50 Fun Facts About Arthropods

Which sea creature's bright blue blood is harvested to test medicines for dangerous bacterial toxins?

The horseshoe crab has blood containing a chemical that reveals bacterial toxins, which drug makers rely on. Despite the name, the animal is closer to spiders than to true crabs.

Which creature that rolls into a ball under garden logs is a crustacean rather than an insect?

The pill bug is a woodlouse, a land-living relative of crabs and shrimp. It loses water quickly, which is why it keeps to damp, dark places such as the underside of rocks.

Which blood-feeding parasite that spreads Lyme disease is an eight-legged arachnid, not an insect?

The tick belongs to the same class as spiders and mites, and an adult has eight legs. Fleas, lice and bedbugs are all six-legged insects.

The first animal ever found with more than a thousand legs lives deep underground in which country?

Australia produced the record-breaker from mining drill holes up to 60 meters deep: an eyeless, thread-like animal with as many as 1,306 legs. The previous record was 750.

Meganeura, an extinct insect with a 70-centimeter wingspan, resembled which living insects?

Dragonflies are the closest living look-alikes of Meganeura, which hunted other insects about 300 million years ago. The biggest modern species is far smaller.

Bagheera kiplingi, a jumping spider that eats mostly plants, is named after a character from which classic?

The Jungle Book supplied the genus name, after Kipling's black panther Bagheera. The spider lives on acacia trees in Central America and eats protein-rich nubs that the trees produce.

The novelty aquarium pets sold to children as Sea-Monkeys are actually which tiny crustaceans?

Brine shrimp lay eggs that can lie dormant for years until they touch water, which is why a dry packet seems to spring to life. The kits were developed in the United States in 1957.

Spiders have no muscles for straightening their legs, so what do they use to extend them instead?

Hydraulic pressure extends a spider's legs: fluid is pumped in to push each joint straight. That is why the legs of a dead spider curl up.

Which crustaceans, long classed as mollusks, did Charles Darwin describe in a series of monographs?

Barnacles are stuck down by cement glands on the head, so each adult lives fixed upside down by its forehead. Darwin's son assumed that every father studied them at home.

Lobsters, crabs and tarantulas have bluish blood because its key pigment contains which metal in place of iron?

Copper gives the pigment hemocyanin its blue color, in the way iron gives human hemoglobin its red. Without its cargo, hemocyanin is colorless.

Which tough carbohydrate, also found in the cell walls of fungi, forms the exoskeleton of arthropods?

Chitin is the second most abundant polysaccharide in nature, behind only the cellulose of plants. An estimated billion tons of it are made every year.

The word arthropod comes from two Greek words that together describe which feature of the animals?

Jointed leg is the meaning, from 'arthron', a joint, and 'pous', a foot or leg. The term first appeared in anatomical descriptions published in 1832.

What is the scientific name for the shedding of an old exoskeleton that lets an arthropod grow?

Ecdysis leaves the animal briefly helpless: the new exoskeleton is so soft and wrinkled at first that its owner cannot support itself and can hardly move.

Which middle section of an adult insect's body carries all three pairs of legs and any wings?

The thorax holds the flight muscles as well as the legs. Adult insects are the only arthropods that ever have wings, with up to two pairs.

Through which small openings along the sides of the body do most insects take in air?

Spiracles exchange gas less efficiently as a body gets bigger, which is one limit on how large an insect can grow.

What name is given to the blood-like fluid that circulates through an arthropod's body cavity?

Hemolymph in insects mainly carries nutrients rather than the gas they breathe, which reaches the tissues through a separate system of air tubes.

Which breathing organs of spiders are built from stacks of alternating air pockets and thin tissue plates?

Book lungs get their name because the stacked layers look like the pages of a folded volume. They are a land version of the page-like gills of sea-dwelling relatives.

An arthropod's heart pumps blood into body spaces rather than sealed vessels, an arrangement called what?

Open circulation means the organs are bathed directly in blood inside one long body cavity. The heart is a muscular tube running just under the animal's back.

Arthropods share with humans, but not with adult starfish, which type of body symmetry?

Bilateral symmetry means a body with a head end, a tail end and matching left and right sides. Starfish begin that way as embryos, then switch to a five-part layout.

In insects with complete metamorphosis, which stage comes between the larva and the winged adult?

The pupa is the third of four life stages, after egg and larva. A butterfly's is known as a chrysalis.

Insects and springtails both belong to which arthropod subphylum, the largest of them all?

Hexapoda, from the Greek for six legs, takes in the true insects plus three small groups of wingless animals that were once counted among them.

Which mouthparts, shaped as fangs or pincers, give spiders, mites and their relatives their subphylum name?

Chelicerae in nearly all spiders are hollow fangs connected to venom glands, used to inject venom into prey or into anything that threatens them.

Which predators, with one pair of legs per body segment, inject venom through modified front legs?

Centipedes never have exactly 100 legs, despite the name: every species has an odd number of leg pairs, from 15 up to 191.

Arthropleura, the largest land arthropod of all time, was a two-meter ancient form of which animals?

Millipedes of this giant kind left fossil trackways up to 50 centimeters wide. Arthropleura reached at least 2 meters in length, and possibly more than 2.5.

Which extinct marine arthropods, common as fossils, died out in the mass extinction that ended the Permian?

Trilobites lived in the oceans for almost 270 million years, and more than 22,000 species have been described. Only a handful were left when the end came.

Which long-legged arachnids, often mistaken for spiders, have no venom, no silk and a body like a single oval?

