|
THE BUTTERFLY LIFE CYCLE - From egg to adult
 |
Paul Smart - The illustrated encyclopedia of the Butterfly world |
|
|
Butterflies are examples of endopterygotes: insects which characteristically undergo a complete change or metamorphosis during the course of their development. Their life cycle includes both a larval and a pupal stage before the adult insect or imago emerges. The larva is completely different from the adult both in appearance and habits. The pupa is an inactive, non-feeding stage which gives rise to the adult. In grasshoppers, cockroaches and earwigs, which are examples of exopterygote insects, development is much more gradual with a series of young stages or nymphs which all resemble the adult and become progressively more like it at each successive stage. In these insects there is no pupal stage.
The egg
Female butterflies usually lay their eggs on or very near to the food plant on which the larva feeds. The oviposition site is carefully chosen, and touch, smell, taste and sight are probably all involved in its selection. Most species lay their eggs singly and cement them to the plant by a sticky secretion. The eggs are usually laid on a particular part of the plant, for example, on the leaves, flower heads or in crevices in the bark. Most frequently they are laid on the under surface of the leaf; the female alights on the upper surface and curves her abdomen under the leaf until a suitable position is found. Here the eggs are protected from rain and sunshine and to some extent from predators. However, a large number of eggs are laid by a single female to ensure that at least some will hatch successfully. The large white (Pieris brassicae) often lays clusters of one hundred or more eggs, whilst a few others lay smaller batches of 5-15 eggs. The arrangement of the eggs varies, they may form regular bands around twigs or pendant strings of eggs. A small number of species merely scatter their eggs at random as they fly over vegetation (usually grassland).
Butterfly eggs are commonly yellow or green in colour although they may darken just before hatching. The shape of the egg varies in different species and may be spherical or oval and flattened. The shell is often elaborately sculptured with regular ribs or pits (reticulations). At the top of the egg is a slight depression within which is a minute opening or micropyle. The micropyle marks the entry point of the male sperm into the egg, and once the egg is laid, air and moisture pass to the developing embryo through this pore. Food is contained inside the egg in the form of yolk and this is gradually consumed as the young larva develops.
Just before hatching the fully formed embryo can be seen curled up within the transparent egg shell or chorion. The young larva gnaws its way through the shell and after hatching continues to eat the shell until only the base is left. In the large white butterfly, where the eggs are laid in dusters, a newly hatched larva may also eat off the tops of other unhatched eggs. The shell contains valuable nutrients and is immediately available to the larva. After this the food plant on which the egg was laid will be devoured.
The larva
Butterfly larvae, or caterpillars as they are commonly called, are quite variable in colour and in shape, although their basic structure is relatively constant. The larva has a head followed by 13 trunk segments of which the first three are regarded as the thorax and the remainder the abdomen. The larval skin or cuticle is soft and flexible., though spines or bristles (setae) which arise from surface tubercles may be present. These are particularly characteristic of the family Nymphalidae.
The head is a hardened round capsule with a completely different array of structures to the adult butterfly. The larvae feed on plant material which is relatively tough to a small insect. Consequently, the mouth parts are modified for biting and chewing. There is a prominent pair of toothed jaws or mandibles which bite off fragments of food and shred them into fine pieces. The maxillae (which form the proboscis in the adult) are very small and used to guide the food into the mouth. The other main mouth part (the labium) is modified to form the spinneret which is used in silk production.
Compound eyes are lacking in the lava, the main visual organs being the lateral ocelli. These are arranged in two groups of six, one group on either side of the head. The ocelli in many ways resemble the single optical unit or ommatidium of the adult's compound eye. Each ocellus has a lens and receptive part or retina. It seems that they are unable to create an image of their visual field and probably only detect the difference between dark and light. The lateral ocelli can therefore be of little use in the location of food, the senses of touch and smell being more important in this context. The head also bears a pair of short, stubby antennae.
