10 Facts About Arthropods

There Are Four Main Arthropod Families

Naturalists divide modern arthropods into four large groups: chelicerates, which include spiders, mites, scorpions, and horseshoe crabs; crustaceans, which include lobsters, crabs, shrimps, and other marine animals; hexapods, which includes millions of species of insects; and myriapods, which include millipedes, centipedes, and similar organisms.

There is also a large family of extinct arthropods, the trilobites, which dominated marine life during the later Paleozoic Era and have left numerous fossils. All arthropods are invertebrates, meaning that they lack the characteristic backbones of mammals, fish, reptiles, and amphibians.

Arthropods Account for 80 Percent of All Animal Species

Arthropods may not be very big, but at the species level, they vastly outnumber their vertebrate cousins. There are about five million arthropod species alive on earth today (give or take a few million), compared to about 50,000 vertebrate species. Most of these arthropod species consist of insects, the most widely varied arthropod family; in fact, there may be millions of undiscovered insect species in the world today, in addition to the millions we already know about.

How hard is it to discover new arthropod species? Well, some astonishingly tiny arthropods are parasitized by even more incredibly tiny arthropods!

Arthropods Are a Monophyletic Animal Group

Just how closely related are trilobites, chelicerates, myriapods, hexapods, and crustaceans? Until recently, naturalists considered the possibility that these families were "paraphyletic" (that is, that they evolved separately from animals that lived hundreds of millions of years ago, rather than having a last common ancestor).

Today, though, molecular evidence demonstrates that arthropods are "monophyletic," meaning they all evolved from a last common ancestor (which will probably remain forever unidentified) that swam the world's oceans during the Ediacaran period.

The Exoskeleton of Arthropods Is Composed of Chitin

Unlike vertebrates, arthropods don't have internal skeletons, but external skeletons—exoskeletons—composed largely of the protein chitin (pronounced KIE-tin). Chitin is tough, but not quite tough enough to hold its own in a millions-year-long evolutionary arms race; that's why many marine arthropods supplement their chitin exoskeletons with much harder calcium carbonate, which they extract from seawater. By some reckonings, chitin is the most abundant animal protein on earth, but it's still dwarfed by RuBisCo, the protein used by plants to "fix" carbon atoms.

All Arthropods Have Segmented Bodies

A bit like modern houses, arthropods have modular body plans, consisting of the head, the thorax, and the abdomen (and even these segments are composed of varying numbers of other segments, depending on the invertebrate family). You can argue that segmentation is one of the two or three most brilliant ideas hit upon by evolution, since it provides the basic template upon which natural selection acts; an added pair of legs in the abdomen, or one less pair of antennas on the head, can mean the difference between extinction and survival for a given arthropod species.

Arthropods Need to Moult Their Shells

At least once during their lifetimes, all arthropods have to undergo "ecdysis," the molting of their shells to allow for change or growth. Usually, with only minimal effort, any given arthropod can shed its shell in a matter of minutes, and a new exoskeleton usually begins to form within a couple of hours. In between these two events, as you can imagine, the arthropod is soft, chewy, and especially vulnerable—according to some estimates, 80 to 90 percent of arthropods that don't succumb to old age are eaten by predators shortly after molting!

Most Arthropods Have Compound Eyes

Part of what gives arthropods their unnervingly alien appearance is their compound eyes, which are composed of numerous smaller eye-like structures. In most arthropods, these compound eyes are paired, either set in the face or on the end of weird stalks; in spiders, though, the eyes are arranged in all sorts of bizarre ways, as witness the two main eyes and eight "supplementary" eyes of the wolf spider. The eyes of arthropods have been shaped by evolution to see things clearly only a few inches away (or a few millimeters) away, which is why they're not nearly as sophisticated as the eyes of birds or mammals.

All Arthropods Experience Metamorphosis

Metamorphosis is the biological process whereby an animal radically transforms its body plan and physiology. In all arthropods, the immature form of a given species, called a larva, undergoes metamorphosis at some stage in its life cycle to become an adult (the most famous example is a caterpillar turning into a butterfly). Since immature larvae and mature adults differ greatly in their lifestyles and diets, metamorphosis allows a species to minimize the competition for resources that otherwise would occur between juvenile and adult forms.

Most Arthropods Lay Eggs

Given the vast (and still undiscovered) diversity of the crustacean and insect kingdoms, it's impossible to generalize about these arthropods' means of reproduction. Suffice it to say that the vast majority of arthropods lay eggs and that most species consist of recognizable males and females.

Of course, there are a couple of important exceptions: barnacles, for example, are mostly hermaphroditic, possessing both male and female sex organs, while scorpions give birth to live young (which hatch from eggs nestled inside the mother's body).

Arthropods Are an Essential Part of the Food Chain

Given their sheer numbers, it's no surprise that arthropods lay at (or near) the base of the food chain in most ecological systems, especially in the deep ocean. Even the world's apex predators, human beings, rely crucially on arthropods: lobsters, clams, and shrimp are a basic food staple around the world, and without the pollination of plants and crops provided by insects, our agricultural economy would collapse. Think about that the next time you're tempted to squash a spider or set off a bomb to kill all the mosquitoes in your back yard!