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The second major group of protostome animals is the Ecdysozoa (Fig. 44.20), which get their name from the process of ecdysis, or molting. Uniquely among animals, all ecdysozoans secrete a protein- or polysaccharide-rich cuticle that covers their bodies. This tough cuticle is like a suit of flexible, lightweight armor, protecting bodies from injury and physical challenges of the environment such as drying. This “armor” has a special property: The hard cuticle can be used to form appendages that function as tools, weapons, or even wings. Like a suit of armor, the cuticle is always a perfect fit for the wearer but does not stretch, and so it must be exchanged episodically during growth for a larger size to fit a larger, growing body. Ecdysozoans, then, are animals that molt their external cuticle during growth, producing a very soft, larger replacement underneath that soon hardens into a new protective covering.

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The Ecdysozoa include eight phyla, but two dwarf the others in abundance and diversity. The most numerous of all animals are the small roundworms known as nematodes (Fig. 44.21). Most nematodes are tiny and so you’ve probably never noticed them, but they occur by the billions all around you and may well be inside your body as you read this. About 20,000 nematode species have been described, but estimates of their true diversity run as high as half a million. Most are microscopic, preying on microorganisms in soils and marine sediments. Their elongate cylindrical bodies have a mouth at one end, an intestine for digestion, and an anus in the rear. Other organs are simple: Nerve cords extend along the main axis from a ring in back of the mouth, rudimentary muscles line the body, and gonads expel sperm and eggs for reproduction. Nematodes are the main pseudocoelomate phylum, their digestive and reproductive organs surrounded by a fluid-filled cavity. As in other ecdysozoans, the body is covered by a tough cuticle that molts during growth. Parasitic nematodes are common and well known to doctors; hookworms in particular are major sources of illness in the tropics.

If nematodes are the most numerous animals, the Arthropoda, including insects, are the most diverse, accounting for more than half of all known animal species. What features have led to the remarkable evolutionary success of this branch?

Perhaps the most obvious feature of arthropods is the one for which they are named: jointed legs (arthro means “jointed,” and pod means “foot” or “leg”). The jointed leg is the most versatile part of the arthropod body; it has been modified through evolution into structures that function as paddles, spears, stilts, pincers, needles, hammers, and more. The jointed leg allowed arthropods to colonize land by providing a multipoint, independent suspension—much like that of an all-terrain vehicle—able to support a relatively large body and move over the most uneven ground. So well adapted for locomotion is the arthropod leg that NASA engineers are using it as a model for planetary exploration vehicles. Researchers have conducted many studies to understand how this one type of limb could give rise to so many diverse structures with different functions (Fig. 44.22).

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The arthropods’ other defining characteristic is the material that forms their hard external skeleton: a strong, lightweight and nearly indestructible polysaccharide called chitin. Chitin, you may recall from Chapter 34, also forms the wall of fungal cells. Next to the cellulose that forms the wall of plant cells, chitin may be the most abundant biomolecule on Earth.

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There are four main groups of arthropods (Fig. 44.23), of which the most diverse by far are the insects. The other three are chelicerates, which include spiders, scorpions, and their relatives; myriapods, which include centipedes and millipedes; and crustaceans, which include lobsters, shrimp, crabs, and their relatives.

Chelicerates, named for the pincerlike claws called chelicerae, are the only arthropods that lack antennae. The chelicerates are mostly carnivores, except for plant-eating mites. Scorpions and spiders are known for their venoms, used to subdue prey. In spiders, the venom is injected through chelicerae modified to form fanglike appendages, while in scorpions the venom is injected through the curved tail segment of their abdomen. Other familiar chelicerates include the harvestmen (often called daddy long-legs in the United States), ticks, and horseshoe crabs. Giant sea scorpions called eurypterids, up to 2 m long, were dominant predators in warm shallow seas 450–250 million years ago. The chelicerates were among the very first animals to invade land, about 400 million years ago, aided by their protective cuticle and supported by their arrays of jointed legs.

Myriapods are named for their many pairs of legs, which make them easy to recognize. Centipedes (“hundred legs”) may have up to 300 pairs of legs and are fast-moving predators with front legs modified into venomous, fanglike organs. Millipedes (“thousand legs”) may have several hundred pairs of legs (though never a thousand) and are slow-moving herbivores and detritivores (eaters of dead organisms). Millipedes often have glands lining their long bodies that produce a substance that contains cyanide, which they secrete to defend against predators.

