Arthropods are an extremely diverse group of invertebrates belonging to the phylum Arthropoda. The term ‘arthropod’ originates from the Greek words’ arthron,’ meaning joint, and ‘podos,’ meaning foot, referring to their jointed appendages. It is the largest phylum in the kingdom Animalia, comprising an estimated 5 to 10 million species (almost 80 percent of all known species on Earth.)
The bodies of these invertebrates are divided into a series of repetitive segments called metameres or somites (metameric segmentation), both internally and externally. A chitinous exoskeleton covers the external segments called the ‘cuticle,’ which is shed during molting. They are adapted to inhabit almost all habitats, from deep seas, high mountains, and deserts to glaciers.
Although most terrestrial arthropods are tiny, aquatic ones are considerably larger. For example, most extinct giant water scorpions were as long as 1.8 meters (5.9 feet). The largest known arthropod species, also a water scorpion, is the fossil Jaekelopterus rhenaniae, which measured nearly 2.5 meters (8 feet) in length.
While large terrestrial arthropods, like adult insects and spiders, do not usually exceed 100 gm (0.22 lbs), sometimes the larvae of Megasoma actaeon, a variety of rhinoceros beetle, weigh more than 200 gm (0.44 lbs). In contrast, the smallest arthropods include beetles of the family Ptiliidae, some parasitic wasps, and tiny mites that measure less than 0.25 mm (0.01 inches) in length. Some giant arthropods, like the Japanese spider crab and the American lobster, weigh around 20 kg (44 lbs).
Arthropods have their bodies externally segmented into either three (in insects), two (in spiders), or no tagmata (in mites). They also bear two extended segments: an ocular somite in the front and a telson at the end, although both are not considered true segments. Their bodies are also segmented internally, with nervous, muscular, circulatory, and excretory system components found in each repeated segment.
Every appendage-bearing segment originally had two distinct pairs of legs: an upper, unsegmented exite and a lower, segmented endopod. However, with time, these two fused into a single pair of biramous appendages, anchored together by a basal structure called a protopod. However, insects, centipedes, and millipedes have retained their single-branched (uniramous) appendages. Some arthropods, like crustaceans and extinct trilobites, have an additional segment known as the exopod (external branch) in their appendages.
In almost all arthropods, the appendages of certain segments undergo functional modification to form gills, antennae, claws, or feeding apparatus. They may also be completely lost from certain segments, such as the abdomen.
The cuticle has three underlying layers: the outer, waxy epicuticle, the chitinous exocuticle, and the inner endocuticle. The exocuticle and the endocuticle are together referred to as the procuticle. In aquatic and some terrestrial crustaceans, the cuticle contains minerals, like calcium carbonate (biomineralization), that help fortify the exoskeleton for defense.
Arthropods have open circulatory systems with a dorsal tubular heart and a network of short, open-ended arteries circulating the body fluid (hemolymph) through the body cavity (hemocoel). A series of paired ostia run along the heart that act as non-return valves, allowing blood to enter the heart but preventing its untimely release.
Many crustaceans, chelicerates, and tracheates use a copper-based respiratory pigment called hemocyanin to transport oxygen to different body tissues. Interestingly, a few crustaceans and insects also possess hemoglobin, the vertebrate respiratory pigment, instead of hemocyanin. As with most invertebrates, these pigments remain dissolved in the hemolymph and are rarely transported in corpuscles like in vertebrates.
Respiratory structures vary among different arthropods, such as book lungs in arachnids, gills in crustaceans, and tracheal systems in insects and myriapods.
Small, external openings called spiracles lead into the tracheoles (terminal endings) that are deeply seated in the tissues and even in muscle cells.
The digestive tract’s structure varies among different arthropods based on their dietary choices and feeding mode. However, the midgut is the most important section of the tract, producing digestive enzymes and absorbing already-digested food. In most arthropods, these enzymes are passed to the foregut, but in spiders, they are also transferred outside the body to the preoral cavity.
Aquatic arthropods, like crustaceans, eliminate nitrogenous waste in the form of ammonia, primarily through their gills. In contrast, terrestrial arthropods, like insects, myriapods, and arachnids such as spiders and mites, possess Malpighian tubules that filter out waste as uric acid from the hemolymph and dump it into the hindgut for excretion as feces.
