Caterpillars undergo total metamorphosis.
Caterpillars, the larvae of Lepidoptera, undergo total metamorphosis. This total metamorphosis makes them different from adult butterflies and moths.
The caterpillar appears round but can be flattened. They are mostly long and spindle-shaped or cylindrical, but some can be flattened. Their head is very well developed. There are a few hairs on the cuticle covering of the head but mostly the head is bare. There are some caterpillars in the group of Lepidoptera that have heads can be drawn back into the segment of the thorax thus making the head disappear. A number of lateral ocelli are similar to the eye location in adult insects. They are small transparent lenses with six on either side. There will be one that is prominent and it will have five behind. There are varieties of caterpillars that have closely connected ocelli. On the back of the head are usually three antennae having tips as sensory receptors that are very short and are on either side of the head. These antennae show only tips at times as they can enter the cavities on either side of the head. The mouth parts are chewing types with parts being used for the production of silk. The labrum carries six pairs of bristles on its upperside with sensory organs on the underside. The jaws are used for preliminary chewing and for seizing food. It will depend on the amount of food they are able to obtain as to the size of the crests and points.
There is spinneret in a part of the labium called the prementum. This elongated tube narrows to a point and the silk comes out. There are hard bans in this but the tub is membranous.
There are three segments of the thorax each with a pair of jointed legs being short or flattened ending with a developed claw. There are some leaf-mining Lepidoptera that have no thoracic legs. For the caterpillars that are able to withdraw the head the prothorax is wider to allow for this passage. Ten abdominal segments of which some carry prolegs (soft, fleshy false legs). These prolegs are under the control of bundles of powerful muscles. These can individually act as suction cups to help the caterpillar to cling as it moves. The foot has hooks in a circle to help with the griping action. The hooks will not be the same on all caterpillars, as the hairs or bristles will not either.
On each side of the body of the caterpillar there will be breathing pores or spiracles with two on each side paired and symmetrical. There are eight pair on the first of the eight abdominal segments and one pair on the prothorax. Due to the colors these are easily visible on the caterpillar. There are some caterpillars that live in the water or in water areas and these breathing pores are necessary. The respiration of the integument of the body is fairly thin and permeable to oxygen. The larvae obtain oxygen by going to the water's surface of derive air from air bubbles of parts of plants that are submerged.
In some caterpillars there are gills. The tracheobranchiae are from the skin, threadlike or flattened. The extensions of the air-tubes will branch into gills. Their tracheal system is a closed system as respiration only occurs through these small air-tubes as oxygen is passed through the body wall. Muscle will allow the movement of the organ in the water to enable an oxygen supply. The blood gills are constructed different as they have a thin cuticle and carry the haemolymph blood liquids and seldom have tracheal branches. There are three species having the blood gills and all belong to the genus Cataclysta.
The digestive system runs in a straight line down the body cavity, is rarely curved and is short and broad. There are three sections of the digestive system. The stomodaeum or the oesophagus is short and will connect the mouth aperture to the mesenteron or middle section of the intestine. The cardiac sphincter is at the point where the two parts of the alimentary canal meet with circular muscular fibres for closure. The pyloric valvule will separate the mesenteron from the proctodaeum making the final section of the intestine. The roctodaeum and stomadacum are of the same origin as their cavity is covered with a cuticle as is the body surface. Mesenteron has no cuticle and is the center of most of the digestive and absorption functions in the caterpillar.
The silk-glands are connected with the alimentary canal. These glands are labial glands as in the salivary glands of other species of insects. These glands in the Lepidoptera are only present in the stage of larval. The silk-glands lead to the labium, are very long, about the length of the entire caterpillar body. There are three parts of this gland performing different functions. The part that is far from the outlet is narrow and produces the silk along with the wider part known as the reservoir. Next to the mouth there is a narrow and winding tube leading to the common duct. There are muscles that are powerful that dilate the cavities and end in the spinneret on the lower lip. The silk solidifies when it reaches air even though it is liquid due to oxidization. The silk will come out as a long thread with inner fibres consisting of fibroin and sericin. The inner fibre of the silk thread has two strands formed by the juxtaposition of threads that are secreted by two paired glands that press together by a common duct known as the press. This duct can cut the silk thread and determines the thickness of the thread.
The osmeterium contains a secretary organ located in the thorax of the caterpillar. It will act as a deterrent and is excretary and can be scented. This gland has an aromatic secretion. The osmeterium pumps a large amount of blood and then becomes brightly color and extends. When it returns to normal there is a retraction of muscles. In the moth family Lymantridae have hairs on this organ that can stiffen and become poisonous. These myrmecophilous organs allow some of the family to live as larva in ant nests so they can feed and live. Three myrmecophilous organs are in a triangle on the rear half of the caterpillar's back and can protrude. Two secretions are produced, one is sweet and the other is scented. The blood pressure causes this gland to swell and secrete drops of the sweet substance. It would not be a good idea to touch a caterpillar as some have dangerous poisonous secretions.
