The planet today is almost completely dominated by a single phylum of animal life. On land, in the sea, even in the air itself, they are the true masters of the Earth.
They are the arthropods. Arthropods are eucoelomate protostomes, dominating the protostome branch of the animal tree, just as vertebrates dominate the deuterostome branch. Arthropods share a common ancestor with polychaete worms, and may even be a direct descendant of polychaetes. But unlike other eucoelomate invertebrates, the arthropod coelom is greatly reduced in the adult animal.
All arthropods have jointed appendages. This evolutionary innovation is probably the key to the stunning success of this diverse group. There are about 10 billion billion arthropods alive at any one time. There are over three times as many species of arthropods as there are of all other animals on Earth, and there may be millions more that we haven't even discovered. Arthropods do everything with legs or modified legs.
They walk, they swim, they creep and crawl, they use legs to sense with the antennae , to bite and sting with, and even to chew with. That's one reason arthropods look so alien when we see them up close. They chew sideways, and it's all done with legs. Their bodies are protected by an tough cuticle made of proteins and chitin , a polysaccharide with added nitrogen groups. A cuticle is a tough outer layer of non living organic material.
The cuticle of arthropods acts as an exoskeleton. Most are very small, though a few lobsters reach up to a meter, and one giant crab grows to 3. Fossil insects were also very large. Ancient dragonflies had wingspans of a foot or more. But living insects are uniformly small. Perhaps smaller insects were better at hiding or escaping from their many predators. Terrestrial arthropods remain small primarily because of the limitation imposed by their exoskeleton.
A large insect would need such a thick exoskeleton to withstand its strong muscles that the weight of the cuticle would be too great for the animal to carry around. For a small animal, having your skeleton on the outside is as logical as having it on the inside. But it poses a fundamental problem for arthropods. They must shed their exoskeleton, or molt , in order to grow.
The exoskeleton splits open. While the animal molts, it is especially vulnerable - just ask a plate of soft-shelled crabs! Arthropods have segmented bodies, like the annelid worms. These segments have become specialized, however, with one pair of jointed appendages added to each segment. Among living arthropods, the millipedes most closely suggest what the ancestral arthropod might have looked like.
Arthropod segments have also fused together into functional units called tagma. This process of segment fusion, or tagmosis , usually results in an arthropod body that consists of three major sections, a head, thorax, and abdomen. Sometimes the head and thorax are fused together into a cephalothorax.
Each of these body sections still bear the appendages that went with it, though these appendages are often highly modified. Arthropods are very highly cephalized, often with intricate mouthparts and elaborate sensory organs, including statocysts , antennae, simple eyes and compound eyes.
Sensitive hairs on the surface of the body can detect touch, water currents, or chemicals. Their nervous systems are highly developed, with chains of ganglia serving various parts of the body, and three fused pairs of cerebral ganglia forming a brain. Aquatic arthropods respire with gills. Terrestrial forms rely on diffusion through tiny tubes called trachea.
Trachea are cuticle-lined air ducts that branch throughout the body, and open in tiny holes called spiracles , located along the abdomen. Insects can open and close these spiracles, to conserve water that would otherwise be lost to evaporation from the open tubes. Their reliance on diffusion for respiration is one of the reasons that insects are small.
Arthropods excrete by means of malphigian tubules , projections of the digestive tract that help conserve water.
Terrestrial forms excrete nitrogen as uric acid , as do birds. Their waste is nearly dry, a superb adaptation to life on land. Arthropods have an open circulatory system, and separate sexes.
Fertilization is usually internal, another adaptation for terrestrial life. Males and females often show pronounced sexual dimorphism.
Order Orthoptera - grasshoppers, crickets, roaches. In chelicerates, the first pair of appendages are called chelicerae, and are modified to manipulate food. They are often modified as fangs or pincers.
Chelicerates lack antennae. Horseshoe crabs have larvae that are very similar to trilobites, and they may be descendants of this long vanished group. Horseshoe crabs are nocturnal, feeding on annelids and molluscs. They swim on their backs, or walk upright on five pairs of walking legs.
