You’ve heard of insects, you’ve heard of arachnids—but what about arthropods? As it turns out, if you’re familiar with insects and arachnids, you’re also familiar with arthropods, because insects and arachnids belong to the Arthropoda phylum. The Arthropoda is a fascinating phylum, which includes some of the smallest organisms and organisms which have a fossil record. Keep reading to learn more about this phylum and about some of the animals that are a part of it.

So, what exactly is the Arthropoda phylum?

Before going into more detail about the Arthropoda phylum itself, it’s worth understanding a little bit about the terms used to group organisms in taxonomy. The animal kingdom contains all the animals—living and extinct—in the world. Each kingdom may h ave several phylum. Phylum include classes, orders, and species. All of these different categories have been created based on shared characteristics. Species is the most specific group, sharing a few specific characteristics that set it apart from other species. A species will also have the same characteristics as other organisms within the same phylum, class, order, and family.

The Arthropoda phylum is the largest phylum in the animal kingdom. By so me counts, there are close to one million different types of recorded arthropods—and some even think this is just a fraction of how many species actually exist.

Arthropod characteristics

What makes arthropods distinct is the exoskeleton, the hard exterior shell where the muscles attach. The exoskeleton is usually composed of the complex sugar chitin, and bound to protein. The exoskeleton is secreted by the arth ropods’ epidermis (which differs from the skin of other animals). Arthropods usually have segmented bodies with jointed appendages on each segment. However, the number of appendages and segments can vary; for example, the millipede is a well-known arthropod famous for having hundreds of legs—though not quite the one thousand legs, despite the number being a common misconception. In fac t, the name arthropod is derived from Latin which means “jointed feet.”

While most arthropods are fairly small, some aquatic forms can grow to be quite large because they have th e support of water through which to more easily move their bodies. For example, spider crabs may grow to weigh as much as 14 pounds, while any of the terrestrial insects have not been known to grow over a quarter of a pound.

Animals within the Arthropoda phylum

As mentioned earlier, insects and arachnids belong to the Arthropoda phylum. Specifically, insects and arachnids are considered classes within the Arthropoda phylum. In each class are orders that you might also recognize. For example, the insec t class includes the Hymenoptera order, which includes ants, bees, and wasps; and, the Lepidoptera order, which includes butterflies and moths. The arachnid class includes spiders, scorpions, ticks, a nd mites. Arachnids are well-known for most in that class having eight legs as adults.

You may have noticed that we mentioned spider crabs in the previous section. Spider crabs are of course ne ither insects nor arachnids—but they are arthropods. Among some of the other groups that make up the huge Arthropoda phylum is the Crustacea subphylum—made up of crustaceans. Crustaceans i nclude familiar animals like shrimp, crab, lobsters, and crayfish. There are both salt-water dwelling and fresh-water dwelling crustaceans. While most crustaceans are marine animals, some have become terrestrial over time, such as the pill bug.

There’s one more group within the Arthropoda phylum that’s worth mentioning: the extinct subphylum Trilobitomorpha. This subphylum includes trilobites, dominant sea-dwelling arthropods, which became extinct almost 300 million years ago at the end of the Paleo zoic Era. They are believed to have first appeared around 540 million years ago during the Cambrian Period. Scientists know this from fossils of trilobites that have been found and studied.

A s you can see, the Arthropoda phylum is vast, varying, interesting, and home to some of the smallest creatures on the planet.