Making up around 75% of all animal species in the world, arthropods are are a vast phylum of the kingdom Animalia. The name comes from the Greek words “arthro”, meaning joint, and “podos”, meaning legs. While they all share segmented bodies with joined legs, members in this phylum vary wildly, from butterflies and millipedes, to scorpions and lobsters. They are also all invertebrates, which means that they do not have backbones. Instead, they use hard exoskeletons to protect themselves, which are made out of chitin. Because these exoskeletons are relatively inflexible, arthropods molt as they grow larger, which means they shed their exoskeletons.
The History of Arthropods: Evolution
During the Cambrian Period (542 million to 488 million years ago), the first arthropods arose. They are believed to have evolved from a closely related phylum, called Annelids, which include worms and leeches. With their hard exoskeletons and the first true-eyes of any animal, arthropods, beginning with a species called trilobites, quickly found their niche in the watery environments that they inhabited. At this time, all life on Earth lived in oceans and seas. Although they are currently extinct, trilobites were once very abundant, and we know this because of the copious fossil evidence that has been found. Fossils also show that another type of arthropod, crustaceans, evolved around this time as well. As Earth entered the Devonian Period (416 million to 359 million years ago), arthropods continued to evolve, producing the wide array of characteristics that we see today. Some of these characteristics made it possible for arthropods to become the first animals to live on land, which happened around 400 million years ago.
The arthropod’s ability to adapt has allowed it to proliferate across many different habitats, which is why they comprise so much of the animal diversity today. In fact, there are five subphylas of arthropods: Trilobitomorpha, Hexapoda , Myriapoda, Crustaceans, and Chelicerata.
Classes of Athropods and Examples
This subphylum is where trilobites fit into. Under a single class, there were once 4,000 different species of trilobites. However, as mentioned earlier, they are all currently extinct. Trilobitomorpha lived during the Paleozoic Era (541 to 251 million years ago), and likely died off at the end of it. All trilobites were aquatic, with gill-like branches on their legs to respire. Their legs also had spiny portions that allowed these scavenger-predators to rip food apart. Although the most primitive arthropod, trilobites have the segmented body parts that can be seen in other arthropods.
The animals that we know as insects are in this subphylum, which contains the classes of Insecta and Entognatha. Entognatha are basically primitive insects, but for the sake of simplicity, we will be focusing on Insecta. All insects have three body parts (head, thorax, abdomen) and six legs. More structurally complex than the trilobites, insects have functioning hearts, brains, and digestive systems as well. Insects are also the only arthropods to have wings, although only some species do. The varying morphology has produced over one million known species of insects, which are found all over the world. Some examples include beetles (the most abundant type), ants, bees, and more.
Most insects undergo a process known as complete metamorphosis. This means that the young born from eggs do not resemble the adults. The organisms must go through metamorphosis in order to end up in their adult state. The most common depiction of this is in butterflies, where the larvae (caterpillars) hatch from the egg and grow, eventually making a pupa (chrysalis) for itself. Inside the pupa, chemical changes occur. This makes it so that when the organism emerges from the pupa, it resembles the adult, not the larvae. A less common form of metamorphosis is incomplete metamorphosis, where the organisms hatched from the eggs are already miniature versions of the adult. This tends to happen in more primitive species.
The 13,000 species in this subphylum include centipedes and millipedes. In fact, the name of the subphylum means many feet. The classes Symphyla and Pauropoda resemble centipedes, but only the class Chilopoda includes actual centipedes. The class Diplopoda is reserved for millipedes. So how are centipedes and millipedes different? The most obvious disparity is their leg count. Centipedes have two legs per segment, while millipedes have four legs per segment. Centipedes are also usually carnivorous, with venomous spikes and a flattened body that allows them to move faster to catch prey. On the flip side, the herbivorous millipedes have rounder bodies, although they are usually poisonous to avoid being eaten by predators.
Likely the most tasty arthropods, crustaceans include lobsters, shrimp, crabs, and barnacles. While most are aquatic, a few species (out of 44,000) live on land. Crustaceans are the only subphylum of arthropods to have two pairs of antennae, and they also have definite brains within their calcium carbonate-enforced shells. Some species also have claws and spines to protect themselves and attack prey. In order to grow, crustaceans must molt, which leaves them vulnerable as they wait for their new shells to harden.
With two body parts (cephalothorax and thorax) and eight legs, members of Chelicerata include spiders, scorpions, and daddy long legs. The most prominent class is Arachnida, which include mites, spiders, scorpions, and ticks. What makes Cherlicerata similar is that they all have chelicerae, which are special mouth parts. Additionally, they have pedipalps, which are modified appendages whose function varies between species. For example the pedipalps of spiders are used to sense to world, like antennae, while scoprion pedipalps are the claws that we can see. Fun fact: most scorpions with large claws have less potent venom compared to their smaller-clawed counterparts, although there are exceptions.
While commonly seen as nasty bugs or pests, arthropods are absolutely necessary for our environment and livelihoods. For example, bees and butterflies help us by pollinating different plants they visit. Without them, we wouldn’t have apples, coffee, peaches, strawberries, and tomatoes, just to name a few. Additionally, arthropods such as ladybugs and spiders help reduce pest populations. Ladybugs eat aphids, which are notoriously known to eat agricultural crops, while spiders help by catching disease-spreading mosquitoes in their webs.
Even though we reap tremendous benefits from certain arthropods, the increased use of pesticides can be detrimental to them, which is why it is important to be aware of these consequences. There are so many arthropods in the world, some that we do not even know about yet, and we should strive to make sure that we are not driving them to extinction.
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