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Topics in Depth

The Theme of Life Takes to the Land in Animal Evolution and Diversity

We see arthropods everywhere we go. Arthropoda, the third protostome phylum, is everywhere. Arthropods live on the land and in the sea. They swim, run, and fly. Arthropods mean a lot to us, too. We eat them, get sick from them, and even eat because of them. Some are pollinators, crucial to the survival of plants crucial to us. Some spread disease among us and the animals and plants we raise. We may sometimes want to, but we can't live without them.

Insects, arachnids, and crustaceans are all arthropods. They have in common:

  • Bilateral symmetry. This means a front, back, top, bottom, left side and right.
      
  • Segmented body plans, often with legs on each segment except the head – Their bodies are divided into sections and there are legs almost everywhere.
      
  • Many pairs of jointed legs. Arthropods legs have more than one section and the legs bend where the sections meet.
      
  • Body segments specialized for specific functions. The segmented worms tend to have functions repeated in each section. The segments of an arthropod each do a different thing.
      
  • Rigid, non-growing exoskeleton made mostly from chitin. Our hard parts—our bones—are inside. Arthopods have their skeleton on the outside.
      
  • Open circulatory system. We've seen this before with mollusks.
      
  • Heads with developed sensory organs like eyes and antennae. The head is the place with most of the tools for figuring out what's going on outside the animal.
      
  • Growth through molting. That hard exoskeleton has to come off and be grown again when the arthropod grows.

Arthropods are the most successful animals on earth, with both the most individuals and the most species. They are also the first animals to come ashore and the first to fly.

Like the mollusks, arthropods are crunchy on the outside and soft in the middle. This armor is one of main reasons some arthropods were able to move to new digs —the land. Arthropods are trailblazers.

Life on earth evolved in the water and still depends upon it. In order to exist outside the water, animal life had to find a way to 1) not dry out and 2) get oxygen without water. Arthropod solutions are: an exoskeleton and learning to breathe air.

An arthropod's exoskeleton is pretty much waterproof, keeping the insides moist. Gases and food are brought in from the outside without breaking this barrier. Exoskeletons have some downsides, but arthropods have found ways around these. Soft-bodied animals can sense their environment through touch with things like tentacles. Arthropods developed complex sensory organs to take in what is going out around them. Many arthropods have setae, which are bristles that sense movement in the environment around them. They also have eyes and other ways to detect light, as well as organs to sense chemicals and vibrations.

Arthropod exoskeletons are rigid, made from chitin and protein, and they don't grow. As the animal grows, it must get rid of its exoskeleton and grow a new one. This is called molting. It is as inconvenient as it sounds. The arthropod is exposed during this process and uses a lot of energy to re-grow a skeleton. Plus, they leave their discarded skeletons everywhere, forcing the rest of us to clean up after them. How rude. However, the benefits of exoskeletons must be worth it, since there are more arthropods than any other animal on earth.

Depending on the habitat, arthropods have a variety of systems for breathing. Crustaceans living underwater have gills. Spiders have their own version of lungs to get oxygen from the air. Insects have a system of tracheae (tubes) that run directly to the cells from pores in the surface called spiracles.

Arthropod means "jointed foot." Imagine walking around with no knees or ankles and toes that don't bend. Or, picture an octopus dragging itself across a field. What works under water, doesn't work so well on land. Moving on land takes a different set of tools and marine arthropods with jointed legs had the right stuff. 

Brain Snack

Jointed legs may have come before hard bodies. This animal looks like a walking plant.

The body of an arthropod is made up of very specialized parts. We are getting further and further away from the sack with pores model that sponges have. Arthropods have segmented bodies, like annelids, but arthropod segments are far more specialized, with certain functions in each segment. Arthropods generally have two to three body parts called tagmata, sections of the body made up of fused segments. Sometimes these segments within a body part are apparent as in the scales of a lobster tail and sometimes the segments are completely fused as in arthropod heads. Crucial functions tend be centralized in one segment. For instance, the insect brain resides in its head. Earthworms had ganglia (like little brains) repeated throughout the body.

Arthropods have an open circulatory system, but it works a little differently than the ones we've seen so far. Instead of fluid flowing around the main body cavity (coelom), arthropods have small cavities called hemocoel around specific organs. The heart pumps a blood-like fluid called hemolymph through the hemocoel. Arthropods have their exoskeleton to hold them up and support muscles, so a big balloon in the middle isn't needed.

Arthropods have a nervous system that looks a bit like a ladder. The brain is in the head and two nerve cords run through the body, connected in each segment. Each segment has a connected pair of ganglia, which are larger bundles of nerve tissue that control certain functions in that segment.

Most arthropods have male and female forms and reproduce sexually. Aquatic arthropods use both external and internal fertilization, depending on the species. The external-internal issue concerns whether sperm and egg meet inside the female or outside. Arthropods who live on land generally use internal fertilization or have a means to protect sperm from dying out before it reaches the female. Eggs and sperm cannot last long outside water.

