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Wednesday, May 2, 2007

Centipede Anatomy

Centipede Anatomy

Exoskeleton

Centipedes, like all arthropods, lack internal skeletons. Instead, they are covered with a hard exoskeleton made of cuticle, which protects the soft internal organs and also serves as an attachment point for the centipede's muscles. Most of the cuticle consists of chitin, which is the same substance found in crab shells.

The arthropod cuticle is constructed in several layers. The inner layer is made up of the epidermis. The cells of the epidermis are alive and secrete the chitin which makes up the outer shell.

Centipedes differ from most arthropods in lacking a waxy outer cuticle. This leaves them vulnerable to water loss and desiccation. Centipedes are thus limited to living in areas with a locally high humidity, such as under rocks or logs. Even in desert areas, centipedes are never found far from moist areas. Centipedes characteristically rest in damp areas that afford them maximum contact with their body surface, a tactic that allows them to conserve moisture. Failure to provide proper humidity and moisture will kill them, and more captive centipedes probably die from being kept in improperly dry conditions than from any other cause.

Like all arthropods, the exoskeleton of a centipede is dead and cannot grow. As the centipede gets larger throughout its life, its cuticle layer becomes tighter and tighter. Periodically, if the centipede is to continue growing, it must shed its exoskeleton and replace it, a process known as ecdysis or molting. When the centipede is ready to molt, a new cuticle layer will grow underneath the old one. During this time, the centipede will stop eating and will retreat to a dark sheltered area for protection. Just before the molt, a thin layer of fluid will form between the old exoskeleton and the new one, loosening it. Using fluid pressure to swell up its body, the centipede splits the old exoskeleton open between the first and second tergites, and climbs out, pulling each leg from its old shell and compressing the old skin like an accordion. The shed skin is a precise replica of the centipede---all of the legs, mouthparts and even the lining of the genital openings -- are shed in one piece. Once the shed is complete, the centipede will remain still for a short time while the new exoskeleton expands, dries and hardens. After this process is finished, the centipede will usually eat the shed exoskeleton.

Young centipedes which are growing rapidly will shed more often than adults who have reached maturity.

Segments

A centipede's body is divided into a series of segments, each of which is virtually identical to the one before it. The first six segments are fused together to form the head, and the mouthparts are formed from the modified legs that once belonged to those segments. In the scolopendromorphs, each body segment contains a pair of walking legs, one on each side. The number of body segments is the same as the number of pairs of walking legs.

The segments are flattened horizontally, giving the centipede a ribbonlike appearence. This allows the animal to wedge itself under rocks and logs in search of prey, and also to spend the day in tightly enclosed areas with locally high humidity. Each segment is protected on the top by a hard plate called a tergite, and another hard plate below, called a sternite. The tergites and sternites are held together by a flexible pleural membrane.

Legs

The number of legs in centipedes ranges from 15 or so to over 150, but all of the scolopendromorphs have either 21 or 23 pairs. Each pair of legs is connected to the ones near it by a band of pliable connective tissue, which helps insure that the legs will not become entangled with each other. In most species, each pair of legs is a little bit longer than the pair immediately in front of it. The legs on each segment move opposite to each other--the leg on one side moving forward while the leg on the opposite side is moving backwards. Some centipedes are capable of running at speeds of almost two feet per second.

Most species of centipede have only one claw at the tip of their feet, and run and walk on tiptoe. The house centipede has a lengthened and flattened foot, which allows it to run faster than most species.

Several species of centipede are capable of dropping some of their legs when threatened by a predator, a tactic known as "autotomy". The detached legs continue to move and thus distract the predator's attention while the centipede escapes. A few species can rub their last pair of legs together to produce a warning noise. Some others can use their rear legs as pincers to deliver painful strikes to attackers.

Legs that are lost can be regenerated and replaced at the next molt.

Trachea

Centipedes breathe through a series of tracheal tubes, in a manner somewhat similar to that used by insects. Each segment of the centipede's body has a network of finely branching tracheal tubes inside it, which carries air inside and allows oxygen to diffuse directly to the body tissues. The tracheal tubes exit the body through a hole known as the spiracle, located on the side of the body segment in the pleural membrane, near the base of the legs. In some groups, each body segment contains spiracles. In the larger scolopendromorphs, spiracles are found on segments 4, 6, 8, 11, 13, 15, 17, 19, and 21. Unlike the insects, centipedes are not capable of closing their spiracles, and these openings are a major source of moisture loss for the animal.

