Arthropod

Arthropods . Invertebrates that have an articulated chitin exoskeleton . They include trilobitomorphs , merostomes , pycnogonids , arachnids , crustaceans , myriapods and insects . They have had great evolutionary success, as proven by the fact that more than 80% of all known animal species belong to this group.

Summary

[ disguise ]

• 1 Origin
• 2 Features
• 3 Exoskeleton
  • 1 Layered structure
  • 2 Ecdysis or molt
• 4 Digestive system
• 5 Breathing
• 6 Circulation
• 7 Excretion
• 8 Nervous system
• 9 Senses
  • 1 Eye models
• 10 Playback
  • 1 Types of development
• 11 Phylogeny
• 12 Taxonomy
• 13 Classification
• 14 Sources

Origin

It may be that the first arthropods were similar to their presumed ancestors, the annelids . Its body was long and soft and had many segments, all of them very similar and equipped with a pair of legs.

Subsequently, the surface of the body hardened to form an external skeleton ( exoskeleton ) or cuticle that contains chitin , proteins , lipids and calcium salts .

Characteristics

Arthropods constitute one of the great divisions of the animal kingdom, subdivided into various classes, some of which have a large number of genera and species .

They are called this way because they are provided with articulated legs. In reality, it is not just the legs, but the entire body that is made up of several segments joined together by joints.

Despite their variety and disparity, arthropods have fundamental morphological and physiological characteristics in common:

• Presence of articulated appendages that show enormous evolutionary plasticity and that have given rise to the most diverse structures (legs, antennae), gills , lungs , jaws , chelicerae , etc.
• Presence of an external skeleton or chitinous exoskeleton that they shed periodically. Since various pseudocoelomate phyla also shed the cuticle, some authors relate arthropods to nematodes and related groups, in a clade called ecdysozoans .
• Body made up of repetitive segments, a phenomenon known as metamerism , whereby the body appears built by repeated modules along the anterior-posterior axis. Segmentation is accompanied by regionalization or tagmatization, with division of the body into two or three regions in most cases.

Due to this character, they have been traditionally related to annelids, which are also metamerized animals; but defenders of the ecdysozoan clade argue that it is a case of evolutionary convergence.

exoskeleton

The exoskeleton of arthropods is a continuous covering called cuticle, which extends even across both ends of the digestive tube and through the respiratory tracts or cavities, and is located above the epidermis (called in them for this reason hypodermis), who is the one who secretes it.

The composition of the exoskeleton is glycopeptide (with a carbohydrate part and a peptide part). The main and most characteristic component belongs to the first of these two types, and is chitin, a polysaccharide derived from the amino sugar N-acetyl-2-D-glucosamine that is also found, for example, in the cell wall of fungi.

In many cases, the consistency of the exoskeleton gains due to the addition of mineral substances, as in the case of crabs and other decapod crustaceans whose cuticle appears calcified, due to the deposit of calcium carbonate.

The thickness and hardness of the cuticle is not the same throughout its entire length. On the contrary, it appears forming hardened areas called sclerites, separated or joined together by thinner and more flexible areas.

The sclerites receive complex names that vary in each group, but those located dorsally are generally called tergites, sternites those located ventrally, and pleurites those laterally.

There may also be ridges of the exoskeleton developed inwards called apodemes, on which the muscles are inserted. The cuticle is also usually crossed by pores.

Layered structure

The exoskeleton is structured in layers:

• Epicuticle. Very thin, stratified in turn and with hydrophobic properties that give it a waterproofing function. It is composed of proteins and lipid substances such as waxes. Where it is thinner, the exchange of substances is facilitated, for example perspiration.
• Procuticle. It is the main and thickest part of the cuticle. It is made up of two layers:
• Exocuticle: This part is the most uneven in thickness and the most rigid. Its hardness derives from the presence of phenolic compounds that bind to the other polymers. It is abundant in the sclerites and is thinner or absent in the articulation areas.
• Endocuticle. Thick but at the same time flexible and more uniform in thickness than the exocuticle.

The cuticle very frequently appears covered with setae (hairs) of various functions, including tactile sensory functions.

The coloration of arthropods usually depends on the cuticle. Colored pigments or guanine crystals are deposited in the procuticle . The epicuticle may have fine striations that produce physical (not chemical) colors, such as the metallic or iridescent appearance of many insects.

