Arthropoda

 
Insects are "the little things that run the natural world."
E.O. Wilson

By almost any measurement—diversity, habitat usage, sheer numbers—arthropods are the most successful group of animals on the planet. The invasion of land by arthropods about 400 million years ago opened up many new ecological niches, which led to their adaptive radiation (a process in which organisms diversify rapidly from an ancestral species into many new forms). Arthropods are found in virtually every known environment including land, sea, and air and account for over 80% of all known living animal species, or about one million in all. Recent studies estimate that 30 to 50 million species may yet be undescribed, especially in tropical rain forests.

 

Phylum Arthropoda

  1. modified metamerism by specialization of body regions for specific functions (tagmatization)
  2. chitonous exoskeleton that provides support & protection, and is modified to form sensory structures
  3. paired, jointed appendages
  4. growth accompanied by ecdysis or molting
  5. ventral nervous system
  6. coelom reduced to cavities surrounding gonads & sometimes excretory organs
  7. open circulatory system in which blood is released into tissue spaces (hemocoel) derived from blastocoel
  8. complete digestive tract
  9. metamorphosis often present


The success of arthropods has been strongly influenced by three characteristics: chitinous exoskeleton with jointed appendages, metamerism and tagmatization, and metamorphosis. Some biologists add the hemocoel to this list. We'll look at each of these in details.


Arthropods are unbelievably diverse, yet the basic body plan is relatively constant. The phylum name, "arthropod", means jointed foot. They are enclosed in a chitinous (and proteinaceous) exoskeleton that provides support for the body and protects the animal from predators. This suit of armor also prevents water loss and provides surfaces for muscle attachment. The exoskeleton is often cited as the single feature most responsible for arthropod success.


Arthropods have segmented (metameric) bodies and various patterns of fusion of body segments (tagmatization) to form distinct functional units (e.g., heads, thorax, etc). This permits the specialization of regions of the body for specific functions (e.g., feeding, reproduction). The acquisition of legs reduced the emphasis on body segments for locomotion (like in annelids).

Metamorphosis is a radical change in body form and physiology as a larva becomes an adult. As a result, they don't compete with each other. For instance, larval crabs are plankton feeders, whereas adults are scavengers or active predators. Larval butterflies are herbivores and the adults feed on nectar.

The main body cavity of arthropods is a hemocoel, derived from the blastocoel. The hemocoel is part of the open circulatory system in that it consists of blood-filled sinuses. The coelom forms only small cavities around the gonads and sometimes the excretory structures.


The arthropod nervous system resembles that of annelids, consisting of a dorsal brain and ventral nerve cord. Most groups have specialized respiratory organs, varying from gills in crustaceans, to book lungs in arachnids, to tracheae and spiracles in insects. They also have specialized excretory organs for eliminating nitrogeous wastes—the most common of which are the Malpighian tubules found in insects.

The ecology and life history of arthropods is difficult to describe because of the incredible diversity of the group. The first arthropods were marine (trilobites lived in the sea), but fossil records show that around 420 mya some type of arachnid (probably a scorpion) made the leap from water to land. So, why would a creature like a scorpion leave the comforts of the sea in favor of land? There are 3 possible factors: abundant food, unoccupied niches, and absence of predators. During the 440 and 410 mya time period that arthropods were moving onto land, the terrestrial landscape was simultaneously being populated by vascular plants, which promoted a close association between plants and arthropods that still exists. The unsurpassed success of arthropods is, in part, due to their adaptations to the ecological challenges associated with a terrestrial existence including smaller body size and respiratory and excretory systems that conserve water.

Living arthropods are divided into four subphyla:
Chelicerata: horseshoe crabs, spiders, scorpions, ticks and mites; body usually divided into prosoma and opistosoma (fused and covered with a single carapace in ticks and mites); 1st pair of appendages piercing or pincerlike used for feeding

Crustacea: shrimp, lobsters, crabs & copepods; most aquatic, head with 2 pairs of antennae, one pair of mandibles, and 2 pairs of maxillae; biramous appendages; mostly dioecious (barnacles are hermaphroditic, an adaptation to being sessile).
Myriapoda: millipedes & centipedes; body divided into head and trunk; 4 pairs of head appendages; uniramous appendages
Hexapoda: insects; body divided into head, thorax, and abdomen; 5 pairs of head appendages; 3 pairs of uniramous appendages on the thorax
Trilobites were a fifth subphylum of arthropods that went extinct during the great Permian extinction 250 million years ago.


