Arthropoda is a phylum of animals that includes many well-known invertebrates such as insects, crustaceans, spiders, centipedes, millipedes and scorpions. The word ‘Arthropoda’ is of Greek origin and translates to ‘jointed foot’. The distinguishing features of arthropods from other invertebrates are their hard exoskeleton or shell and jointed limbs.

How many species of arthropods are there?

Arthropods are the most diverse phylum of animals and more than 1.2 million species of arthropods have so far been described. This accounts for approximately 80% of all animal species and most of the diversity is found in insects. Species of insect and other arthropods are however being lost to extinction every day due to threats such as habitat destruction and invasive species. Many of the world’s arthropods have yet to be discovered and described by science and due to extinction rates currently being very high, many will go extinct before they are.

Where are arthropods found?

Species of Arthropoda have found their way into almost every habitat on Earth. They are hugely abundant in both water and on land although most living arthropods are primarily found on land. Insects are the most abundant group of animals on land-based environments and are also common in many freshwater ecosystems. Crustaceans are found mostly in marine environments, many have also ventured out into freshwater environments and only a few have successfully colonised land.

How did arthropods evolve?

It is believed arthropods evolved from Annelids which are a phylum of invertebrates that includes earthworms, tubeworms and leeches. This divergence is believed to have occurred more than 500 million years ago during the Cambrian Explosion, a period on Earth when many of the current groups of animals first evolved. The evolution of arthropods occurred following the loss of body segments, development of specialised legs and a more developed head.

The exoskeleton of arthropods

The chitinous exoskeleton acts as a form of armor for arthropods. Unfortunately, the chitinous shell is unable to grow and arthropods must moult their exoskeletons seasonally as they would otherwise limit their growth. The exoskeleton also serves as an anchor point for the internal muscles of arthropods which allow the animals to perform precise movements.

Last edited: 15 November 2015

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