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The caterpillar
Unlike an insect with an incomplete metamorphosis, the caterpillar looks nothing like the insect in its adult state. The purpose of a caterpillar is to collect energy and nutrients for the development of the butterfly or moth, for this purpose a caterpillar is simple but well equipped, a head followed by thirteen segments.
The head
The head of the caterpillar consists mainly of a head capsule of chitin, at the bottom of the head capsule is an opening where the mouthparts stick through. At the lower end of the head capsule are six pairs of simple eyes (ocelli) which can’t create an image but are perfect for detecting differences in light intensity. The antennae that are placed just below the ocelli, are used to feel and smell. At the bottom centre of the head are the spinnerets with which they can spin a silk thread. This thread is used to spin a cocoon or can be used to fix themselves prior to moulting or pupation. With two large sharp jaws, the caterpillar is able to cut leaves into small pieces. Just behind the jaws are the maxillae, these take care of transport to the mouth and are used to taste with.
The body
As mentioned earlier, the body of a caterpillar has thirteen segments. The first three segments are the thorax segments, each containing a pair of legs, these are the true legs and like any insect, there are six in total. The true legs are not used for crawling because they are usually too short, their primary function is taking a tight grip on food while eating.
The abdomen starts with the fourth segment. Segment six to nine usually contain prolegs or false legs, these provide good grip and are used to move around. Not all caterpillars have prolegs on segments six to nine, geometer moths for instance, only have one pair of prolegs on segment nine. A Caterpillar never has prolegs on segments four and five, larvae that do are usually sawfly larva. On the last segment an anal proleg (clasper) is present, it provides a very firm grip and also helps moving around. On each segment, the caterpillar has two spiracles, one on the left and one on the right side of each segment, these are openings through which the caterpillar can breathe. The spiracles of many caterpillars can be seen clearly because they often have a different colour than the ground colour of the caterpillar, they look like little mouths and can be opened and closed to control evaporation, oxygen- and carbon dioxide levels.
Many caterpillars have fleshy protrusions on their body, these are called tubercles and can be present in different sizes. Tubercles are often equipped with hairs or spines. For example, caterpillars of some Automeris species have tubercles like tiny Christmas trees, complete with spines that when touched, can cause a burning pain like the sting of a nettle (Beautifull larvae but I prefer the sting of a nettle by far). Caterpillars of the Brahmaea hearseyi have tubercles that grow longer after each moult, after the third moult they look like two inch long corkscrews, after the fourth moult they have completely disappeared. The body of the caterpillar may be hairless but can also be covered with hair, some larvae have so much hair, they look like a brush or tuft of wool.
As you can see, the appearance of a caterpillar may vary enormously from species to species, but that's not all. There are larvae that won’t grow larger than a tenth of an inch, the giants among the caterpillars may reach a length of 7 inches. Also the enormous variety in colour and pattern (sometimes within a single species), creates an endless variety of combinations.
Behaviour and food
Food is cut in small pieces by the jaws, with the maxillae it’s transported to the digestive tract where nutrients and moisture are subtracted, then it leaves the body as hard, brown and relatively large droppings. Caterpillars mainly eat leaves with a few exceptions, some species eat prey, wood, grain or other plant materials (clothes moth). Since leaves generally do not contain many calories, a caterpillar has to eat a lot to grow. Therefore the digestive tract consists of a very large proportion of the total weight of the caterpillar. During his life of a few weeks, a caterpillar manages to grow 1000 to 30,000 times heavier than a newly hatched larva. Species that eat wood like the Goat Moth may take three years to reach final size.
Growth
A caterpillar doesn’t do much more but eating and taking a rest occasionally. This causes a tremendous growth which can’t continue for long, because the caterpillar has an external skeleton and would burst if it keeps eating. For this reason the caterpillar is forced to shed skin once in a while and replace the exoskeleton for a larger version. When a caterpillar reaches its maximum size, it will stop eating for some time and hold still on a branch of leave fixing itself with some silk . Due to the influence of hormones, some things will start changing inside the caterpillar. Under the existing skin, a new soft and wrinkled up skin is created, the old exoskeleton will be dissolved and re-created. The new skeleton is still soft and flexible until it has hardened. After one to three days the skin will tear open just behind his head and the old skin is stripped of, the old and way to small head capsule will fall off as well. After this arduous task, the caterpillar will sit still for a while to fill up its tracheae and trunks with air. Because the new skeleton is soft and flexible, the caterpillar will get considerably thicker in a couple of minutes. After that is done, a period follows which is used to harden the new exoskeleton. Many caterpillars eat their old skin because they contain a high amount of nutrients, other species leave their skin behind for what it is. Between moults, the caterpillars grow primarily in length and won’t get much ticker. The number of cells from which a caterpillar is made, remains the same throughout its life. All cells will grow larger but cell division doesn’t take place during the larval stage. The phases that larvae undergo during development, are expressed in instars. A just hatched larva is a first instar larva and after the first moult, it’s called a second instar larva. This is often expressed as L1, L2 and so on.
The final stage
A caterpillar sheds its skin depending on the species, four to five times. During the last instar, the appetite and growth is enormous. When the last skin is stretched to the max, the caterpillar will be triggered by hormones and stop eating, it’s time to prepare for the final moult. In order to dispose of all waste, it will empty it’s intestines as a slimy blob, his life as glutton has ended. The caterpillar will start looking for a suitable place to pupate, it will sometimes travel considerable distances to find such a place, this is the stage that many caterpillars are found. The place where the caterpillar will pupate varies from species to species. Butterflies pupate hanging from, or against a stem or branch. There are species that just hang upside down fixed with a small tuft of silk but some have an extra strap around their abdomen and tie themselves straight up against a stem. Many moths will spin a cocoon of silk in which they pupate, this is often used in combination with sticks, branches or other materials such as leaves for camouflage and reinforcement. Other caterpillars dig a burrow underground and another group will look for a suitable place among the fallen leaves in the litter layer. After finding a suitable place, the ability to crawl will slowly disappear, the caterpillar changes in appearance and will get shorter and thicker. Much of the moisture that is collected in the larval stage is lost to evaporation, making the nutrients that ware gathered more concentrated. Under his skin a newly formed skeleton will be developed but not like one as during previous moults. Most of the body parts that were useful during the larval stage like muscles of prolegs, clasper and complete digestive tract won’t be needed any more and will be broken down into a nutritious soup. After that, the caterpillar will start its final mould.