Classes Diplopoda (Millipedes) and Chilopoda (Centipedes)
Millipedes are grouped together with the centipedes into the subphylum Myriapoda.
Millipedes do not bite, pinch or sting, but some emit foul-smelling or irritating defensive chemicals. Some exotic, large centipedes will bite defensively, and should not be handled.  [4]
The house centipede pictured here is considered harmless to humans or their pets. [2]
Live adult centipedes and millipedes photographed at northern Illinois, USA.

House Centipede
House Centipede, Scutigera coleoptrata [1]

The 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 as well as deserts. 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 have life spans over 10 years. [4]

Centipedes and millipedes normally live outdoors, in moist places such as leaf litter, or under rocks and decaying wood. The house centipede, however, also 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. [2]

Figure 1. Head and Antennae Detail
Figure 1. Head, Antennae & Tergites Detail: Well-developed compound eyes are sensitive to ultraviolet as well as visible light [2]

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. [4] 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.

House Centipede
This centipede is missing one of its back legs. Size: Body = 25mm, overall = 7cm

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.
 

Millipede, Parajulidae sp.
Female Millipede, Parajulidae sp.

Millipedes are arthropods in the class Diplopoda. Diplopods have two pairs of legs per segment (except for the first segment behind the head which does not have any appendages at all, and the next few which only have one pair of legs). Each segment that has two pairs of legs is a result of two single segments fused together as one. 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.

Millipedes are detritivores and slow moving. Most millipedes eat decaying leaves and other dead plant matter, moisturizing the food with secretions and then scraping it in with the jaws. They can also be a minor garden pest, especially in greenhouses, where they can cause severe damage to emergent seedlings. Signs of millipede damage include the stripping of the outer layers of a young plant stem, and irregular damage to leaves and plant apices.

This class contains approximately 10,000 species. There are 13 orders and 115 families. [3]

Millipedes can be distinguished from the somewhat similar and related centipedes (Class Chilopoda), which move rapidly, and have a single pair of legs for each body segment. Chilopods are active, carnivorous predators, in contrast to the slow-moving, docile millipede. I might add: When you are trying to take their picture, some millipedes can move pretty fast.

Female Millipede, Parajulidae sp.

Female Millipede, Head Detail
Millipede's antennae are short, clavate

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 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. [4]

Each segment bears two pairs of legs, rather than just one, as in centipedes (Chilopoda). This is because each is actually formed by the fusion of two embryonic segments, and is therefore properly referred to as a "diplosegment", or double segment. The first few segments behind the head are not fused in this fashion, and the first segment is legless, while the second to fourth have one pair each. In some millipedes, the last few segments may also be legless. The final segment bears a telson. [3]

Female Millipede, Head Detail
Figure 2. Two pairs of legs per segment are easily visible. Millipedes have very flexible bodies and curl like a watch spring in defense.

References
  1. Bugguide.net, Scutigera coleoptrata
  2. William F. Lyon, Ohio State University Extension Fact Sheet HYG-2067-94
  3. Wikipedia, The Free Encyclopedia, Millipede
  4. Lenny Flank, Lenny Flank's Herp Page, Keeping and Raising Millipedes and Centipedes
 
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