Introduction to Phylum Arthropoda

The word "arthropod" comes from Greek words meaning "jointed foot." This refers to the jointed legs found on most arthropods.

Superlatives are always used when describing the phylum Arthropoda, which comprises crabs, shrimp, spiders, millipedes, centipedes, and insects.

Arthropods make up more than 85% of all animal species; over a million insect species have been identified, and estimates of the total number of species vary up to 30 million (based on extrapolation from tropical forest studies). The phylum has spread to every environment, including the ocean floor and very high elevations, and its members have incredibly varied morphologies and evolutionary trajectories. Arthropods have enormous biomass; according to some estimates, they have more than all other creatures put together.

Classification of Phylum Arthropoda:

The phylum Arthropoda is commonly divided into four subphyla of extant forms:

1. Chelicerata (Arachnids)
2. Crustacea (Crustaceans)
3. Hexapoda (Insects and Springtails)
4. Myriapoda (Millipedes and Centipedes)

1. Chelicerata (Arachnids)

• They may be found largely on land.
• The abdomen and cephalothorax of the body are distinct.
• There are no antennas.
• There are 13 parts in the abdomen.
• There are four sets of interior appendages on it.
• They use their gills or trachea to breathe.
• In excretion, the Malpighian tubules are helpful.
• Example: Limulus, Aramea

The following classes make up the subphylum Chelicerata:

• Arachnida
• Merostomata
• Pycnogonida

2. Crustacea (Crustaceans)

• They can be parasitic, aquatic, or terrestrial.
• The skull and the area of the thorax known as the cephalothorax are joined.
• Through the gills or the entire body surface, one breathes.
• The body is encased in a solitary, enormous carapace.
• They have five pairs of appendages in addition to two pairs of antennae.
• They expel waste by antennal or green glands.
• They have gonopores as well as a pair of compound eyes.
• Indirect development occurs. Stage of larvae is present.
• Palaemon, Daphnia, etc.

The subphylum Crustacea is divided into six classes-

• Branchiopoda
• Remipedia
• Chephlocarida
• Maxillopoda
• Ostracoda
• Malacostraca

3. Hexapoda (Insects and Springtails)

• They usually inhabit the land.
• The head, thorax, and abdomen are the three distinct parts of the body.
• the pre-segmental acron is on the head.
• Three parts make up the thorax.
• 7–11 parts make up the abdomen.
• There are three sets of appendages on them.
• Its eyes are compound eyes.
• They use their gills and trachea to breathe.
• The excretory organ is the Malpighian tubule.
• The larval stage is present and indirect development is occurring.
• Including Tabernus, mosquitoes, and ants.

The Hexapoda subphylum is organized into two classes:

• Insecta
• Entognatha

4. Myriapoda (Millipedes and Centipedes)

• Most of these are terrestrial.
• The body is segmented into many lengths.
• Two sets of jaws, two pairs of simple eyes, and antennae are all present on the head.
• They have a lot of legs.
• The mouth's top lip is made up of the epistome and labrum, and the lower lip is made up of two maxillae.
• There are two mandibles inside the mouth.
• They exhale through their tracheas and urinate through Malpighian tubules.
• such as Julus and Scolopendra

The following classes make up the subphylum Myriapoda:

• Chilopoda
• Diplopoda
• Pauropoda
• Symphyla

Characteristics of Arthropoda

Arthropods are distinguished by their segmented bodies coated in a tough cuticle that is frequently hardened to create an exoskeleton. Joints are created by flexible cuticle between limb and body segments, which also permits muscles linked to the cuticle to move. The chitin carbohydrate, the second most common polymer on earth after cellulose, is the main component of the cuticle. Arthropods also have a segmented body with bilateral symmetry, a variable number of body segments, and paired appendages that serve various purposes depending on the section of the body from which they are derived.

The central nervous system is made up of an anterior supra-oesophageal center or brain and a ganglionated ventral nerve cord, while the circulatory system is made up of a dorsal heart and vascular compartments that make up a hemocoel. The muscles mostly consist of striated fibers, and ciliated epithelial cells are typically absent.

A rolled-up common Asian millipede (Trigoniulus corallinus) on the green grass.

