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. Centipedes dwell in damp places under old logs and stones. They are carnivorous, eating mostly insects.
They are highly segmented, and have one pair of legs per segment. The first trunk segment bears poison fangs. Centipedes are very dangerous, and their bite is extremely painful.
Class Diplopoda - 10, sp. Millipedes share the same habitat as centipedes, but they are mostly herbivorous, feeding on decaying vegetation in the leaf litter. Animals that feed on detritus are called detritivores.
Each segment of the millipede is actually two segments fused together hence the double set of legs. They can secrete a defensive fluid that smells bad, and a few species actually secrete tiny amounts of cyanide gas to protect themselves! Class Insecta - , sp. If we knew all the different insects on Earth, there could be as many as 30 million species.
Insects evolved about mya, with cockroaches and dragonflies among the first to appear. Insects have a head, thorax, and abdomen, with three pair of legs 6 legs on the thorax. Crustaceans have legs on the abdomen as well as on the thorax. Most insects have one or two pairs of wings. They are the only invertebrates that fly. Most have compound eyes , and can communicate by sound and scent, using powerful chemical hormones called pheromones. Insects have extremely elaborate mouthparts, consisting of pairs of appendages fused into a lower lip labium , and an upper lip labrum , with other appendages called maxillae aiding in chewing.
These mouthparts are highly modified in various groups for chewing, sucking, and piercing. Insects undergo metamorphosis as they develop, changing from one form to another as they mature. The juvenile stages look like tiny versions of the adults. Their larvae are often radically different from the mature adult like the butterfly and the caterpillar.
They not only look different, they live in different places and eat different food. Observe the preserved arthropods on display. How do the various groups use their legs to walk, swim, feed or mate? Watch the way the millipede moves. Look at the legs. See how the waves of muscle contraction pass down through the segments? The polychaete worm Nereis moves in exactly the same way.
Handle the millipedes very gently. They are someone's pets. They also make great pets for dorm rooms - they need little care, don't take up much room, and don't make noise or messes, unlike your roommate. Disturb the centipedes to get them moving around. Can you see the poison fangs? Notice how flat the body is, and contrast the number of legs with those of the millipede. Why does each container hold only a single centipede? Don't open the jars unless you have a thing for extreme pain.
Play around with the roly-polys. Oh, go ahead, it's cool. They won't bite. Watch the way they roll up into a ball when disturbed. Not all isopods can do this, but rolling up into an armored ball is a great defensive tactic. Compare our teeny tiny terrestrial version with the enormous preserved marine isopods. Look at the live brine shrimp, hermit crabs and fiddler crabs. Treat them gently more pets. Watch the way they use their legs, including the modified legs that form their mouthparts.
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. Observations on the Microfauna of the Duke Forest. Ecological monographs, Volume Sabrosky, C. How many insects are there? Selected References: Erwin, T. May, R. How many species are there on earth? Science, Volume Andrew's Cotton Stainer.
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