They live in the deep ocean, migrating inshore in large numbers in the spring to mate on the beaches during moonlight and high tide - much like undergraduates on Spring Break. This very successful group of arthropods have four pair of walking legs 8 legs.
The first pair of appendages are the chelicerae , and the second pair are pedipalps , appendages modified for sensory functions or for manipulating prey.
They are mostly carnivorous many mites are herbivores. Most secrete powerful digestive enzymes which are injected into the prey to liquify it. Once dissolved in its own epidermis, the prey is sipped like a root beer float. Order Scorpiones 2, sp. Scorpions date back to the Silurian, about mya, and may be the first terrestrial arthropods. Order Araneae 32, sp. Not all spiders spin webs. Wolf spiders are the tigers of the leaf litter, and the common jumping spider leaps several times its body length to catch its prey.
Spiders use pedipalps as copulatory organs. Spiders breathe by book lungs. Order Acari - 30, sp. Most are very tiny, less than 1 mm long.
The thorax and head are fused into a single unit cephalothorax. Order Opiliones 5, sp. It has an oval body with extremely long legs, which they frequently lose in various accidents and brushes with predators.
They are predators, herbivores, and scavengers. Look at them closely next time you see one. They carry their eyes atop a little tower on their back weird! Crustaceans are mostly marine, and dominate the ocean to the same degree that insects dominate the land and air.
Despite their aquatic diversity, there are very few terrestrial crustaceans, just as there are very few truly aquatic insects. Crustaceans have biramous appendages. Each leg has an additional process, like a little miniature leg branching off from the main leg. Many groups of crustaceans have lost this extra appendage during subsequent evolution.
The Order Decapoda have five pair of walking legs, and include the familiar crabs, lobsters, and crayfish. The first pair of appendages are usually modified as antennae. Crustaceans have two pair of antennae. Another set of anterior appendages are modified as mandibles, which function in grasping, biting, and chewing food.
Male crayfish also use one pair of legs as a copulatory organ. All crustaceans share a common type of larva called a nauplius larva. They are one of the few successful terrestrial crustaceans.
They feed on decaying vegetation in the leaf litter. Uniramians have a single pair of antennae, and uniramous appendages. They probably evolved from oligochaete worms. Class Chilopoda - 2, sp. The largest numbers of described species in the U. Several enlightening studies have been conducted involving the numbers of individual insects in a given area.
In North Carolina, soil samples to a depth of 5 inches yielded a calculation that there were approximately million animals per acre, of which 90 million were mites, 28 million were springtails, and 4.
A similar study in Pennsylvania yielded figures of million animals per acre, with million mites, million springtails, and 11 million other arthropods. Even specific insect species have been found to be quite numerous, with calculations of from 3 to 25 million per acre for wireworms larvae of click beetles. Certain social insects have large numbers in their nests. An ant nest in Jamaica was calculated to include , individuals. A South American termite nest was found to have 3 million individuals.
Locust swarms are said to hold up to one billion individuals. These great numbers of insect species and individuals were created by a number of factors including their long geological history, the capability of flight, their small size that allows survival in many various habitats, their ability to store sperm for delayed fertilization, and their general adaptive abilities to the environment.
Insects have remarkable fertility and reproductive abilities, which have usually led to the vast numbers of individuals in nature. East African termite queens have been recorded to lay an egg every two seconds, amounting to 43, eggs each day. To appreciate the population potentials of insects the example of the housefly is sometimes used, stating that the descendants of one pair of this insect, provided that they all survived during a five month season, would total quintillion individuals.
Recent figures indicate that there are more than million insects for each human on the planet! A recent article in The New York Times claimed that the world holds pounds of insects for every pound of humans. Erwin, T. Tropical forest canopies: the last biotic frontier. Bulletin of the Entomological Society of America, Volume Janzen, D. Why are there so many species of insects?
Pearse, A.
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