Anatomy and Physiology of Phylum Arthropoda

Other than squids and other molluscan cephalopods, arthropods have the most intricate internal structures of any group of invertebrates. They belong to the coelomate phylum and have an open circulatory system that carries oxygen that is taken in through the gills, tracheae, book lungs, and/or surface respiration. Aquatic animals typically excrete waste as ammonia. Arthropods have a highly evolved nervous system that includes a cephalic brain and several sophisticated sensory organs, some of which include image-forming eyes. Other than vertebrates and cephalopods, no other living thing has a brain as sophisticated as the most advanced arthropod. As a result, arthropods frequently have a rather complex behavioral repertoire and communicate chemically, optically, and through touch.

The chitinous and protein-based exoskeleton offers support, protection, and a variety of locomotor possibilities and is frequently strengthened with calcium carbonate (walking, burrowing, swimming, and flying, when modified appropriately). Muscle attachments are made possible by internal exoskeleton projections in this mode of movement. For terrestrial stages, the exoskeleton also lowers water loss and offers small to major protection from predators. Unfortunately, the expense of growth was coupled with this beneficial external skeleton. The exoskeleton must be regularly shed during a molting process known as ecdysis in order to increase the body size or change components like appendages. The new exoskeleton must be formed throughout this process, and if the old one is not subsequently devoured, it must also be strengthened by absorbing the necessary salts. Additionally, eating could stop briefly before and after ecdysis. Even if a predator is not waiting nearby to pounce on the momentarily soft shell animal, the actual act of shedding the old skin can be hazardous since removing the body from the old exoskeleton can be difficult and even deadly.

Feeding Behavior of Phylum Arthropoda

In aquatic habitats, arthropods can be found at all trophic levels, from herbivores and detritivores to top aquatic predators. They play a crucial role in the decomposition of dead organic materials, especially leaf litter that enters headwater streams, and they are the main eaters of benthic and planktonic algae. Aquatic vascular plants are consumed by certain species, such as larvae of aquatic moths and burrowing dipteran "miners," although the majority of this organic material enters the food chain as detritus. Because of their size, predaceous arthropods most frequently prey on other arthropods, while tadpoles and tiny fish can occasionally be attacked and eaten by giant beetles, bugs, dragonflies, and decapods. Fish are the main predator and the main consumer of decapods in the majority of temperate zone ecosystems. Crabs and huge Macrobrachium shrimp, however, are frequently the biggest creatures in some tropical streams (particularly on islands), and they may then hunt upon lesser fish! Depending on the relative sizes of the predator and prey as well as the arthropod's mouthparts, the victim is either consumed whole, ripped into bits, or its fluids are sucked out once it has been subdued.

Reproduction and Life History of Arthropoda

Hermaphroditism and even parthenogenesis are possible in freshwater arthropods, which are typically dioecious (having two sexes). In all subphyla of arthropods, courtship and brood care are infrequent and, when they do exist, are often straightforward. Aquatic or semiaquatic larval stages are present in all freshwater arthropods. Most are not as long-lived as adult stages, however, most insects are the contrary. Even pre-adult stages of crustaceans may appear remarkably identical to the adult except in size and elaboration of specific parts (e.g., juvenile crayfish when connected to the mother's abdomen). Crustaceans normally grow as larvae within the egg/embryo rather than as a free-living stage. However, the shape and size of branchiopods and copepods in their nauplius larval stages are very different from those of the adult forms.

Critical evaluation

Links between arthropods and other animal phyla, as well as relationships within the phylum, remain unclear. For a long time, it was thought that arthropods and annelids were closely related, with arthropods perhaps deriving from annelid ancestors or vice versa. That notion is questioned by contemporary assessments, which contend that their similarly segmented body designs would have had to develop separately.

Many of the relationships inside the group are also in a state of flux. For instance, it is generally accepted that insects and myriapods, which are terrestrial arthropods, are closely related. Both groups could have descended from the same progenitor. On the other side, growing molecular evidence links the myriapods to arachnids and horseshoe crabs and ties insects more closely to other crustaceans and crabs.

Additionally, the Arthropoda have integrated several animal species. Pentastomids, a genus of parasitic worms, is an example of current examples of highly modified crustaceans. The microscopic water bears (tardigrades), which are closely linked to arthropods, and the onychophorans, which include Peripatus, will likely stay in one or more different phyla, on the other hand, are two additional groups of creatures.