Subphylum Myriapoda

SUBPHYLUM MYRIAPODA

The term “myriapod,” meaning “many footed,” describes members of four classes in subphylum Myriapoda that have evolved a pattern of two tagmata—head and trunk—with paired append ages on most or all trunk segments. Myriapods include Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda (pauropods), and Symphyla (symphylans). Myriapods use tracheae to carry respiratory gases directly to and from all body cells in a manner similar to onychophorans and some arachnids, but tracheal systems have likely evolved independently in each group. Excretion is usually by Malpighian tubules, but these have evolved independently of Malpighian tubules found in Chelicerata.

Class Chilopoda

Chilopoda (ki-lop +o-da) (Gr. cheilos, margin, lip, + pous, podos, foot), or centipedes, are land forms with somewhat flattened bodies. Centipedes prefer moist places such as under logs, bark, and stones. They are very agile carnivores, living on cockroaches and other insects, and earthworms. They kill their prey with their venom claws and then chew it with their mandibles. The largest centipede in the world, Scolopendra gigantea, is nearly 30 cm in length. Common house centipedes Scutigera (L. scutum, shield,+ gera, bearing), which have 15 pairs of legs, are much smaller and often seen scurrying around bathrooms and damp cellars, where they catch insects. Most species of centipedes are harmless to humans, although many tropical centipedes are dangerous. There are about 3,000 species worldwide.

Centipede bodies may contain from a few to 177 segments. Each segment, except the one behind the head and the last two in the body, bears a pair of jointed legs. Appendages of the first body segment are modified to form venom claws. The last pair of legs is longer than the others and serves a sensory function. The head appendages are similar to those of an insect. There are a pair of antennae, a pair of mandibles, and one or two pairs of maxillae. A pair of eyes on the dorsal side of the head consists of groups of ocelli.

The digestive system is a straight tube into which salivary glands empty at the anterior end. Two pairs of Malpighian tubules empty into the hind part of the intestine. There is an elongated heart with a pair of arteries to each segment. The heart has a series of ostia to provide for return of blood to the heart from the hemocoel. Respiration is by means of a tracheal system of branched air tubes that come from a pair of spiracles in each segment. The nervous system is typically arthropodan, and there is also a visceral nervous system. Sexes are separate, with unpaired gonads and paired ducts. Some centipedes lay eggs and others are viviparous. The young are similar in form to adults and do not undergo metamorphosis.

Class Diplopoda

Diplopoda (Gr. diploo, double, two, + pous, podos, foot) are commonly called millipedes, which literally means “thousand feet”. Millipedes are not as active as centipedes: They walk with a slow, graceful motion, not wriggling as centipedes do. They prefer dark, moist places under logs or stones. Most millipedes are herbivorous, feeding on decayed plant matter, although sometimes they eat living plants. Millipedes are slow-moving animals and may roll into a coil when disturbed. Many millipedes also protect themselves from predation by secreting toxic or repellent fluids from special glands (repugnatorial glands) positioned along the sides of the body. Common examples of this class are Spirobolus and Julus, both of which have wide distribution. There are more than 10,000 species of millipedes worldwide.

The cylindrical body of a millipede is formed by 25 to more than 100 segments. Their short thorax consists of four segments, each bearing one pair of legs. Each abdominal segment has two pairs of legs, leading to the impression of a thousand feet. The millipede exoskeleton is reinforced with calcium carbonate.

Their head bears two clumps of simple eyes and a pair each of antennae, mandibles, and maxillae. The general body structures are similar to those of centipedes. Two pairs of spiracles on each abdominal segment open into air chambers that connect to tracheal air tubes. There are two genital apertures toward the anterior end.

In most millipedes the appendages of the seventh segment are specialized as copulatory organs. After millipedes copulate, females lay eggs in a nest and guard them carefully. Interestingly, larval forms have only one pair of legs to each segment.

Class Pauropoda

Pauropoda (Gr. pauros, small, + pous, podos, foot) are a group of minute (2 mm or less), soft-bodied myriapods, numbering almost 500 species. Although widely distributed, pauropods are the least well known myriapods. They live in moist soil, leaf litter, or decaying vegetation and under bark and debris. Representative genera are Pauropus and Allopauropus. Pauropods have a small head with branched antennae and no true eyes, but they have a pair of sense organs that resemble eyes. Their 12 trunk segments usually bear nine pairs of legs (none on the first or the last two segments). They have only one tergal (dorsal) plate covering each two segments. Tracheae, spiracles, and circulatory system are lacking. Pauropods are probably most closely related to diplopods.

Class Symphyla

Symphyla (Gr. sym, together, + phylon, tribe) are small (2 to 10 mm) and have centipede-like bodies. They live in humus, leaf mold, and debris. Scutigerella (L. dim. of Scutigera) are often pests on vegetables and flowers, particularly in greenhouses. They are soft bodied, with 14 segments, 12 of which bear legs and one a pair of spinnerets. The antennae are long and unbranched.

The mating behavior of Scutigerella is unusual. Males place a spermatophore at the end of a stalk. When a female finds it, she takes it into her mouth, storing sperm in special buccal pouches. Then she removes eggs from her gonopore with her mouth and attaches them to moss or lichen, or to walls of crevices, smearing them during handling with some of the semen, thereby fertilizing them. Young at first have only six or seven pairs of legs and development is direct.  Symphylans are eyeless but have sensory pits at the bases of the antennae. Their tracheal system is limited to a pair of spiracles on their head and tracheal tubes to anterior segments only. Only 160 species are described.

---

Useful External Links

• Myriapods, the Many-Legged Arthropods by Laura Klappenbach
• Myriapods by Paul E. Marek
• Subphylum Myriapoda, Class Diplopoda by Jean-Jacques Geoffroy