19 Arthropods - Myriapoda

Myriapoda Overview

General Characteristics
- Contains ~16,000 species, primarily terrestrial.
- Comprises several subgroups:
- Hexapoda (insects, springtails, diplurans, and proturans)
- Crustacea (includes crabs, lobsters, shrimp, isopods, water fleas, barnacles, etc.)
- Pauropoda
- Diplopoda (millipedes)
- Chilopoda (centipedes)
- Symphyla
- Arachnida (spiders, mites, ticks, scorpions, etc.)
- Chelicerata which includes Eurypterida (sea scorpions) and Xiphosura (horseshoe crabs)
- Pycnogonida (sea spiders)
- Trilobites (extinct group)

Body Structure
- Characterized by a single pair of antennae.
- Mouth located under the head.
- Possesses a large number of repeating body segments (metameric).
- Each segment bears one or two pairs of legs.
- Appendages are all uniramous, meaning they are not branched.

Respiration and Circulation
- Myriapoda utilize spiracles that connect to a tracheal system for respiration.
- Have a complete digestive system.
- Circulatory system is poorly developed.
Reproductive System
- Two distinct sexes (dioecious).
- Fertilization may be internal or through the use of a spermatophore.
- Egg development involves considerable maternal care. Juveniles may have fewer segments than adults.

General Information
- Characterized by two pairs of legs on each body segment.
- Generally terrestrial with a total of approximately ~12,000 species, making them the largest group within Myriapoda.
- Notably, no species has been recorded with 1,000 legs.
Physical and Behavioral Characteristics
- Many body segments are fused into pairs known as diplosegments, giving the appearance of having two pairs of legs.
- The last segment of the body is called the telson.
- Millipedes have a cluster of ocelli, however, these do not constitute a compound eye.
- Antennae are typically short.
- Some species are chemically defended and produce toxic compounds for protection (examples include hydrogen cyanide and phenol).
Defensive Mechanisms
- Structural formulas for defensive compounds include:
- A. Hydrogen cyanide
- B. Trans-2-dodecanol
- C. Benzaldehyde
- D. Mandelonitrile
- E. Benzoyl cyanide
- F. Phenol
- G. o-cresol
- H. p-cresol
- I. Guaiacol
- J. 1,4-benzoquinone
- K. Glomerin
- L. Homoglomerin
- M. Polyzonimine
- N. Nitropolyzonamine
Notable Species
- A female Illacme plenipes has been recorded with 618 legs.
- The fossil Pneumodesmus newmani is the oldest known land animal from the Devonian period.
- Arthropleura, an extinct genus of millipedes, represents the largest known land invertebrates growing up to 2.5 m long during a period with high atmospheric O₂ content.
- The Giant African millipede can grow up to ~40 cm in length.

General Information
- Comprises approximately ~3,000 species, all terrestrial.
- Characterized by one pair of legs per body segment.
- Notably, there are no centipedes with 100 legs; the species Orya barbarica serves as a reference example.
Morphology and Reproduction
- Centipedes have a distinctive structure known as forcipules, which are modified legs used to capture prey.
- The final segment is also termed a telson.
- Centipedes engage in reproduction where males deposit a spermatophore and females lay eggs.
- Some species exhibit a growth pattern where the number of legs increases with age, whereas others are born with all their legs (e.g., house centipedes are born with 4 pairs of legs).
Example Species
- Scolopendra gigantea is a notable species known for its predatory behavior.

Role in Ecosystems
- Both myriapods and their respective groups play important roles in ecosystems:
- Millipedes are crucial detritivores, contributing to soil health and nutrient cycling.
- Centipedes serve as important predators in their habitats.
Human Interactions
- Myriapods may act as minor garden pests, but they also serve as interesting pets for enthusiasts,
- Understanding both groups can have implications for pest management and ecological studies.