Segmented body is a defining feature of arthropods and is closely tied to the use of jointed appendages.
The study of segmentation helps explain the diversity of arthropods and how they accomplish different ecological tasks using different segments.
What is a segment?
A segment is one of a series of serially repeating body units.
Segment repetition and variation across taxa illustrate how segmentation can take many forms (febrization).
The hypothetical ancestral arthropod likely looked like a velvet worm (Onychophora): similar segmentation with appendages and a body plan that could have given rise to arthropods.
Almost all segments have associated structures (e.g., a small heart, a nerve cord), and often a small amount of gut and other tissues distributed along the body.
Velvet worms (Onychophora) exhibit a form of segmentation: each segment bears a nearly identical pair of appendages, except the head (antennae).
This shews why some researchers connect onychophorans with the ancestral arthropod state.
Segmentation across the animal kingdom
Arthropods show the greatest elaboration of segmentation, but other segmented groups exist: annelids (earthworms, leeches, polychaetes) are clearly segmented.
Vertebrates are also segmented during development (somites) even though adult vertebrates do not show segmentation in the same way (e.g., heart location).
Early development (e.g., frog embryos) reveals somites: mesodermal blocks that become the spine, ribs, and associated tissues.
Segmentation appears to be a case of convergent evolution; it has arisen independently in multiple lineages rather than evolving once from a common segmented ancestor.
Some phyla lack segmentation entirely (e.g., mollusks, Platyhelminthes).
Velvet worms (Onychophora) and their biology
Onychophorans are commonly known as velvet worms.
They are not arthropods, but they are crucial for understanding the hypothetical ancestral condition for arthropods.
Biology includes: open circulatory system, tracheal respiration, damp-habitat preference (tropical rainforests in regions like Costa Rica, Africa, Southeast Asia).
Predatory lifestyle; mouth contains salivary glands that eject glue to immobilize prey; prey can be smaller than the velvet worm.
In talking about the ancestry, the velvet worm-like morphology is used to describe the hypothetical ancestor to all arthropods.
Ancestral state and phylogeny
The common ancestor of arthropods is hypothesized to resemble onychophorans (segmented, paired fleshy appendages, no hardened exoskeleton).
A simplified phylogeny (not exhaustive) shows a branching of Arthropoda, Onychophora, and Tardigrades as separate lineages.
In arthropods, the segments become hardened and specialized (typhosis/tagmosis) into functional units.
Tagmosis (typhosis) and the evolution of body regions
Tagmosis is the evolutionary process by which adjacent segments become functional units (head, thorax, abdomen in many arthropods).
In the hypothetical ancestral arthropod, the head, thorax, and abdomen emerged as distinct functional regions through segment specialization.
In spiders, the head is fused with the thorax (cephalothorax) and wings are absent; in other groups, segments have diversified markedly.
Insects show three main tagmata: head, thorax, abdomen.
Insect head: formed from ancestrally six fused segments; many appendages were lost or repurposed into mouthparts or sensory structures (protocephalon development).
Palps on the mouthparts (labial and maxillary palps) are derived from ancestral appendages; mandibles and maxillae are key mouthparts.
Evolution of insect thorax: first segment (prothorax) bears legs but usually no wings; mesothorax and metathorax bear wings and legs; wings are largely a feature of the thorax.
Abdomen contains digestive and reproductive organs; variation in the number of abdominal segments among insects relates to genitalia arrangement.
Functional roles of the three regions in insects: head for sensing and ingestion, thorax for movement, abdomen for digestion and reproduction.
Telescoping terminology for head orientation (nathus terms): prognathous (mouthparts forward), hypognathous (mouthparts downward), epognathous (mouthparts projecting backward).
Mouthparts can be classified by their visibility and positioning: endognathous (internal mouthparts hidden) vs ectognathous (external mouthparts visible).
Entognatha vs Ectognatha: Entognathous insects (e.g., springtails) have mouthparts folded inside the head; most other hexapods are ectognathous.
Tagmosis and mouthparts are sometimes used in keys to identify taxa; many variations exist across orders.
Exoskeleton: structure and composition
Insects possess an exoskeleton that is largely made of chitin (a polymer of
N-acetylglucosamine) that forms a β-linked structure: eta ext{-linkage} with proteins and water.
Chitin is a polysaccharide with amine groups; it can take many forms depending on its interaction with proteins and water content.
Variation in cuticle rigidity comes from the ratio of chitin