Ants, flies, cockroaches, shrimp, crabs, spiders and scorpions are examples of arthropods.
The three main classes of arthropods are: insects (cockroaches, ants, flies, bees, beetles and butterflies), crustaceans (crabs, lobsters, shrimp and barnacles) and arachnids (scorpions, spiders and mites). Other classes are onychophorans (velvet worms), diplopods (millipedes) and chilopods (centipedes).
Select any question to share it on FB or Twitter
Challenge your Facebook and Twitter friends.
Arthropods present three distinguishing features: they are metameric organisms (segmented body), they have an exoskeleton made of chitin and they have articulated limbs.
Arthropods are triploblastic (they have three germ layers) and are coelomate organisms.
Given their metameric structure, arthropods are evolutionarily closer to annelids, as they also have segmented bodies. In the embryonic development of some arthropods the fusion of metameres takes place, forming structures such as the cephalothorax of arachnids, for example.
Phylum Arthropoda - Image Diversity: arthropod cephalothorax
The external shell of arthropods is called the exoskeleton. The exoskeleton of an arthropod is made of chitin, a nitrogen-containing polysaccharide.
Due to the presence of the exoskeleton, the growth of arthropods is periodical. During the growth period, the animal loses the exoskeleton, grows and develops a new exoskeleton. This process is called ecdysis or molting.
Since they have an exoskeleton and periodic ecdysis, the growth of arthropods is limited to lessen the organisms' vulnerability to environmental harm. However, some arthropod species contain relatively large-sized species, such as “giant” cockroaches, crabs and spiders.
In the exoskeleton of arthropods, there is a layer of waterproof wax. This feature was fundamental in allowing primitive arthropods from the sea to survive on dry land without losing excessive water to the environment.
The digestive tract of arthropods is complete, containing a mouth and an anus. Arthropods are protostome animals, meaning that the blastopore produces the mouth, during their embryonic development.
Arachnids can inject poison to paralyze or kill their prey by using structures called chelicerae. The prey is partially digested outside the body of the arachnid by digestive enzymes injected together with the venom or afterwards. After this extracorporeal digestion, the food is ingested and enters the digestive tract of the predator, where the extracellular digestion continues.
As typical aquatic animals, crustaceans contain highly vascularized gills that are in contact with water and permit gas exchange. In terrestrial insects, respiration is tracheal and gases flow inside small tubes that connect the animal's external surface with tissues and cells without the participation of blood. In arachnids, in addition to tracheal respiration, book lungs (thin folds resembling the pages of a book) may also exist.
In arthropods, the impermeability of the exoskeleton makes the passage of gases through it difficult. In addition, the new methods of respiration present in arthropods were preserved by evolution because they were more efficient for those animals.
In arthropods, the respiratory system is open (lacunar). Blood, also known as hemolymph, is pumped by the heart and enters into cavities (lacunas), irrigating and draining tissues.
All arthropods have a heart. Crustaceans and arachnids have respiratory pigments. Most insects do not have respiratory pigments, since their blood does not carry gases (in insects, gases reach tissues and cells through tracheal structures). However, some insects do have the respiratory pigment hemoglobin (a contribution from Jacob Campbell).
Respiratory pigments are molecules able to carry oxygen and other respiratory gases present in circulatory fluids.
In crustaceans and in arachnids, hemocyanin is the respiratory pigment. In humans, the equivalent pigment is hemoglobin.
Despite having a circulatory system that works at low speed and low pressure, since it is a lacunar (open) circulatory system, insects perform extremely fast and exhaustive movements with their muscle fibers, such as beating their wings. This is possible because, in these animals, respiration is separate from open circulation. Gas exchange is carried out with great speed and efficiency by the tracheal system that puts cells in direct contact with air. Muscles can then work fast and hard.
In crustaceans, a pair of excretory organs called green glands exists. The green glands collect waste from the blood and other parts of the body. They are connected to excretory pores located under the base of the antennae via ducts. These pores excrete this waste outside the animal.
In insects, small structures called malpighian tubules remove waste from the blood. moving it into excretory ducts that open into the intestine. In these animals, excretions are eliminated together with feces.
In arachnids, in addition to malpighian tubules, there are coxal glands located in the cephalothorax near the limbs that are also involved in excretion.