Class Insecta
To reach adulthood, an insect must periodically shed its exoskeleton in a process called ecdysis. Periods of ecdysis alternate with periods of growth and development of immature stages during which the insect is termed an instar. The evolutionary trend is for insects to spend a greater part of their lives in immature stages. The different degrees of divergence between immatures and adults are divided into the following three categories:
(1) There are a few insects (less than 1%) that hatch from an egg as a miniature adult. A number of molts allows the animal to grow into adulthood. Examples are springtails and silverfish. This is known as direct development. All other insects undergo some type of metamorphosis.
(2) In incomplete metamorphosis, the immature insect (nymph) resembles the adult—antennae, compound eyes, similar mouthparts—and is gradually transformed, through progressive molts, into a sexually mature individual. E.g., dragonflies, mantids, cockroaches, crickets.
(3) The most common developmental pattern is complete metamorphosis, in which the larva, [also known as a caterpillar (butterflies and moths) or maggot (flies)] bears no resemblance to the adult. Different life stages live in different habitats with different behaviors so they do not compete with each other for environmental resources. The larvae undergo a radical change into adulthood in a pupa or chrysalis (butterflies and moths), or it may occur in decaying logs, in prepared cells (honeybees) or within the bodies of living animals (parasitoids). Beetles, butterfles, ants, flies (see orders of insects below for more examples).
*recall that we saw mayfly nymphs (incomplete metamorphosis) and caddisfly larvae (complete metamorphosis) in Cherry Creek.

Insect flight
Insects were the first animals to fly. Flight was a major adaptation for arthropods opening up new realms including the ability to locate food and shelter, to find mates, to escape predators, to disperse to new habitats, and to migrate long distances. Wings are extensions of the exoskeleton, so unlike flying vertebrates, insects can fly without sacrificing any walking legs. The explosion in insect diversity 350-250 mya coincides with the ability to fly.

Insects and People
Insects, as a group, have both negative and positive impacts on humans. They can be vectors of diseases (e.g., mosquitoes, tsetse flies), damage crops (e.g., aphids), sting (e.g., bees and hornets), serve as human parasites (e.g., bed bugs), and cause general damage (e.g., termites, carpenter ants). On the other hand, insects serve a vital function in the recycling of materials, pollination of plants, and in biocontrol of other insects. According to famed entomologist E.O. Wilson, "If all mankind were to disappear, the world would regenerate back to the rich state of equilibrium that existed ten thousand years ago.. If insects were to vanish, the environment would collapse into chaos."

Evolution
Arthropods are the most diverse animal phylum and their phylogenetic relationships have been debated for centuries. With the advent of molecular techniques, arthropods were found to be monophyletic and placed within a clade of molting animals, the ecdysozoans, with nematodes and six other phyla (see the Current Phylogenetic Tree of Animals).
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The closest arthropod relatives are Phyla Onychophora and Tardigrada. Members of the phylum Onychophora are known as velvet worms. Extant forms are terrestrial and found in humid, tropical, and subtropical regions. Several fossils have been found in marine fossil formations, including the ~500 year old Burgess Shale.

The members of the phylum Tardigrada are known as "water bears" because of their teddy bear appearance. They are meiofauna, meaning they live in marine interstitial areas (space between sand grains), freshwater detritus, and in the water film on terrestrial lichens, liverworts, and mosses. Tardigrades can enter a period of suspended animation termed cryptobiosis. This ability offers great survival benefit to these animals, which live in habitats where conditions can suddenly become adverse. When a tardigrade begins to dessicate, it contracts into a shape that produces an ordered packing of organs and tissues to minimize damage. Rehydration reverses these events. Repeated periods of cryptobiosis can extend a life span of approximately 1 year to 60 or 70 years. The Onychophora and Tardigrada have a cuticle that is shed by ecdysis, a characteristic they share with arthropods. See cladogram of arthropods (link below).

In spite of the success of the crustaceans in marine and freshwater environments and the importance of chelicerates as the first terrestrial animals, the insects are the dominant arthropods in terms of both numbers of species and numbers of individuals.

Orders of insects.

Cladogram

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