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Arthropods: Characteristics, Anatomy & Groups

Instructor: Meredith Mikell
Arthropods are a very diverse group of animals that make up a very large part of Earth's ecosystems. Here we will learn about their characteristics, anatomy & physiology, and classification.

Description

You have probably seen many different types of arthropods...just in the last day! Insects, spiders, crabs, and shrimp are all examples of members of Phylum Arthropoda, a group of invertebrates making up the largest phylum in the Animal Kingdom. These critters are so diverse and numerous they comprise 80% of all known animal species! It is estimated that there are between 2 and 10 million different species of arthropods living today, and the fossil record indicates that the number of extinct species is much, much higher. They live literally anywhere and everywhere on our planet: land, oceans, rivers, lakes, mountains, hot springs, you name it. They fly in the air, swim fast in the water, drift with currents, dig down in the dirt, stow-away on other animals, weave webs, and more. The ways in which arthropods have adapted to life on Earth are limitless!

A beetle is an insect, belonging to one subgroup of Arthropods.
beetle

Characteristics & Anatomy

The word arthropod literally translates into jointed foot. That might sound like a strange name to give to such a large group of living things, but it actually describes a key characteristic shared by all arthropods: jointed appendages. Their many legs, claws, and antennae have joints, allowing for flexible and effective movement. This trait is key to their evolutionary success, making them both speedy predators and nimble prey. Arthropods also feature segmented bodies, typically consisting of a head, thorax, and abdomen, though in some cases the head and thorax are fused into one segment.

The basic body plan of arthropods: a head, thorax, and abdomen, as shown on this insect.
arthropod anatomy

Body segmentation is seen elsewhere in the Animal Kingdom, but it has shown to be particularly advantageous for arthropods by diversifying their range of movement. These segments are covered by a chitinous exoskeleton, which also comprises the appendages. This exoskeleton has a similar thickness and strength as your fingernails, and is shed, or molted by the animals as they grow. It provides some body protection but is also flexible enough not to restrict their movement. All arthropods are also ectotherms, meaning they cannot produce their own body heat, relying on the environment for temperature regulation.

A cicada molting.
arthropod molting

Arthropods also have a wide spectrum of very strong sense organs, making them very responsive to changes in their environment. These sensing superpowers can include:

- Compound eyes (think of flies or spiders)

- Antennae

- Setae, tiny hairs covering the body that act as touch or taste sensors

Some members of this phylum, such as the mantis shrimp, have such complex compound eyes that they can see more colors than we can! Some can fly, some have very powerful pincers, some have spinnerets. It's no wonder that arthropods are sometimes nicknamed the Swiss Army Knives of living things by biologists.

Examples & Classification

There are five different sub-phyla of Arthropods, the first four are still around today:

1. Arachnids - spiders, mites, scorpions. All have eight legs; most are terrestrial.

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