The three segments of the thorax each have a pair of short jointed legs which end in a single claw. The abdomen has ten fleshy rings or segments, five of these bearing a pair of false legs or prolegs. These are soft structures, without joints, present on the third to sixth segments. A final pair on the last segment are called claspers, and some larvae will be tom in half rather than release their hold with them. The end of each proleg is flattened and has a series of hooks or crochets which help the larva in locomotion. When not being used the prolegs can be withdrawn into the body. The arrangement of the hooks on the prolegs is a useful guide in the classification of larval types.
Since the skin of the larva is so it is not able to provide a suitable skeleton for the attachment of muscles (as the exoskeleton does in the adult). Consequently the body has to be kept turgid by the pressure of the body fluid (haemolymph) rather like an earthwonn. The characteristic crawling movement consists of a co-ordinated interaction between the muscles of the body wall and the internal pressure of the haemolymph.
As the larvae grow they entirely fill their skin, which becomes very tight. In order that a further size increase may occur this skin is shed from time to time, exposing a new and larger one which has formed beneath it. Rapid expansion of the 'new' larva occurs before the skin becomes toughened. This shedding of the skin is known as moulting or ecdysis and usually takes place four or five times before the larva is full grown. Each growth stage between moults is known as an instar. Moulting in insects is very carefully and precisely controlled by hormones, although environmental conditions and the availability of food may cause variations in the duration of the instars.
|
|
|
|
Larva and Pupa of butterfly - Picture: Lê Văn Nông |
|
The larvae feed mainly on the leaves of flowering plants and trees. Ferns or mosses appear never to be eaten. They’re extraordinarily specific in their feeding habits and will usually only feed on a small number of closely related plant species. If a suitable food plant is not available then larvae will starve to death rather than eat something else. A larva recognizes its food plants by certain aromatic vegetable oils which they contain. It is generally thought that selection may depend upon the detection of chemical attractants in the food species and of repellents in other plants.
The larva is the main feeding stage in the life cycle, and when they are present in sufficient numbers caterpillars can defoliate large areas of vegetation. This means that those butterfly species whose larvae feed on agricultural crops can be a serious pest. Single larvae may consume an entire leaf before moving on to the next, but more often only a part of the leaf is eaten. The way in which a larva tackles a leaf is often characteristic of the species ; some eat holes in the leaf, whilst others attack the leaf margin. In some species feeding occurs at night and in others by day. Generally periods of active feeding alternate with periods of rest. Feeding ceases a day or so before a moult, but is resumed as soon as the new skin is fully developed. A diet of plant leaves, in addition to requiring mouth parts for chewing the food, necessitates some modifications of the digestive tract. A large quantity of food is passed through the alimentary canal which is consequently a wide straight tube. The stomach is the largest region and has muscular walls which maintain continual churning movements, causing thorough mixing of the food material.
The droppings of the larvae are discrete oval structures and are referred to as frass. Since much of the plant material consists of indigestible celulose a large amount of frass is produced and may even form a distinct layer under small trees or shrubs on which many larvae have been feeding.
The internal organization of the other systems closely resembles that found in the adult insect. However, the salivary glands, which In the adult produce substances to accelerate digestion, take on a new role in the larva, that of silk production. Several species are known to produce silk to make webs or cocoons. The silk is manufactured in the salivary glands which are a pair of long tubes, particularly conspicuous in the Satumiid moths. The spinning apparatus lies in the spinneret (part of the labium) and consists of a silk regulator and a directing tube. The characteristic side to side movement of the head of the larva draws the silk out into a fine thread which adheres to the substrate wherever it is touched by the directing tube. During spinning silk is continually passed from the regulator to the directing tube and hardens on exposure to the air. The silk net forms a protection for the larva, especially when it pupates.
Butterfly larvae are rather inactive sluggish creatures and this together with the soft outer covering makes them an ideal target for a wide range of predators. To protect themselves the larvae have evolved various structures and habits to either serve as camouflage or to make them appear unattractive or unpalatable to a would be predator. Some of these devices will be discussed in mote detail in later chapters.
