Arthropods, including insects, arachnids, crustaceans, and millipedes, are a diverse and successful phylum with a segmented body and appendages. They make up 80% of animal species, have a dorsal heart and ventral nervous system, and are covered in an exoskeleton. Oxygen levels limit their size, but they have existed since the Cambrian period and pursue various food strategies. They thrive in heat and humidity and are likely nearby, with spiders being the most common.
Arthropods are an important category of animals, which includes insects, arachnids, crustaceans, millipedes, and a few others. In Greek, arthropoda means “jointed foot,” a reference to one of the phylum’s defining characteristics: a segmented body with appendages. Arthropods are the most successful and diverse animal phyla, in terms of number and ubiquity. They make up about 80% of all animal species and over a million have been given scientific names.
All arthropods have a dorsal heart (top) and a ventral nervous system (bottom). They are covered in an exoskeleton made of chitin, a rigid polysaccharide that acts as armor and protects arthropods from desiccation. As an arthropod grows, it has to shed its layers. In today’s world, arthropods tend to be relatively small, generally smaller than half a meter, although some, such as the Japanese spider crab, have leg spans of up to 3.5 m (12 ft). The smallest arthropods are plankton around 300 microns in size.
Some ancient arthropods, such as Jaekelopterus, a water scorpion, and Arthropleura, a relative of millipedes, reached a size of 3 m (10 ft) and weighed more than today’s largest arthropod. In general, oxygen levels are the limiting factor on arthropod size, as their open circulatory systems are not as efficient at circulating blood as the closed circulatory systems of many other animals. During the Carboniferous period, 300 million years ago, the Earth had higher oxygen levels, which allowed for huge arthropods, especially insects, such as the dragonfly’s enormous relative, Meganeura.
Arthropods are a very ancient phylum, dating back to the Cambrian period (535 million years ago) and possibly even earlier. Trace fossils produced by arthropods date back to 535 million years ago, and Spriggina, an enigmatic trilobite precursor that may be an arthropod, dates back to 555 million years ago.
Arthropods thrive on heat and humidity. They pursue every possible food strategy, from herbivore to carnivore, from scavenger to parasite. As one of the most numerous life forms on Earth, you’re probably within a few feet of some arthropods right now, most likely spiders.
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