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Wednesday, May 2, 2007

Millipede Anatomy

Millipede Anatomy

Exoskeleton

Like all arthropods, millipedes lack an internal skeleton, and instead are protected by a tough outer shell called an exoskeleton or cuticle, made of 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. The middle layer of the exoskeleton is made up of soft, flexible chitin which provides an elasticity, allowing the millipede to absorb impacts. The outer layer is also made of chitin, but it has been stiffened and hardened by the addition of calcium carbonate crystals--the same substance that makes up clamshells -- to form a durable exterior. Millipede exoskeletons are so tough that they can support up to 25,000 times the millipede's own weight before being crushed.

The exoskeleton is dead and cannot grow. As the millipede gets larger, it must periodically shed its skin and grow a new one. When the old exoskeleton gets too tight, the millipede will secrete a new one underneath it. It will then split the old cuticle open and pull itself out, carefully extracting all of the legs, antennae and mouthparts. The newly emerged cuticle is soft and flexible, and takes several hours to dry and harden. During this stage the millipede is extremely vulnerable to predators, and it will usually seek out a secure hiding spot to carry out the molting process. Some species dig themselves a special molting chamber in the soil.

After the new exoskeleton has completely hardened, the millipede will usually eat the old cuticle to recycle the nutrients and calcium it contains. The large millipedes found in the pet trade often have tiny light-colored mites living on their exoskeletons. These do not appear to cause any harm, and it is generally believed that they help the millipede by eating and removing wastes and detritus that cling to the exoskeleton.

Segments

Like centipedes, the millipede's body is divided into a long series of segments, each more or less identical to the one before it. Each segment is actually formed by the fusion of two sections during embryonic development, and thus each millipede segment has two pairs of legs, as well as armored plates above (known as tergites) and below (known as sternites). The tergites and sternites are held together by a tough flexible pleural membrane.

Each segment also contains a spiracle, four on each segment in the sternite near the leg bases, through which the millipede breathes. Millipedes do not have lungs and do not breathe through their mouth like vertebrates. Instead, air is carried directly into the body through a finely branching system of tubes known as trachea, and these open to the outside through the spiracles. The fine network of trachea allows oxygen to diffuse directly into the body tissues and carbon dioxide to diffuse out through the spiracles. This arrangement is inefficient, and because millipedes cannot oxygenate their body tissues rapidly they tire very easily, but this is not a disadvantage since they move slowly all the time anyway, and depend on their protective cuticle and metathoracic glands for defense rather than speed.

Young millipedes typically have far fewer segments than adults of the same species (most species have six segments and three pairs of legs when they hatch). With each successive molt, three or four new segments are added until the young millipede reaches adult size. Large "giant millipedes" may have over 100 body segments as adults.

Legs

Although a millipede may have as many as 400 legs, they are not built for speed. The legs are very short and have a limited mobility. Rather than being predatory sprinters like centipedes, millipedes are designed for pushing their way through soil and leaf litter in search of rotting vegetation and other sources of food.

Most of the segments on the millipede's body have two pairs of legs. The only exceptions are the three segments immediately behind the head, known collectively as the millipede's "thorax". These have only one pair of legs each, but they contain the millipede's sexual organs. In males, the rear segment of the thoracic region contains a pair of sexual organs, which are used to transfer sperm from the testes to a small gelatinous packet called a spermatophore. During breeding, this packet is transferred to a pair of highly modified legs on the male's seventh body segment, called gonopods, which are then used to place the spermatophore into the female's genital opening.

During walking, each pair of legs on a segment is lifted at the same time. The pairs move oppositely to each other -- the pair of legs on the left side moving forward while those on the right side move backward. The millipede walks by using successive waves of motion along its body. You will easily be able to see this "rippling" action as the millipede moves along.

Eyes

Millipedes do not have the large compound eyes found in insects. Instead, they have "simple" eyes, known scientifically as ocelli, that can sense light and dark but are not very good at forming images. As a result, the eyesight of millipedes is very poor, and they are practically blind. Since nearly all millipedes are nocturnal, though, lack of good vision is not a problem for them. They get most of their information concerning their surroundings from their antennae, which are loaded with chemical sensors. Millipedes are also very sensitive to vibrations that they pick up through the ground.

Metathoracic glands

Most species of millipede have a number of glands running along the sides of the body which produce noxious chemicals to drive away predators. In some species, these metathoracic glands produce a tiny cloud of hydrogen cyanide gas, which to humans has a pleasant almond odor, but which has a deterring (and potentially lethal) effect on such millipede predators as beetles and shrews. In other species, the chemical arsenal may include quinones, hydrochloric acid, aldehydes, toluquinone, benzaldehyde or benzoquinones. These chemical secretions are usually harmless to humans, although some millipedes can produce a blue or purple stain on human skin. Millipedes of the genera Rhinocricus and Spirostreptus, however, release stronger chemicals that can produce blisters on some people. The secretions of most millipede metathoracic glands can cause painful burning and itching if they get into the eyes, nose or an open cut.

In addition to their metathoracic glands, a few millipedes produce chemicals in their cuticle that glow in the dark, a phenomenon known as "bioluminescence". In the United States, the genus Motyxia, which is found in California, is bioluminescent. Since millipedes are virtually blind and have no use for light while they are active, it is assumed that this glow serves as an anti-predator mechanism, perhaps giving a visual warning that the millipede, with its chemical arsenal, is distasteful to predators.

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