Ecdysis or molting

The external skeleton has a disadvantage and that is that, in order to grow, the animal must get rid of it. It does so in a hormonally controlled process of ecdysis or molting.

The hypodermis secretes enzymes that partially soften and digest the lowest layer of the cuticle (the endocuticle), causing the rest to fall off.

The secretion of a new cuticle immediately begins, first the exocuticle and then, below it, the procuticle. Until this new covering hardens, the animal is relatively defenseless, with less possibility of escape or resistance.

The entire molting process is hormonally controlled; Ecdysone or “molting hormone” is the hormone responsible for these changes to occur .

The successive phases of the animal’s existence between molt and molt are called stages or instars. This trait is shared by arthropods with some other phyla, such as nematodes, which also have a cuticle and molt; There is a theory that now classifies them together in a subkingdom Ecdysozoa.

Digestive system

The digestive system of arthropods is divided into three distinct regions, the stomodeum, the mesodeum and the proctodeum. Stomodeum and proctodeum are the regions located at the anterior and posterior ends, respectively; They are of ectodermal origin so they are covered with cuticle that is renewed every time the animal molts.

The middle part of the digestive tube: the mesodeum, derives from the endoderm (second blastodermal sheet) and is what produces digestive secretions and where most of the absorption of nutrients takes place; It frequently presents derivations or lateral cecums that expand its surface.

Breathing

Many arthropods are too small to have or need respiratory organs. Aquatic arthropods, which include the first members of the group, usually have gills, appendages that are more internally vascularized than other organs.

They are found in crustaceans, as specializations of the dorsal branch of the thoracic appendages, and in the same way in xiphosurs or in eurypterids or the first fossil scorpions. Secondary gills (derived from tracheae) are also found in the aquatic larvae of some insects, such as mayflies.

As is general in animals, members of the aerial life group breathe through internalized organs, which in arthropods can be of two types:

• Tracheas. Insects, some orders of arachnids, myriapods or scale insects (crustaceans of the order isopods adapted to terrestrial life) have a network of ducts (tracheal system) that communicate with the outside through holes called spiracles, frequently equipped with openings. provided with valves that regulate their diameter.

The cuticle extends over them in a very thin and permeable version, which in any case comes off when the molt arrives. In some cases, active ventilation is observed, with cyclical movements of inspiration and expiration.

• Lungs in book. They have a highly folded internal structure (book lungs), which multiplies the surface area through which gas exchange takes place, and they open to the outside through their own independent openings. Book lungs are found in several orders of arachnids, most notably spiders and scorpions.

Circulation

The circulatory system of arthropods is open. That is, there is no closed circuit of vessels through which a differentiated liquid circulates, which could properly be called blood.

What exists is a pumping motor that is a specialized vessel in a dorsal position, called the dorsal heart, which moves the internal body fluid, hemolymph, which it receives from open posterior vessels and propels forward through equally open vessels.

The network of vessels is always poorly developed, except in the gills of aquatic arthropods. There are no blood cells specialized in oxygen transport, although, as in all animals, there are amoebocytes (amoeboid cells) with functions of cellular immunity and hemostasis (coagulation and healing). Yes, there may be respiratory pigments, but dissolved in the hemolymph.

Excretion

Crustaceans have antennal and maxillary glands for excretion, at the base of these appendages. Arachnids usually have coxal glands, which open at the base of the locomotor legs. In insects and myriapods, characteristic tubular organs appear, called Malpighian tubes or ducts, which open between the midgut and the hindgut (proctodeum); Their products add to the composition of the feces.

Terrestrial arthropods are usually uricotelic, that is, they do not produce ammonia or urea for nitrogen excretion, but rather uric acid or sometimes guanine.

In arthropods, excretion by accumulation is common, as an alternative or complement to excretion by secretion. In this case, excretion products accumulate in nephrocytes, pericardial cells or directly in the cuticle. The accumulation is usually of urates or guanine, very poorly soluble nitrogenous bases that form solid deposits. In the latter case, the molts serve the added function of getting rid of these excreta.

Nervous system

As corresponds to Protostomes , the nervous system develops on the ventral side of the body and, as corresponds to metameric animals, its organization is segmental.

