Arthropod

Arthropods Definition

An “arthropod” is an invertebrate animal that has an exoskeleton, a segmented body, and jointed appendages. The following families of organisms are all examples of arthropods:

• Insects such as ants, dragonflies, and bees


• Arachnids such as spiders and scorpions


• Myriapods (a term which means “many feet”) such as centipedes and milipedes


• Crustaceans such as crabs, lobsters, and shrimp

It may help to remember that the term “arthropod” comes from the Greek words for “jointed foot.” If the organism has an exoskeleton with joints between its feet and its body, it is probably an arthropod!

Arthropods are a lineage of life that developed skeletons on the outside – their hard shells, made of a material called “chitin” – instead of on the inside for structural support.

Arthropods’ bodies also have other important differences from those of vertebrates like ourselves – their organ systems are simpler and less efficient, which limits the size arthropods can attain.

An ant the size of a human, for example, would not be able to pump oxygen through its blood to feed all its tissues, since the arthropod circulatory system is simpler and less efficient than a humans’.

All arthropods are thought to have evolved from a single common ancestor, though scientists are not sure what this common ancestor looked like, or exactly when it lived.

Arthropod Characteristics

Characteristics shared by all arthropods include:

• Exoskeletons made of chitin


• Highly developed sense organs


• Jointed limbs (the limbs must be jointed like the joints in a suit of armor, since the exoskeleton is rigid and cannot bend to allow movement)


• Segmented bodies


• Ventral nervous system. “Ventral” means “in front,” so this means that arthropods’ nervous systems run along the front of their bodies, near their stomachs, instead of along their backs like the spinal cords of animals.


• Bilateral symmetry. This means that the left and right sides of an arthropod are the same – it will have the same number and arrangement of legs, eyes, etc. on the right side of its body as on the left.

Types of Arthropods

Trilobites

Trilobites were an ancient family of marine arthropods that went extinct during the Permian-Triassic extinction event. Today, they are known to us mostly through fossils like the one below.

They lived on the ocean floor and occupied ecological niches similar to those occupied by crustaceans today.

Asaphus platyurus

Chelicerates

Chelicerata are a branch of the arthropod family tree that, at first glance, may not appear related to each other.

This family includes arachnids (such as spiders and scorpions), sea spiders (which look similar to arachnids but have some important differences), and horseshoe crabs (which, despite their name, have important differences from other crustaceans).

Myriapods

The term “myriapod” means “many legs” – so it is not surprising that centipedes, milipedes, and other many-legged creatures are part of this family.

Myriapods can have anywhere from less than ten legs – to over 750! That just seems excessive.

Myriapods are typically found in forests and other ecosystems where there is lots of decaying plant and animal material for them to feed on.

Crustaceans

Crustaceans are a family of primarily aquatic arthropods that include lobsters, crabs, shrimp, crayfish, barnacles, and the odd one out – wood lice, also known as pill bugs or “roly polys.”

Unlike their aquatic cousins, wood lice live mostly on dry land and are found in environments such as gardens and forests, where they survive by eating decaying plant and animal material.

It may also surprise you to see barnacles included on this list: adult barnacles develop hard shells that stick them to their surroundings, such as the bottoms of boats or other underwater surfaces.

But earlier in their lives before they freeze in place, barnacles have bodies with legs much like the other crustaceans!

Hexapods

The term “hexapod” literally means “six feet.” It might not surprise you to learn that insects – which all have six legs – are hexapods.

Insects include most “bugs” with six legs, such as flies, ants, termites, beetles, dragonflies, mosquitoes, cockroaches, butterflies, and moths.

There are also three much smaller groups of animals in the “hexapod” category. Collembola, Protura, and Diplura were all once considered to be insects, but later found to have small differences that set them apart from other insects.

Examples of Arthropods

Ants

When you think of a stereotypical arthropod body, you probably think of an ant. Ants have hard exoskeletons and jointed legs. They also have bodies which are clearly segmented into a head, thorax, and abdomen.

Ants show one type of social organization that has been developed by arthropods. Ants, bees, and termites are all what is called “eusocial” organisms – organisms living in extreme degree of cooperation, with “colonies” that almost operate like a single organism themselves.