In arthropods, the nervous system has more sophisticated sensory receptors with advanced cephalization. In the anterior region of the body, there is a group of ganglia which form a brain connected to two ventral ganglial cords that contain motor and sensory nerves.
The increased development of the sensory system of arthropods provides more possibilities for these animals to adapt and, as a result, a greater ability for them to explore many different environments.
Arthropods have compound eyes made of several visual units called ommatidia. Each ommatidium transmits visual information through the optic nerve to the brain, which interprets the image. These ommatidia are round and numerous, and their external surfaces point in different directions creating independent images. This causes arthropod eyes have a visual field larger than that of vertebrates. Some insects have one or more simple eye in addition to their pair of compound eyes.
Reproduction in animals of the phylum Arthropoda is sexual, with a larval stage in some insects and crustaceans (arachnids only go through direct development).
Some species of arthropods reproduce through external fertilization whereas others use internal fertilization. Internal fertilization is predominant.
Fertilization in insects is internal, with copulation.
In crustaceans, some species undergo direct development whereas others undergo indirect development. In insects, some species do not have a larval stage (ametabolic insects), whereas others go through indirect development beginning with an egg stage followed by a nymph stage (hemimetabolous insects). Moreover, other insects go through indirect development beginning with a larval stage (holometabolous insects).
The transformation of a larva into an adult insect is called metamorphosis. Hemimetabolous insects undergo incomplete metamorphosis while holometabolous insects undergo complete metamorphosis.
Nymphs are the larvae of hemimetabolous insects (such as grasshoppers). They are very similar to the adult insect, although smaller. In holometabolous insects (such as butterflies), the larva makes a cocoon (chrysalis, pupa) where it lives until transforming into the adult form. Imago is the name given to the adult form of insects with indirect development.
The period during which the larva is within its cocoon is a time of intense biological activity since the larva is being transformed into an adult insect.
Crustaceans and arachnids do not have wings. Most insects have wings.
In addition to the phylum Arthropoda, another animal phylum with flying creatures is the chordate phylum, as birds and chiropteran mammals (bats) have wings. It is possible that the reptiles that were the ancestors of birds also had wings. There also exist amphibians and fish that jump high to explore their aerial environment.
Crustaceans have two pairs of antennae; insects have one pair; arachnids do not have antennae.
In crustaceans and arachnids, the head is fused with the thorax to form the cephalothorax. Their body is therefore divided into cephalothorax and abdomen.
Insects have a head, thorax and abdomen.
Most crustaceans have five pairs of limbs. Insects have three pairs and arachnids have four pairs of limbs.
Insects are the animal group with the greatest diversity of species. Almost 750,000 insect species are known, making up approximately 55% of the total already recorded species of living organisms (compare this with mammals, with no more than 4,000 known species). However, scientists estimate that the number of unknown species of insects may be over 2 million. The insect population on the planet is estimated to be more than 10 quintillion (10,000,000,000,000,000,000).
The evolutionary success of insects is due to factors such as their: small size and diversity of foods, which make it possible for them to explore many different ecological environments; wings, which provided them with a greater area to spread across; tracheal respiration, which makes them highly agile; high reproductive rates, which produces a large number of offspring.
Insects such as some species of bees, wasps, ants and termites form societies that contain hierarchies and the division of jobs among their members. Spiders build sophisticated external structures, their webs, mainly to serve as a trap for capturing prey. Another example is the communication mechanism in some bees known as the bee dance, through which one bee tells others information about the location of flower fields and other sources of nectar.
Examples of representative species: cockroaches, flies, crabs, lobsters, shrimp, spiders, scorpions and mites. Basic morphology: segmented body (metameric), articulated limbs, chitinous exoskeleton, periodic ecdysis. Type of symmetry: bilateral. Germ layers and coelom: triploblastic, coelomates. Digestive system: complete. Respiratory system: tracheal in insects, branchial in crustaceans, tracheal and book lungs in arachnids. Circulatory system: open, hemocyanin in crustaceans and arachnids. Excretory system: malpighian tubules in insects, green glands in crustaceans, malpighian tubules and coxal glands in arachnids. Nervous system: ganglial. Types of reproduction: sexual, with or without a larval stage in insects and crustaceans, metamorphosis in some insects, no larval stage in arachnids.
Now that you have finished studying Arthropods, these are your options:
Give access to Biology Q&As to someone you like. Click here.