The brain and the paired ventral nerve cords constitute the backbone of the nervous system of all arthropods. While the brain is located above the esophagus, the nerve cord runs all along the length of the body, with each body segment containing a pair of ganglia connected by a large bundle of nerves (commissures), which makes the nervous system look ‘ladder-like.’
Ganglia from the acron and one or two of the foremost segments of the head constitute the brain. In insects, ganglia from the other head segments combine into a pair of subesophageal ganglia lying under the esophagus, whereas in spiders, all the segmental ganglia fuse, occupying the maximum space of the cephalothorax.
While some arthropods have simple eyes or pigment cup ocelli that can only detect the direction of light, most of them possess compound eyes comprised of fifteen to thousands of hexagonal sensory units called ommatidia. Each of these units contains photosensitive cells, lenses, and a cornea. Insects, like bees, also have receptors for various wavelengths of light that facilitate color vision.
Until the late 20th century, scientists argued for the polyphyly of arthropods, indicating that their members arose from different ancestral lineages. The polyphyletic theory gained support due to the observed differences among its members, especially in segment arrangement, types of limbs, the structure of compound eyes, and the chemical processes used for cuticle hardening.
However, after remarkable advances in molecular phylogenetic studies during the 1990s, arthropods were finally placed within the broader superphylum, Ecdysozoa, characterized by species that typically undergo molting. This group also includes nematodes, priapulids, and tardigrades, excluding annelids.
Such classification indicates that the segmentation in arthropods and annelids has either arisen independently or from an extremely old (thus inconsiderable) common ancestor.
Around 1,170,000 species are currently divided into four major groups or subphyla and a separate extinct one.
While many crustaceans live in depths as low as 4,000 meters (13,100 feet), arachnids like jumping spiders and hexapods such as collembolans thrive at heights exceeding 6,700 meters (22,000 feet) in Mount Everest. Mites, like oribatids, thrive in the cold Antarctic region, whereas tiger beetles are often found in boiling hot springs.
Most insects live for less than a year, whereas crustaceans, like American lobsters, may live as old as a hundred years.
While most arthropods are distinctly male or female, some species, including barnacles, are hermaphroditic, possessing both male and female reproductive organs within a single individual. Usually, reproduction occurs sexually, but some species, like bees and aphids, reproduce asexually (without fertilization) through parthenogenesis.
While aquatic arthropods, like horseshoe crabs, get fertilized both internally and externally, all terrestrial arthropods solely resort to external fertilization. Most terrestrial males transfer their sperm indirectly through spermatophores (packets of sperm), which the females receive using their genitals. In arachnids like scorpions and pseudoscorpions, the spermatophores are deposited on the ground, where the females are either attracted by pheromones or courted by the males. On the other hand, crustaceans, millipedes, and insects like hemipterans and dipterans directly transfer sperm to females instead of packing them in spermatophores.
Most arthropods lay yolk-laden eggs (oviparous), but scorpions retain the eggs in their mother’s body until they are ready to hatch (ovoviviparous). After hatching, the newborns may take multiple forms, such as caterpillars in butterflies, hatchlings in silverfish, or nauplis larvae of crustaceans.
Since the exoskeleton cannot be stretched, a newborn’s growth requires constant shedding of the old cuticle and its replacement by a new one, a process known as molting or ecdysis. At the onset of this process, an arthropod stops feeding, and its epidermis releases an enzyme-rich fluid that digests the old endocuticle. This digestion starts after the secretion of a new epicuticle has begun, following which the arthropod pumps in large quantities of water. As its body swells, the old cuticle splits along the areas where it was the thinnest and exposes the fresh cuticle. At this point, the new layer is extremely soft and gradually hardens with time. This cycle of shedding and growth is repeated through multiple instars (developmental stages) until the arthropod reaches sexual maturity.
Arthropods are preyed upon by birds, frogs, lizards, and mammals, like bats and opossums. They are also attacked by some species of their own phylum, such as spiders and insects.
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