Harvestmen are the subject of an urban legend that calls them the most venomous animals alive. In fact none of the 6,650-plus known species has venom glands at all.

Genetic studies show that termites, once given their own order, are a social branch of which insects?

Cockroaches turn out to include termites deep inside their family tree, with wood-eating roaches as the nearest kin. Termites are not ants, despite the old nickname white ants.

Which insects, with about 400,000 described species, make up roughly a quarter of all known animals?

Beetles are set apart by front wings hardened into protective cases. Estimates of the true number of species run from 0.9 million to 2.1 million.

Which soft-bodied close relatives of arthropods snare their prey by squirting sticky slime?

Velvet worms, named for their velvety texture, squirt a milky glue from glands beside the mouth. About 232 species have been described, though many more probably exist.

Which branch of zoology, sometimes confused with the study of word origins, is devoted to insects?

Entomology once covered spiders, crustaceans and other arthropods as well, because the word insect used to be applied far more loosely.

Which crustacean has the greatest leg span of any arthropod, reaching about 3.7 meters from claw to claw?

The Japanese spider crab disguises itself by sticking sponges and other animals to its shell. At up to 19 kilograms, it is the second-heaviest living arthropod.

Which crustacean, the largest arthropod living on land, has been filmed climbing trees?

The coconut crab is a giant hermit crab that stops carrying a borrowed shell once it grows up. An adult will drown if it is held under water for too long.

Which tarantula is the largest spider in the world by mass, though only second by leg span?

The Goliath birdeater rarely eats birds: the name goes back to an 18th-century engraving by Maria Sibylla Merian that showed a tarantula eating a hummingbird.

Which shrimp-like crustaceans form Antarctic swarms with a total biomass similar to that of all humans?

Krill of the Antarctic species have been estimated at roughly 500 million metric tons in all, which makes it one of the most abundant animals on the planet.

Which marine crustacean has eyes with 12 to 16 types of light-detecting cell, against four in humans?

The mantis shrimp is also a violent hunter: some species club their prey so fast that vapor-filled bubbles form in the water around the blow.

Males of which crustacean wave one hugely oversized claw in a beckoning display to win over females?

The fiddler crab is so named because its small and large claws together suggest a fiddle. A male that loses the big claw grows a new one on the same side after his next molt.

Where in the Indian Ocean do millions of red land crabs carpet the ground each year as they march to the sea?

Christmas Island once held an estimated 43.7 million adult red crabs. An invasive ant introduced by accident is thought to have killed about a third of them.

Which arachnids, found on every continent except Antarctica, glow under ultraviolet light?

Scorpions carry fluorescent chemicals in the exoskeleton. The oldest fossil one found so far lived during the Silurian period in what is now Scotland.

In Ovid's telling, Arachne became a spider after challenging the goddess Minerva to a contest in which craft?

Weaving was Arachne's gift, and her tapestry mocking the gods so enraged Minerva that the goddess beat her with a shuttle. Her name is the Greek word for spider.

Which term, from the Greek for a body of soldiers, describes a body region made of grouped arthropod segments?

A tagma may have its segments fused together, as in an insect's head, or left free to move independently, as in the abdomen of most insects.

What are the tiny independent units, each with its own lens, that together make up an insect's compound eye?

Ommatidia can number in the thousands in a single eye, each pointing in a slightly different direction. The picture the animal sees is a combination of all their inputs.

Which organs opening into an insect's gut filter waste from the body fluid, doing the job of kidneys?

Malpighian tubules are named after Marcello Malpighi, a seventeenth-century anatomist. The waste ends up as uric acid mixed into the droppings, once the rectum has reclaimed the water.

In flies, the hind wings have evolved into which small club-shaped organs that sense rotation in flight?

Halteres beat along with the wings and work like vibrating gyroscopes, picking up small changes in body position. The name comes from weights that Greek athletes held when leaping.

What term describes the way non-crab crustaceans have repeatedly evolved a crab-like body plan?

Carcinization was named in 1916 by Lancelot Borradaile, who summed it up as nature's many attempts to evolve a crab. King crabs, for example, arose from hermit crabs.

Opabinia, a five-eyed Cambrian animal on the arthropod family tree, was found in which fossil deposit?

The Burgess Shale of British Columbia has yielded every known specimen. Opabinia looked so strange that the audience laughed when a full analysis of it was first presented.

A long-debated hypothesis links giant prehistoric insects to unusually high atmospheric levels of which gas?

Oxygen was first put forward as the reason in 1911 by a railway engineer, Édouard Harlé. Doubters point out that giant insects were still flying after its levels had dropped.

Which appendages on a spider's abdomen push out silk made by as many as six different types of gland?

Spinnerets are covered in microscopic spigots, each producing a single filament, so a spider can combine filaments into different kinds of silk for different jobs.

Mature male spiders transfer sperm to females using a structure on which second pair of front appendages?

Pedipalps also serve male spiders in courtship, where they are used to shake the web, make sounds or put on visual displays.

Unlike a vertebrate's spinal cord, an arthropod's ladder-like nerve cords run along which side of the body?

Ventral means the belly side, the reverse of the vertebrate layout. The paired cords swell into a pair of nerve knots, called ganglia, in every segment.

Crabs, lobsters, shrimp and crayfish all belong to which class of about 40,000 crustacean species?

Malacostraca means soft shell, which is a misnomer: the shell is soft only just after the animal has shed its old one, and is usually hard.