Most arthropod species are not eusocial, but eusocial colony life is one of the fascinating roads that arthropod evolution has taken.

Spiders

Spiders are also arthropods, possessed of hard exoskeletons, segmented bodies, and jointed limbs.

Spiders typically eat smaller arthropods, such as gnats and flies – though they will eat any living thing they can catch, and some particularly huge spiders have been known to eat birds or rodents!

Spiders have evolved a variety of strategies for catching their prey – some spin sticky, nearly invisible webs that prey animals wander into and get stuck. Others are active hunters, including jumping spiders which can jump at extreme speeds using special mechanisms in their legs.

Some spiders combine these two strategies, such as “trap door” spiders, which set traps by creating hiding places for themselves – and then jumping out to grab unsuspecting prey animals that wander by!

Lobsters

With lobster being considered a luxury food today, it’s easy to forget that lobsters are in the same family as spiders and ants.

Crustaceans can grow bigger underwater than on land – and lobsters can grow to weigh nearly 50 pounds!

Lobsters’ body design has changed little in the last 100 million years, and their anatomy is spectacularly weird. The lobster’s kidneys are located in its head, its brain in its throat, and its teeth in its stomach. Its “ears” for picking up sound are located in its legs, and its tastebuds, like those of insects, are in its feet.

Butterflies

Butterflies are the most famous example of arthropod metamorphosis.

At some point in their lifecycle, all arthropods go through a drastic change from their larval stage to their adult form. But butterflies are the only ones whose adult forms are so beautiful that we pay attention to this change.

The common features of exoskeleton, jointed limbs, and segmented body can be seen in adult butterflies.

Facts About Arthropods

• Arthropods colonized land about 100 million years before vertebrates did. It’s thought that colonizing land was easier for them for several reasons – including the fact that they had already evolved legs, which they used for walking on the bottom of the sea.


• About 80% of all animal species are arthropods! We don’t see them very often in our daily lives, but all the species of bugs and crustaceans on Earth add up!


• All arthropods undergo metamorphosis – a process where their bodies change radically as they pass from their larval to adult stages. Butterflies are the best-known for entering cocoons as caterpillars and coming out quite different, but all arthropods do something similar!


• When arthropods outgrow their old exoskeleton, they have to molt – leaving behind their former skin and growing a new one. All arthropods have to do this at least once in their lives.


• Crustaceans and arachnids – two types of arthropods – have blue blood instead of red blood!This is because their blood uses a blue copper compound to carry oxygen, instead of the red iron compound used by animals.


• Arthropods’ hard exoskeletons is made of chitin – which is made of a derivative of the sugar glucose! But chitin would not taste sweet, and you wouldn’t be able to eat it; to make it hard and strong, the glucose is modified so that our bodies no longer recognize it as sugar.

Related Biology Terms

• Common ancestor – A common ancestor is an individual or species from which multiple individuals or species evolved.


• Evolution – The process by which populations change over time, due to random mutation and the pressures of natural selection.


• Extinction – The process by which a species ceases to exist after the death of its last member. Most species that have lived on Earth to date are now extinct.

Quiz

1. Which of the following is NOT true of arthropods?
A. They have exoskeletons made of chitin.
B. They are symmetrical, having the same features on one side of their body as the other.
C. They colonized land long before vertebrate animals did.
D. None of the above.

Answer to Question #1

D is correct. All of the above are true of arthropods!

2. Which of the following is NOT a type of arthropod?
A. Hexapods
B. Crustaceans
C. Cephalopods
D. Myriapods

Answer to Question #2

C is correct. The term “cephalopod” has the Greek word “pod” for “foot” in it – but that doesn’t mean they’re an arthropod! Cephalopods are the family of squids, octopuses, and other creatures that decidedly do not have exoskeletons, jointed limbs, or segmented bodies.

3. Which of the following is NOT an arthropod?
A. A scorpion
B. A snail
C. A dust mite
D. A crab

Answer to Question #3

B is correct. A snail is not an arthropod. Although it has a shell which could be argued to be a sort of exoskeleton, its shell is not jointed. The snail lacks a true exoskeleton, jointed limbs or a segmented body. The other organisms on this list have all of those things.

Arthropod