Arthropods are transformers. As they grow, they go from crawling to flying, or from looking like a blob to looking like a lobster. The process of changing from one form to another is called metamorphosis. A baby insect does not look like its parents. Caterpillars change into butterflies and lobster larvae change into beautiful, tasty creatures.

Arthropods are divided into five sub-phyla. Within these are many classes of arthropods that have similar characteristics.

Trilobitomorpha

Trilobites were a large group of marine arthropods that became extinct before the dinosaurs were around. (Arthropods are older than any vertebrates.) They looked a little like the chitons and also like some current arthropods such as pill bugs, which you may know as rolly-polys. Arthropods evolved from ancestors that had bodies with many similar segments. Trilobites had a head and then many repeated segments, with a couple legs on each segment. The legs were pretty much all the same.

Even though they don't look like much, trilobites are important for what we know about evolution and the history of Earth. Trilobite fossils in rocks tell us how old the rocks are, since we know trilobites all lived during the Cambrian period 500 million years ago.

Over time, arthropod bodies became more modified. Many segments fused into fewer and body functions became more specialized to specific segments. Repeated, paired legs remained, but not all legs are the same.

Chelicerata

The Chelicerata includes spiders, scorpions, mites, and horseshoe crabs. This group is named for its mouth. They have "claw horns," the chelicerae. These are modified legs that have claws to bring food into the mouth. The green parts in this picture of a spider's head are the chelicerae.

Chelicerata have ten appendages overall: two chelicerae, two pedipalp ("foot feeler"), and four pair of walking legs. The chelicerae have claws. The pedipalp are used for sensing things and for reproduction and the walking legs, well, walk.

Here is the basic anatomy of a spider:


(1) Four pairs of legs, (2) Cephalothorax, and (3) Abdomen.

The first two parts of the basic arthropod body are fused in a spider, creating a cephalothorax, a fancy word for head and thorax. If we look more at the inside of a Chelicerate, in this case a spider, we can see how differentiated the segments are. In other words, different parts of the body specialize in different things.

Chelicerates have a unique way to breathe. Spiders and scorpions on land have book lungs and horseshoe crabs in the water have book gills. Book lungs and gills look like the stacks of pages you see on the unbound side of a book. The pages are really layers of tissue with spaces in between. There is a small opening to the outside so air can flow into the spaces. Hemolymph flows through the tissue and gases are exchanged between the hemolymph and the open spaces. Having lots of spaces means more surface area for more gas exchange. This all happens inside the exoskeleton of a land chelicerate to keep moisture inside. Marine chelicerates have gills structured similarly, but because they are in water, the structure is on the outside of the animal.

There are two classes of animals in this subphylum: Arachnida (spiders, ticks, scorpions, mites) and Merostomata (horseshoe crabs).

Horseshoe crabs are funky animals that are not really crabs; they just look kind of like them. A shell covers them, but unlike crustaceans and some of the other shelled creatures we've met, they have two eyes that stick out the front, a tail that sticks out the back, and eight jointed legs underneath. They actually have ten eyes, but the others are under the shell (or carapace). The two eyes on top are compound eyes—meaning they're made of many lenses that produce images. The other eyes are light sensors.

Spiders, ticks, mites, and scorpions are arachnids. These are the bugs of our nightmares. (Arachnophobia, anyone?) Most are carnivorous. They can vary in size from tiny mites to Aragog. Well, actually, no, but J.K. Rowling wasn't as far off as we'd like.

Arachnids are generally hunters. They have modified chelicerae, with poison glands and a pointy end for injecting it into prey. Spiders have a famous adaptation for catching prey as well: webs. Silk glands in their abdomen secrete a liquid protein that comes out through spinnerets and then dries immediately in the air to form strands. Spiders make webs with these strands and each species has a distinctive web pattern. Silk also comes in handy for dropping lines to escape predators, tying up prey in the web, and covering eggs. They're also useful for fighting the Green Goblin. Unfortunately for little pigs, they can't really write messages with it.

Here's a juicy fact. Spiders can't chew, so don't expect them to be great dinner guests. After they've attacked prey, they drool digestive fluid onto their victim, which dissolves it. They then drink their prey.

Ticks and mites are parasitic arachnids. They feed only on blood and attack their prey from the outside. They cut a hole into the skin and then stick a mouthpart (called a hypostome) into the newly created incision to drink its blood. This doesn't usually harm the host, except when the tick is carrying a disease. Ticks cause Rocky Mountain spotted fever and Lyme disease. Gross.

Scorpions add to the legend of scary arachnids. They look a bit like crabs, with large claws on the front legs, but have a tail that arches up over the back to face forward and show off a large stinger. Scorpions have three parts, adding a tail to the arachnid cephalothorax and abdomen. The claws are on the pedipalps, which are much bigger than in spiders, horseshoe crabs, and ticks.