As the caterpillar emerges it is very smal. There is a cuticle around the caterpillar that cannot expand to contain internal organs of the body as they grow so skin is cast off and replaced with a new and larger skin. This is called molting and can happen as much as ten times. The caterpillars who produce females will molt more than the males. The caterpillar attaches to a surface before the molting process as an anchor with their silk threads. The exuvia or old skin after being cast off is usually eaten by another insect. It depends on the sex of the caterpillar as to the amount of molting that takes place. Hereditary factors, food, and temperature will be factors in the moulting process. The hypodermis secretes a liquid that dissolves the inner layers of the cuticle and ends connection with it and the rest of the body forming a chamber that lies between the hypodermis and the cuticle. This makes the slippage of the old skin easier as there is lubrication. New skin forms as the old skin is shed. The brain controls molting through the nervous system as a part is actively secreted while keeping nervous functions working properly. The secretion goes through the nervous system to the other organs in symmetrical spherical glands composed of nerve cells that secrete and are close to the heart. A cerebral hormone is discharged in the circulatory system that will make the prothoracic glands active in the first thoracic segment. The hormone secretion is called ecdysone or the molting hormone. There are corpota allata glands behind the corpora cardiaca that produce a hormone that controls the formation of the larval cells at each molt. When this hormone is active the larval form is retained when it is not the adult insect is produced. Molting hormones will cause molting and this juvenile hormone decides the form of the insect. It acts as a stimulant to mature the sexual glands of the young adult.
The caterpillar will appear larger after the required molting is over and will seek a place to transform into a chrysalis or to enter pupation. It will then attach itself to a surface, sometimes alone but mostly by making the protective silk cocoon. Three are categories of these chrysalises. The protected crysalises will be enclosed above or below ground or inside a cocoon and the naked chrysalises will be either suspended or girdled. The chrysalis will shed its larval exuvia completely during pupation and the shrunken skin will remain attached to the base of the pupa.
There are contortions to disentangle this shrunken skin. These suspended chrysalises soon to be caterpillars are head down as they hang from a silk on anal prolegs. When the moth or the butterfly emerges, the back of the chrysalis will stay attached to the silk pad. With girdled chrysalses the caterpillars will be attached to the surface with a silken thread for safety around the thorax or the head. When the old skin is detached the chrysalises will remain attached to the surface by the girdle and tip of the abdomen. During this pupal stage the chrysalis are lacking motion more or less although some can move abdomens or appendages when it is almost time to emerge.
The chrysalis will not look as the caterpillar and will look like the adult insect. You can easily see the three sections of the body of the adult that will be divided. It will have spines, outgrowths, colors will vary and it will be tough, hairless and very hard. You will see the compound eyes on the head and the antennae that are folded along the underside between the legs and the wings. In the thorax you will see the characteristic organs of the adult insect. They are usually in a tight group and sometimes the feet will appear before the legs. The forewings will be seen on the second segment of the thorax. They are usually shorter and smaller than adult wings. The wings will be tightly folded in a mature chrysalis. The chrysalises will have ten abdominal segments. You will see the spiracles and you can see the last abdominal segment that looks like a hooking device. The anal orifice is closed as only will be presented as a groove. The male genital orifice is a single groove on the outside of the chrysalis. The female has two grooves.
In some of the Leipidoptera there will be free chrysalises that all but the last three abdominal segments, the wings, the legs and other appendages are free rather than soldered together. The incomplete chrysalis will show wings, appendages and some addominal segments as free. Another type particularly the Rhopalocera has the wings and appendages firmly glued together, to the body and the abdominal segments are soldered together making the chrysalis immobile.
When the chrysalis reaches maturity you will see all the internal structure and the folded wings of the butterfly or moth a few days before emergence of the insect. The pupal case will get progressively darker. The butterfly or moth in its pupal case will break the skin of the chrysalis and force the head out first. Some species even have pointed outgrowth on their heads to rupture the cuticle. This emergence is not simple as it takes time. When the cuticle has been ruptured, the adult insect will emerge. Usually, the cuticle of a chrysalis remains whole, apart form the fracture line, while that of an incomplete chrysalis must crumble before emergency occurs. The head will allow the insect to breathe in air through the mouth parts and then with its legs and by twisting it will complete the emergency operation is a short time. As it works little by little the
Wings dry and expand with the help of the air and of internal liquids forcing under pressure into the wing veins. The cuticle covering the wings stretches and grows harder from contact with the air, and then in minutes or hours, the butterfly or moth reaches fully maturity. Thus a complete metamorphosis is finished and a beautiful insect emerges.