In each segment there is a pair of ganglia, more or less ventrolateral in position, with the two ganglia of one pair soldered or joined by a transverse commissure and those of consecutive pairs joined by connective nerves.

Senses

Most arthropods are equipped with eyes, of which there are several different models.

Dragonfly eyes.jpg

eye models

• Simple eyes are spheroidal cavities with a simple retina and covered frontally by a transparent cornea. Its optical performance is very limited, with the exception of the large eyes of some families of spiders, such as salticids.
• Compound eyes are made up of multiple equivalent elements called ommatidia that are arranged radially, so that each one points in a different direction and together they cover a more or less wide angle of vision.

Each ommatidium contains several sensitive cells, retinal, behind transparent optical elements, fulfilling the function that the cornea and lens play in the eyes of vertebrates. There are also cells that surround the ommatidium, sealing it against light. Not all groups have compound eyes, which are absent, for example, in arachnids.

The vision of many arthropods has advantages that are often lacking in vertebrates, such as the ability to see in an extended spectrum that includes the near ultraviolet, or to distinguish the direction of polarization of light. Color vision is almost always present and can be very rich; The crustacean Squilla mantis, the galley, has thirteen different pigments with different sensitivity to color, which contrasts with the poor trichromatic system (three pigments) of most primates, including our species.

Distributed throughout the body, sensilla can be found, which are receptors sensitive to chemical stimuli, such as those of taste or smell, and tactile receptors, associated with antennae and palps and also with tactile setae, hairs that are associated with a sensitive cell. .

Some insects have a sense of hearing, which is revealed by the existence of auditory signals of intraspecific communication, such as in crickets. Many are sensitive to ground vibrations, through which they detect the presence of prey or predators; Others, like flies, have trichobothria capable of perceiving minimal changes in environmental pressure.

Arthropods are usually equipped with simple but effective position sensors that help them maintain position and balance, such as the chordotonal organs that a dipteran has in its halteres.

Reproduction

They always reproduce sexually. The females, after being fertilized by the males, lay eggs. Development from the egg can be direct or indirect.

Types of development

• In direct development, an individual similar to the adult is born, although, logically, smaller in size.
• In indirect development, a larva is born that involves a series of profound changes called Metamorphosis .

There are frequent cases of parthenogenesis, especially in crustaceans and insects. There are also rare cases of hermaphroditism that appear especially in parasitic or sessile species.

Phylogeny

For many decades, the phylogenetic relationships of the Coelomates were based on Cuvier’s conception of the Articulates, a clade formed by Annelids and Arthropods. Numerous modern morphological analyzes based on cladistic principles have corroborated the existence of the Articulates clade, for example, Brusca, Nielsen, among others.

However, various cladistic analyzes are reaching the conclusion that annelids and arthropods are not directly related. The presence of metamerization in annelids and arthropods should therefore be considered as an evolutionary convergence.

On the contrary, these studies propose the Ecdysozoa clade in which arthropods show close phylogenetic relationships with pseudocoelomate groups, such as nematodes, nematomorphs, priapulids and chinorhynchuses, due to the shared presence of a chitinous cuticle and a molting process (ecdysis) of the same.

The phylogeny of Arthropods has been very controversial, with a conflict between those in favor of monophyletism and those of polyphyletism. Snodgrass and Cisne have defended monophyletism, although the first considers the Arthropods divided into Arachnates + Mandibulates, and the second interprets them divided into Schizorrameans and Atelocerates. Tiegs and Manton defended diphyletism, with the Arthropods divided into Schizorrameans + Unirrameans and the Onychophorans as a sister group of Myriapods + Hexapods. Later, Mantón and Anderson maintained the polyphyletism of the group.

With the appearance of the first studies based on molecular data and combined analyzes of morphological and molecular data, it seems that the old controversy about monophyly and polyphyly has been overcome, since all of them corroborate that Arthropods are a monophyletic group in which they also include the Tardigrades (the clade has been called Panarthropods); most also propose the existence of the Mandibulates clade. However, new controversies have arisen, especially around two mutually exclusive alternative hypotheses that are being debated in numerous articles on the phylogeny and evolution of Arthropods: Atelocerates (Myriapods + Hexapods).