Can't get enough of fun, deadly stuff? Scorpions can sometimes glow, which helps them hunt in the dark.

Arachnids reproduce sexually. Yes, it is true that some female spiders eat the male at some point during or after mating. Lucky for the guys, males of some species have found ways to avoid this fate. Male spiders use the pedipalps to transmit sperm to the female, who later lays eggs. Spider moms may protect their eggs until hatching, though some spider moms die after laying eggs. Think Charlotte's Web.

Crustacea

Crustaceans include favorite food items for many other animals, including humans: crabs, lobsters, shrimp, barnacles, and krill. Crustaceans mostly live in the ocean. There are freshwater ones too, like crayfish. Most move around using ten pairs of legs. Barnacles, however, are sessile and use their six pairs of legs to grab prey.

Crustaceans have two or three body segments and jointed legs, as do all arthropods. Crustaceans, however, have a unique leg structure. The rest of the arthropods have legs made up of segments that lie end to end. Crustacean legs can have one segment with two at the end, giving them very specialized feet. This leg is called biramous, meaning two branches.

Where spiders had chelicerae, crustaceans have mandibles. Mandibles are modified appendages right at the mouth that can crunch up food. Crustaceans bite—don't ask how we know.

Crustaceans generally have males and females, but many can also reproduce through parthenogenesis. This is when females can produce eggs that grow into viable adults without needing sperm. Poor crustacean men—they really hit the glass ceiling in the arthropod world.

The largest group of crustaceans is probably far less familiar, but crucial to ocean life. Copepods are everywhere in fresh and salt water. They are tiny in size, but big in their impact. Animals from fish to whales depend upon copepods as food. Copepods depend upon algae and microscopic plants for their food, making them the first animal in the food chain.

Myrapodia: Chilopoda and Diplopoda

This section is all about the millipedes (class Chilopoda) and the centipedes (class Diplopoda). Millipedes look like armored, segmented worms with lots of legs. Their name means "a million legs," but it only looks that way. There are two pairs of legs on each segment, which can make up to a few hundred legs, depending on the species. Millipedes live on land and eat decaying plants. A millipede ancestor may have been the first creature on land.

Centipedes are millipedes' tough cousins. Centipedes are carnivorous. The front pair of appendages has toxic claws that inject venom in to prey. Centipedes can even be harmful to humans. Centipede means "one hundred legs." In this case, the name is a bit more accurate. Centipedes can have far fewer and far more than 100 legs, with one pair on each segment.

They may be small, but up close, these look very tough.

Hexapoda: Insecta

Insects (class Insecta) are everywhere on land and fresh water. They live fast and furious, spreading everywhere and reproducing quickly and in quantity.

Insects have the three arthropod body parts, but add a whole new structure—wings. Insects may have one or two pairs of them. The wings are not modified, segmented appendages, but outgrowths of the exoskeleton (in contrast to bird and bat wings). Insects also have three pair of legs coming from the thorax.

Insect features:

  • Hard exoskeleton
  • Segmented body
  • Six legs in three pairs
  • Compound eyes
  • Three body parts
  • Many have wings
  • One pair antennae on the head

Insects have keen sensory abilities. Inside their head is a brain that leads to a central nerve cord that runs through the body. Outside the head are two compound eyes for seeing movement and antennae for smelling and touching. Additional light sensing organs are also on the head. Insects can hear using membranes on the abdomen that sense vibration. Many insects also produce pheromones, chemicals that cause a behavioral reaction in members of their own species, for instance, to mate.

Insects have also developed a wide range of ways to eat. Mouthparts are highly modified and may bite, suck, or pierce depending on the insect and its food. There is an insect that eats just about everything.

Insects have specialized organ systems. As with all arthropods, insects have an open circulatory system. The digestive system has different parts specialized for various aspects of breaking down food. The lower part of the digestive system sports Malphigian tubes, which take wastes away to the outside. For respiration, insects use a system of trachea (Greek for "windpipe"), tubes that branch throughout the body and link to many pores in the exoskeleton (spiracles).

Insects have males and females and fertilization is internal. Insects lay eggs, often only once in a lifetime. Most insects go through some kind of metamorphosis, with the young larvae starting out with a different body form from an adult. Check out a few clips of metamorphosis in action.

With insects, we get the first examples of social behavior, where members of a species live and work together. Most insects are solitary, but some (like ants, bees, and termites) exhibit complex social behavior. There are even different physical forms for different purposes within the group.

There are about 26 orders of insects.

Insects were the first creatures to evolve flight and the only invertebrates to do it. This allowed them to take over whole new environments. Flying insects can disperse quickly and range widely to find what they need: food, mates, someplace to live. Insects really began to diversify when flight evolved and continued to diversify with the success of flowering plants.

Insects eat and pollinate most of the plants on earth. While we often think of insects as pests, spreading disease and destroying our food crops, without insects, we wouldn't have those food crops.

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