serve as a deterrent to the usual predators and parasites of the butterfly larvae. The ants never damage the larvae, but merely lick the gland, enjoying the secretion it produces. Some larvae seem almost dependent on the ants and their development is impaired if the ants are absent.
The final relationship to be considered is that between butterflies and man. Many species of butterflies are nectar feeders and thus frequently visit flowers. In moving from one to another they undoubtedly perform a beneficial activity in assisting with plant pollination, although very little is known of their precise role in this context. On the other hand the damage caused by the larvae as they feed on crops is considerable. For example the larvae of many of the Satyridae feed on grass and cereal crops whilst some Lycaenidae feed on the flowers and seed pods of leguminous plants, many of which are of commercial value. One possibly beneficial outcome of the intense feeding activity of the larvae is that some have weed species as their favoured food plant, and so these larvae may play a useful part in weed control. Thus butterflies are both beneficial and detrimental to the human economy, and the control of the harmful species therefore needs to be done selectively.
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