|Classes Diplopoda and Chilopoda -|
Millipedes and Centipedes.
Thousand and hundred-leggers, respectively.
Not many of either are dangerous.
Some exotic, large centipedes (not this one) will bite defensively, and should not be handled.
The house centipede pictured here is considered harmless to humans or their pets. Size: Body = 25mm, overall = 7cm.
Centipedes are some of the oldest terrestrial animals, and some of the very first creatures to crawl from the sea onto the land were probably very similar in appearance to modern centipedes. All centipedes are nocturnal predators which live by actively hunting down insects and other small animals. They are found mostly in tropical forests, but have also established themselves in temperate forests, deserts, and human habitations. Commonly called "hundred-leggers", most centipedes have between 15 and 30 pairs of legs, one pair on each body segment.Some of the larger centipedes can live longer than 10 years. 
Centipedes and millipedes most often live outdoors in moist places such as leaf litter, or under rocks and decaying wood. The house centipede, however, lives indoors, especially in damp basements, bathrooms, or crawl and other excavated spaces under a house. They can move very quickly on their many legs, but they cause no damage to structures or their contents. They do not infest food or "eat books" as is commonly believed.
Centipedes are long-lived as arthropods go. House centipedes live, mate, and lay eggs in dark, moist cracks and crevices. Eggs hatch into larvae with four pairs of legs. Each larva molts 5 or more times, with the number of legs increasing with each. Fully mature adults have 15 pairs.
Unlike most other centipedes, house centipedes and their close relatives have well-developed compound eyes which are sensitive to ultraviolet as well as visible light. S. coleoptrata has developed automimicry in that its hind legs present the appearance of antennae, and there are false eyes on its rump. House centipedes feed on spiders, bedbugs, termites, cockroaches, silverfish, ants and other household arthropods. They administer venom through legs modified as fangs. 
Figure 1. Head, Antennae & Tergites Detail: Well-developed compound eyes are sensitive to ultraviolet as well as visible light 
Some of the plates (tergites) covering the body segments fused and became smaller during the evolution to S. coleoptrata's current state. The resulting mismatch between body segments and tergites is the cause for this centipede's relatively inflexible body.  The two tergites just behind this centipede's head are fused. The 2 evolved, modified legs just under the antennae function as poison fangs, or forcipules.
Stone Centipede in the Order Lithobiomorpha. Size: 20mm
|Class Diplopoda - Millipedes|
Millipedes do not bite, pinch or sting, but may emit foul-smelling or irritating defensive chemicals. Live, adult millipedes photographed at northern Illinois, USA.
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|Los ciempiés son artrópodos de la clase Diplopoda, que contiene aproximadamente 10.000 especies en 13 órdenes y 115 familias. . DIPLOPODI tiene dos pares de patas por segmento (excepto el primer segmento).|
Millipedes are arthropods in the class Diplopoda, which contains approximately 10,000 species in 13 orders and 115 families. . Diplopods have two pairs of legs per segment (except for the first segment behind the head). Each segment that has two pairs of legs is a result of two single segments fused together. Most millipedes have very elongated cylindrical bodies, although some are flattened dorso-ventrally, while pill millipedes are shorter and can roll into a ball like a pillbug.
Most millipedes slow moving detritivores, that is, they eat stuff that's just lying around - decaying leaves and other dead plant matter. However, some species eat live plants. Moisturizing their food with digestive secretions and then scraping it into their mouth with the jaws, they can be an agricultural pest, especially in greenhouses where they cause damage to seedlings.
Millipedes can be distinguished from the somewhat similar and related centipedes (Class Chilopoda) which have a single pair of legs for each body segment. Chilopods are active, carnivorous predators, in contrast to the slower, docile millipede.
A millipede's head is typically rounded above and flattened below and bears large mandibles. The body is flattened or cylindrical, with a single chitinous plate above, one at each side, and two or three on the underside. In many millipedes, these plates are fused to varying degrees, sometimes forming a single cylindrical ring.
Like all arthropods, millipedes lack an internal skeleton, and instead are protected by a tough outer shell called an exoskeleton or cuticla, 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 impact. 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. 
Female Millipede, Parajulidae sp.
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