Crustaceans also have distinctive appendages, notably their branched legs. They navigate their world with the aid of two pairs of highly sensory antennae, the larger of which has two branches. The unique crustacean larva, called a nauplius, swims with the aid of the smaller antenna and has a single eye that typically disappears in the adults. In the sea, these versatile arthropods fill many of the ecological roles that insects play on land, eating plants, other animals, or detritus. The tiny shrimplike plankton known as krill occur in vast numbers in polar seas, supporting ecosystems of fish, birds, and mammals such as whales. One Antarctic krill species is estimated to total over 500 million tons of biomass, more than the global total for humans. Lobsters and crabs are largely detritivores, scouring the ocean bottoms for dead and dying animals. Some crustaceans are even parasites, attaching themselves to the skin or inside the mouths of fish or sea mammals in order to feed. The strange creatures known as barnacles begin life resembling shrimp but soon settle on a surface for a life of filter-feeding. As the barnacles mature, their bodies become covered with thickened plates of armor and their legs become long and develop long brushes that sweep food into their mouths.

Insects were the first animals—and the only arthropods—to evolve wings, nearly 350 million years ago, and modern-day mayflies and dragonflies still bear structural similarities to these first fliers. The earliest-branching members of this group, the silverfish and rockhoppers, are referred to as primitively wingless because they evolved before the origin of wings. Some other insects, like fleas, have lost wings that were present in their common ancestor, presumably because their parasitic lifestyles are better served by jumping than flying. Like all arthropods, insects repeatedly shed their exoskeleton as they grow into adulthood. No insect can molt wings, however, and so only adults can fly.

Within the insects, the main evolutionary dividing line is between those that undergo metamorphosis, a major change in form from one developmental stage to another, and those such as grasshoppers and water bugs that do not. Insects that do not undergo metamorphosis look much like miniature adults when they hatch, and the main change in body form involves the appearance of wings. Some, like dragonflies, may also change the form of their legs or eyes as they leave the aquatic environment.

However, for those insects undergoing metamorphosis, the body changes from a wormlike larva specialized for feeding to a pupa (Fig. 44.24). During the pupa stage, the body tissues undergo a remarkable transformation from the relatively simple larva to a very different looking adult such as a fly, butterfly, wasp, or beetle, usually specialized for reproduction. Insects undergo a nearly complete dissolution of their muscles and other body parts, and so require the immobile, coffinlike pupa in which to transform their bodies from something as simple as a maggot, which lacks even an obvious head or tail, into an animal as sophisticated as a fly, able to walk on water, hover motionless in front of a flower, or fly at 60 mph. This evolutionary step has little parallel elsewhere in life and is one of the keys to insect diversification.

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Unlike other terrestrial arthropods, all insects, whether or not they undergo metamorphosis, have highly specialized eggshells that can withstand desiccation while still allowing gas exchange. This shell allows insect eggs to survive in dry settings such as the surfaces of leaves, and so they can occupy habitats that are difficult for other terrestrial arthropods to access.

Insects have evolved one other critical adaptation for life on land. Aquatic arthropods obtain oxygen through gills, which don’t work well in air because their large surface area dries out. Some terrestrial chelicerates have evolved simple lungs, called book lungs: modified appendages filled with hemolymph that have a large surface area for gas exchange, but are kept moist inside a protective pocket. Some crabs are able to make extended forays on land by keeping their gills moist underneath their carapace. In contrast, insects exchange gases through small pores in their exoskeletons called spiracles (Chapter 39). The spiracles connect to an internal system of tubes, the tracheae, that directs oxygen to and removes carbon dioxide from respiring tissues, much like air exchange ducts in a building.

Insects are extraordinarily diverse, with approximately 1 million described species making up 80% or more of all known animal species. Their success may be a function of the three adaptations that allow them to live in diverse habitats: dessication-resistant eggs, wings, and metamorphosis. The ability of insect eggs to resist drying out means that they can be placed on virtually any surface. Their wings allow insects to reach habitats and feed in ways other animals cannot. Metamorphosis allows them to have two bodies in their life cycles: a larval form specialized for feeding and an adult form specialized for mating and reproduction. These three adaptations enabled insects to feed directly on flowering plants. In turn, many insects pollinate plants, and so pollinating insects are in part responsible for flowering plant diversity (Chapter 33).