Taxonomy

Arthropods form the phylum Arthropoda (Arthopoda), which is divided into four subphylums. The subphylum Unirrámeos (Uniramia) comprises five classes: Diplodos (Diploda), the millipedes; Chilopoda (Chilopoda), centipedes; Pauropods (Pauropoda), small eyeless animals with a cylindrical body that have nine or ten pairs of legs, Symphyla (Symphyla), the garden centipedes; and Insects (Insecta).

The subphylum Crustaceans (Crustaceae), which is primarily marine (although not uncommon on land) and abundant in fresh water, includes animals such as lobsters, shrimps, and crabs. The subphylum Chelicerates (Chelicerata) is made up of three classes that are characterized by presenting the first pair of appendages modified into chelicerae and by lacking antennae. Members of the class Pycnogonidae (Pycnogonida) (sea spiders) have long legs and feed by slurping the juices of marine animals.

The only living representatives of the class merostomata (Merostomata) are pot crabs. The arachnid class (Arachnida) includes animals such as spiders, scorpions, ticks and mites, which are generally terrestrial. Of the groups of extinct arthropods, the best known is the trilobites, which constitute the subphylum Trilobites (Trilobitomorpha).

Classification

Currently, within the group of arthropods we tend to distinguish the euarthropods, or typical arthropods, to which all the general characters mentioned above refer, and the pararthropods (onychophorans, tardigrades and pentastomids), which have a series of characteristics. intermediate between annelids and euarthropods.

Euarthropods are divided into three subphyla: trilobitomorphs, chelicerates and mandibulates. In trilobitomorphs the body is divided into three lobes; In the chelicerates, the first postbuccal appendages, although at the end of embryonic development they are prebuccal, are the chelicerae, appendages that have two or three mobile appendages; In the mandibulated, the first post-buccal appendages are the mandibles, simple pieces with a chewing function.

Within the chelicerates, three classes are distinguished: the merostomes, the arachnids and the pycnogonids. Among the mandibulates we distinguish: crustaceans, insects, symphyles. the pauropods, the diplopods and the chilopods; The common name of myriapods is usually applied to these last four groups.

• Onychophorans. They are terrestrial animals about three centimeters long, looking like a worm with legs. They show external segmentation that does not correspond to internal metamerization. They have chitinous cuticle, so they undergo shedding. They have a pair of antennae, a pair of pincer-shaped jaws and 13 or 14 pairs of non-articulated legs. Respiration is tracheal, excretion is carried out by a pair of nephridia per segment. They live in tropical areas and are carnivores.
• Tardigrades. Most live in mosses. They measure less than a millimeter, have an elongated and non-segmented body and have four pairs of short legs. They feed by sucking the internal liquids of the moss using oral stylets and a suction pharynx. They can withstand desiccation by going through a state of dormant life ( cryptobiosis ) in which the animal adopts the shape of a barrel, and can remain that way for several years.
• Pentastomids. They are parasites of the lungs and respiratory tracts of vertebrates, mainly reptiles. They have an elongated body with successive rings. They have five protuberances: four have hooks and the other opens the mouth. They lack respiratory, excretory and circulatory systems.
• Trilobitomorphs. They are arthropods that lived during the Primary Era, they were marine and benthic, they became extinct about 250 million years ago. They had the body divided into three lobes; In them you can distinguish three tagmas: head, thorax and pygidium. On their heads they had a pair of antennae, a pair of compound eyes, a mouth and four pairs of biramous appendages. The thorax, with a pair of appendages per segment, could be folded on itself.
• Merostomes. They are gill-breathing aquatic chelicerates. They include the eurypterids or giant water scorpions, up to two meters in length, which became extinct during the Primary Era, and the xiphosurs, currently represented by only three genera. The best known species is Limulus polyphemus, commonly called “Moluccan saucepan”.
• Pycnogonids. They are small chelicerate arthropods. They have two regions: the prosoma (junction of the cephalon or head and segmented thorax) and the opisthosoma or abdomen, which is not segmented. The thorax usually has four pairs of long legs, which is why they are called “sea spiders.” The integument is very thin, which allows them to lack a respiratory and excretory system. The intestine has cecums that run along the inside of the legs.