The pupa
The end of larval life is marked by another moult which gives rise to a pupa or chrysalis. Fully grown larvae often select special sites to undergo this transformation and may for. example leave their food plant and enter the soil. The digestive tract is emptied and the larval skin shrivels and eventually splits to expose the pupa. During the pupal period much of the larval tissue is remoulded to give rise to adult structures, particularly the wings, mouth parts and reproductive organs.
The pupa is immobile and neither eats nor drinks since the mouth and anus are sealed over. The only functional openings in the pupal case are the spiracles which permit the exchange of respiratory gases. The legs and antennae are firmly stuck down and cannot be moved. Externally the pupa usually appears brown or green, and the abdominal region, with prominent segmental rings and tapering posteriorly, is very distinct from the thoracic part. All the major features of the adult can be seen within the pupal skin, but one. specialized structure is to be found' at the end of the abdomen, where a number of hooks form the cremaster. This is used for the attachment of the pupa to the substrate.
It is often desirable to distinguish the sex of a pupa before the adult emerges, and this is not usually difficult. In the male there is a single genital opening on the ninth abdominal segment whilst in the female there are two such openings, one on the eighth and another on the ninth segment of the abdomen.
Since the pupa is immobile it is particularly vulnerable to attack by predators and frequently pupation proceeds within a silken cocoon. This may take the form of a hollow of earth lined with silk, or a roll of leaves fastened together with silk threads (eg Hesperiidae and some Satyridae). Silk cocoons are generally much better developed among moths. In some butterflies the pupa is naked but it is then usually protectively coloured. The naked pupa may hang upside down attached only by the cremaster (many Satyridae and Nymphalidae) or it may be attached by the terminal cremaster but also supported head upwards by a silken girdle (Lycaenidae, Pieridae and Papilionidae). In each case the larva spins a little silken pad into which the hooks of the cremaster are firmly embedded..
The adult
The emergence of the adult or imago is preceded by the colour pigment appearing in the wing scales, so that the wing patterns of the adult can be seen through the pupal case. The skin of the pupa splits behind the head, the insect first frees its legs and antennae and after a short while withdraws the rest of its body. Those species pupating within a cocoon have to free themselves from this as well as the pupal skin. In some Saturniid moths there may be a special arrangement of the silk to facilitate this, whilst in others a special softening fluid is produced.
|
|
|
|
The Common archduke Lexias pardalis - Picture: Phùng Mỹ Trung |
|
Immediately after emergence the wings are soft and crumpled. The butterfly moves to a place from where its wings can hang downwards and blood is forced into them. The wings expand by the flattening of the numerous tiny folds and soon become the typical sheets supported by hollow veins. Once they have reached their full size the insect holds them apart until they are completely dry and hardened. The excretory material which has accumulated in the dosed digestive tract during the pupal period is ejected from the anus. The time of adult emergence varies, it may occur early in the morning or in the evening; in the latter case they rest until the next day before becoming active.
Interruption of the life cycle
The duration of the life cycle varies in different species. Some may have a single complete generation in a year whilst others have two or even more. Unfavourable climatic conditions such as the winter in temperate regions or the dry season in the tropics often necessitate an interruption in the life cycle. Any of the developmental stages - egg, larva or pupa - may enter a period of arrested development (or diapause). This delay in development is induced by environmental conditions such as day length, but cannot then be terminated until a pre-determined period has passed. This ensures that the insects do not emerge too- early during temporary favourable periods only to be caught out by a resumption of harsh conditions. The eggs and pupae are protected by their outer shell or skins respectively, whilst the overwintering larvae usually shelter at the base of their food plant or in a specially produced larval cocoon formed from plant leaves and silk. The stage in the life cycle which regularly undergoes hibernation (or aestivation in the tropics) varies in different species and also within the same species in different parts of its geographical range.
|
