Introduction to Arthropods
This is not, as the Victorians called it, the Age of Mammals. 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.
There are over 800,000 named species in the Phylum Arthropoda, named from the Greek arthros (= jointed) and poda (= foot), including the familiar arachnids, crustaceans, and insects, together with a host of less familiar critters, like centipedes, millipedes and sea spiders. All arthropods have jointed appendages. This evolutionary innovation is probably the key to the stunning success of this diverse group. There are about 1018 (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 3.5 meters long.
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. the animal emerges and swells to a larger size until the newer, larger exoskeleton is hardened. 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.
Phylum Arthropoda:
Subphylum Chelicerata
Class Merostomata - horseshoe crabs,
Class Arachnida - spiders, scorpions, ticks, mites
Subphylum Crustacea - crustaceans
Subphylum Uniramia
Class Chilopoda - centipedes
Class Diplopoda - millipedes
Class Insecta - insects
Order Hymenoptera - ants, bees, wasps
Order Coleoptera - beetles
Order Lepidoptera - butterflies, moths
Order Diptera - flies, mosquitoes
Order Orthoptera - grasshoppers, crickets, roaches
Order Odonata - dragonflies
Order Isoptera - termites
Subphylum Chelicerata:
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.
Class Merostomata - horseshoe crabs (Limulus)
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.
Class Arachnida - (57,000 sp.), spiders, scorpions, ticks, mites, and daddy longlegs
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 (2,000 sp.) - Scorpions have pedipalps modified as pincers, along with a venomous sting in their tail. Scorpions date back to the Silurian, about 425 mya, and may be the first terrestrial arthropods.
Order Araneae (32,000 sp.) - Spiders have special modified posterior appendages called spinnerets, which they use to spin their webs. 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 - (30,000 sp.) - Ticks and mites are the largest and most diverse group of arachnids. Most are very tiny, less than 1 mm long. The thorax and head are fused into a single unit (cephalothorax). Ticks are bloodsucking parasites, and can carry diseases like Rocky Mountain Spotted Fever and Lyme Disease.
Order Opiliones (5,000 sp.) - Daddy Longlegs is a familiar arachnid. 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.
Order Isopoda, Isopods have many common names, such as Pill bugs, Roly-Polys, Woodlice, Bibble Bugs, Cheesybugs, Cud-worms, Coffin-cutters, Monkey Peas, Penny Pigs, Sink-lice, Slaters, Sowbugs, Tiggyhogs, and (in New Orleans) Doodlebugs. 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 - (2,500 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. Despite the name, the number of legs comes out to considerably less than one hundred (centi = 100). The first trunk segment bears poison fangs. Centipedes are very dangerous, and their bite is extremely painful.
Class Diplopoda - (10,000 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. They have two pair of legs per segment, (less than a thousand [= milli], but lots more than a centipede). 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 - (750,000 sp.) If we knew all the different insects on Earth, there could be as many as 30 million species. Insects evolved about 200 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. Some (about 10%) show simple metamorphosis, in which there is no resting stage. The juvenile stages look like tiny versions of the adults. Most (90%) show complete metamorphosis, in which one stage is an inactive pupa, like the cocoon of the moth or the chrysalis of the butterfly. 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. You may see the male fiddler crabs raise their large claw and wave it about to claim a territory inside the tank, in the hopes of attracting a mate (Can you blame them?).
Observe the live crayfish. What does the crayfish do when it feels threatened? How does it use its swimmerets when it is stationary?
Observe the diversity in insect mouthparts etc. Don't worry about being able to identify the individual slides. Try to get a feel for the way modified legs are employed in these animals for a wide variety of sucking, sponging, piercing and biting.
Observe the insects on display. You should be familiar (for lab and lecture) with the common orders of insects listed in this guide.
Crayfish are relatively easy to dissect. Many of you have had ample practice dissecting them at Jazz Fest. Your first task is to determine whether you have a male or female crayfish. Turn the animal on its back, and examine the area of the thorax where the legs join the body. Female crayfish have a circular opening, like a tiny doughnut, which is their seminal receptacle. Male crayfish have a hardened pair of swimmerets (legs on the abdomen) that extends back towards the head, and fits neatly into the groove between the walking legs. These modified legs are stiff, like hard plastic. They are curved like half a soda straw, and when they are joined together, they make a tiny tube through which the sperm travel during copulation. Crayfish literally copulate with their legs.
Observe their external anatomy. Identify the following structures: rostrum, antennae, eyes, thorax, carapace, chelae (claws), cheliped, walking legs, abdomen, swimmerets, telson, and uropod. Examine the various appendages and modified appendages closely. Note that some are biramous (ex. uropods, most swimmerets), while some are uniramous (ex. cheliped). The uniramous appendages result from the evolutionary loss of the second branch. Note that each pair of antennae are biramous appendages. Examine the telson and uropod. How does the crayfish use these biramous appendages to escape predators? Using a probe, try to find the mouth and anus. Note the thick triangular mandibles, a primary trait of crustaceans.
Place the crayfish in the pan with its dorsal side up. Carefully cut the carapace just to one side of the midline with your scissors, and down along both sides. Peel it back to expose the gills. Notice how the gills interface with the legs, and observe the second underlying row of gills. Cut away the gills where they join the body. Try to find the tiny heart (good luck!). Just under the heart are the gonads (ovaries or testes). Look for the esophagus and stomach (you can always insert a probe through the mouth to see where it emerges). Carefully remove the internal organs, and look for the tiny brain near the base of the antennae.
The many ways that arthropods help us and hurt us are almost too numerous to mention.
They provide seafood, and pollinate fruit crops.
The also cause billions of dollars a year in crop damage.
They cause or carry a host of diseases, such as malaria and the plague.
Ecologically, they are critically important herbivores. Arthropods are the primary converters of plant tissue to animal tissue on the planet!
How do segmentation and tagmosis account for the success of arthropods?
Why aren't bugs the size of Buicks?
Trilobites were among the most successful arthropods on Earth, once numbering over 10,000 species. Why are they all gone?
How does the smooth flow of muscle contractions in the moving millipede relate to the evolution of segmentation in annelids and arthropods? (Hint: Why is a segmented body plan useful for a burrowing animal?)
There are hundreds of arthropods waiting quietly in obscure corners of the web, ready to pounce on the unwary undergraduate and grab your attention. Where to start? The UCMP server is great for all groups except crustaceans: