Arthropods

The phylum Arthropoda is the largest phylum in the animal kingdom. Estimates of the number of distinct species in the phylum range from 1.17 to 10 million. Eighty percent of all Earth's species belong to the phylum Arthropoda. The phylum Arthropoda consists of myriad animals belonging to the subphyla Trilobita (extinct trilobites with biramous antennae, probably all marine), Chelicerata (horseshoe crabs, scorpions and pseudoscorpions, spiders, harvestmen, mites, ticks, and marine pycnogonids; all lacking antennae), Crustacea (all with biramous antennae), and Uniramia (insects, centipedes, millipedes; all with uniramous antennae). All these subphyla are united by a common body plan similar to that of the annelids, in which the body comprises a series of repeating segments, each bearing a pair of appendages. In contrast to annelids, however, arthropods have fused and modified segments for specialization. This fusion of groups of segments is known as tagmatization. The names for specific tagmata vary from subphylum to subphylum, but they are most commonly recognized as the head, the thorax, and the abdomen. Appendages consist of articulated joints, moveable by muscles that insert onto specialized structures called apodemes attached to the cuticle.

Arthropods are found in all terrestrial and freshwater habitats and on all continents. They are also located in all marine realms, including abyssal depths and hydrothermal vents, and are the only invertebrates to have conquered the aerial realm. Because of their widespread occurrence and ability to occupy all niches on earth, they are considered the most successful animal phylum ever.

External Structure and Function

The first and last segments of the arthropod body differ in embryological origin from all the rest and are considered special segments. The first is called the acron and is preoral (lies before the mouth). The last is the telson and is postanal (occurs after the anus). An increase in segment numbers takes place in a growth zone immediately in front of (anterior to) the telson. Thus, the oldest segments are toward the head, while the youngest are toward the telson. The exoskeleton of each segment consists of two thickened plates, which are connected by a membranous cuticle. The cuticle is a nonliving, secreted material composed of three layers: an inner endocuticle, an exocuticle, and an outer epicuticle, which is usually waxy. Despite this, the cuticle has many living structures that penetrate it and project beyond its surface. Chemical and mechanical sensory organs (setae) generally emerge as hollow projections from the cuticle and are typically hairlike in structure. Gland pores also penetrate the cuticle.

The cuticle is secreted by the epidermis and becomes hardened during a process of protein tanning and can be further strengthened by depositing calcium carbonate in the endocuticle (in Crustacea and Diplopoda). However, this hardened structure restricts growth and must be shed in a process called ecdysis (commonly called molting) for the animal to grow larger. Ecdysis is under neural and hormonal control. In preparation for ecdysis, the old cuticle is partially digested, particularly at joint locations and along specific lines of weakness. A flexible, untanned new cuticle is laid down under the old cuticle, and then the old cuticle cracks along the weak points, so the animal can emerge. The period between successive molts is known as intermolt, and the animal in each of these periods is called an instar; during this time, cellular growth within the confines of the exoskeleton occurs. Some species of arthropods have terminal molts; that is, they molt a specific number of times and then cease all molting and growth. Other species retain the ability to molt and grow throughout their entire lives.

Internal Body Plan

The major, tubular blood vessel (heart) is dorsal, with blood flowing toward the head rather than toward the body. The circulatory system is greatly enlarged and occupies the space of the coelomic cavity, which is vestigial. In most arthropods, the circulatory system is open, so all internal organs are bathed directly in blood rather than fed by smaller vessels (as is common in closed systems). The heart is pierced by a series of holes (ostia), which close when the heart muscle contracts so that blood can be propelled forward. Upon relaxation of the heart muscle, the ostia open, and blood reenters the heart from the sinuses that feed into the sac surrounding the heart. The blood contains clotting agents, amoebalike cells that act as phagocytic cells to attack pathogens and oxygenating pigments.

Aquatic arthropods possess gills through which blood is circulated for gas exchange. Terrestrial forms have either book lungs or a series of cuticle-lined tubes called tracheas, which branch into smaller and smaller tubes to reach nearly all cells of the body. The tracheas open to the outside of the body via spiracles, which possess valves to control their opening and closing to avoid water loss.

The arthropod gut has three divisions: a cuticle-lined foregut, a midgut, and a cuticle-lined hindgut. The foregut is often specialized for food storage and grinding and is separated from the midgut by a complex valve. The midgut has a large absorptive surface, called either the hepatopancreas or the digestive caeca. The hindgut serves as the area for water resorption and the formation of feces. Food and wastes are moved through the digestive system via muscular action due to the cuticular lining of the foregut and hindgut.

Osmoregulation and excretion of nitrogenous wastes are accomplished via one of two kinds of organs: the Malphigian tubules, or the nephridial coxal, antennae, or maxillary glands, all of which filter the blood for wastes. Marine forms excrete ammonia, while terrestrial forms conserve water through the excretion of uric acid.

The nervous system is of a ladderlike chain form, highly segmental, and centrally located. Two longitudinal ganglia run along the midline of the animal. Paired ganglia arise in each segment, which are connected via lateral cords. The brain, composed of three pairs of ganglia, is found anteriorly above the esophagus; these ganglia then connect to the fourth set immediately below the esophagus. Sensory systems are highly evolved and include chemoreception (taste and smell), mechanoreception (vibration, touch, and deformation), and vision. Vision is accomplished via simple eyes (ocelli) or compound eyes, made up of a series of subunits called ommatidia. Compound eyes break up the image before it reaches the retina, and each ommatidium samples only a small part of the complete image.

Reproductive systems and strategies are highly variable. Sexes are usually separate, although hermaphroditism and parthenogenesis are known in some crustaceans and a few insects. Some arthropods display elaborate mating rituals; others do not. Some arthropods use direct copulation, while others use indirect methods. Many arthropods pass through juvenile stages that are highly vulnerable to predators; hence, the reproductive output of most arthropods is great. A few species, however, display direct development, where miniature adults are produced from eggs.

Phylogeny

Arthropods and annelids share a common ancestor, but whether arthropods arose from an annelid ancestor or shared an ancestor from another phylum is unclear. Based on molecular evidence, some argue that annelids and crustaceans are not closely related. Traditionally, arthropods have been treated as a monophyletic group—all arose from a common arthropod ancestor. However, this idea is the subject of debate, with some arguing for a polyphyletic ancestry where each subphylum arose from a nonarthropod ancestor and then formed similar body structures via convergent evolutionary forces. Unfortunately, the fossil record does little to illuminate the phylogeny of arthropods, and current molecular evidence is contradictory.

Due to the size of the phylum, scientists continue to make new discoveries regarding arthropods. In 2022 alone, over 300 new species were identified, and eighty-five were wasps, which are arthropods. Over 1,000 species of arthropods have been discovered in the Appalachian region of the United States, and many are new. Fossils discovered in Morocco indicate that arthropods were dominant in the world’s oceans 470 million years ago, allowing scientists to better understand the development of Earth’s biodiversity. Arthropods preserved in amber deposits help paleontologists better understand the evolution of animals on Earth before dinosaurs as well. Discoveries made about arthropods have been instrumental in understanding how to best protect farmland and crops from other insects considered pests. 

Principal Terms

Biramous: Having two rami; antennae are bifurcated

Cuticle: A noncellular, secreted body covering

Ecdysis: Molting; the process of removing (escaping from) the old exoskeleton

Exoskeleton: A protective system of external layers and joints that allows for antagonistic action of muscles

Rami: The branches of an arthropod limb or appendage

Setae: Hairlike organs, typically sensory in nature, arising from the cuticle

Tagmatization: Functional specialization of groups of segments

Uniramous: Having only one ramus; antennae appear as a single, nonbifurcated structure

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