In some species of house centipedes, tracheal tubes surround the heart in a dense network, allowing some oxygen to diffuse directly into the blood before it is pumped throughout the body. In other centipedes, the blood carries only nutrients and does not transport oxygen to the body cells.

Breathing doesn't require any muscular action on the part of the centipede. Instead, the air flows directly through the trachea into the body tissues, where gas exchange takes place. Since air cannot diffuse very far, the distance between the interior of the body and the tracheal opening cannot be very far--the long thin body shape of the centipede is adapted to allow the maximum amount of oxygen to diffuse into tissues as efficiently as possible.

Because this respiratory system is rather inefficient, however, the body tissues cannot be rapidly and consistently supplied with oxygen. When the centipede is very active and using up energy, such as when running away from a predator, the oxygen reserves are used up quickly and cannot be rapidly replaced. Thus, centipedes are capable of only short bursts of intense activity before they tire, and must stop and rest for a time to reoxygenate their body tissues.

Heart

The centipede's heart is a long hollow tube that runs along the length of the back, just beneath the cuticle. Like most arthropods, the centipede has an "open" circulatory system, in which the blood is not confined to vessels. Instead, it fills the entire body cavity and covers and surrounds all of the internal organs. Contractions of the heart cause the blood to swish around inside the body cavity, moving nutrients from one end to the other and carrying wastes away from the body cells.

A cut or opening in the body wall is a serious matter for a centipede, since it may allow a significant amount of blood to pour out of the body before the clotting agents can stop it, causing the centipede to bleed to death.

Eyes

All of us are familiar with the large multifaceted eyes of insects such as houseflies and dragonflies. These large compound eyes are actually a gathering of many hundred smaller units, each with its own lens and optic nerve. The image formed by these eyes consists of a "mosaic", a gathering of multicolored dots that forms a color image. The larger the number of facets in the compound eye, the more sharp the image that is formed will be. Visual hunters like dragonflies can have several thousand facets in their compound eyes, giving them very sharp images and excellent vision.

Centipedes differ from insects in having only simple eyes, not compound. The simple eyes, known as occeli, are clustered in two groups on the head, however, making them appear as if they are compound eyes. In simple eyes, there is only one lens structure, and the image is not as clear or sharp, nor is it as sensitive to movement. Their poor vision does not present many problems for the centipede, however, as nearly all are nocturnal and are active during the night, where they are far more dependent on sensing vibrations and air currents than on visual cues. Many centipedes, including all of the geophilomorphs, do not have eyes at all and are completely blind. The scutigeromorphs, by contrast, have large clumps of occeli called "Pseudocompound eyes", which form good images and allow these animals to hunt their prey visually, unlike most centipedes.

Centipedes receive most of the information they get about their surroundings from their antennae, which are used for sensing chemical information, rather than their eyes.

Fangs

All centipedes are predators. The first body segment contains a pair of clawlike appendages called maxillipeds, consisting of a modified pair of legs which are connected to venom glands inside the body. The sharp tips of the claws are used to penetrate the exoskeleton of a prey insect, and muscles in the centipede's body squeeze the venom glands to force poison through the hollow fangs into the victim's body. These claws can be used to kill prey ranging in size from insects and other arthropods to small vertebrates such as rodents or lizards. Although the venom is not particularly powerful in most species, punctures from a poison claw can be painful to humans, and some of the larger tropical centipedes can produce systemic effects and are potentially dangerous. There is one report in the literature of the death of a seven year old girl from the bite of a large tropical centipede in the Phillipine Islands (probably the species S. subspinipes).

Repugnatory Glands

Some centipedes have a series of glands running along the body known as repugnatory glands. These are defensive in nature. If threatened, the centipede can secrete a noxious chemical from these glands, which then runs down the legs. It has been reported that, when some centipedes are handled, the sharp claws on the feet can puncture the skin of the handler and allow the secretions from the repugnatory glands to enter, causing a trail of inflamed and painful eruptions. The scolopendromorph centipedes are believed to lack repugnatory glands.

1 comment:

Gururaj M S said...

thank you for your information. I live in mysore ,south india, in my backyard there is a plenty of centipedes appears. Coz my neighbours kept many plywoods. So i have seen upto 5 inches long centipede. If i get bit, can it be painfull? If yes can i use boric poweder to kill them? Plz let me know.