4.2.7. Phylum – Arthropoda

This is the largest phylum of Animalia which includes insects. Over two-thirds of all named species on earth are arthropods.

Arthropoda is recognized as the largest phylum within the animal kingdom. A phylum is a group of organisms that share a common structure and characteristics. Within this group, there are more than two-thirds of all animal species known to us. This means that arthropods, such as insects, arachnids (spiders), and crustaceans (crabs and lobsters), are incredibly diverse and numerous.

Think about your backyard. If you closely observe, you might find insects like ants, butterflies, and grasshoppers. These tiny creatures represent just a small fraction of the total arthropods, illustrating how diverse and ubiquitous these organisms are.

They have organ-system level of organisation. They are bilaterally symmetrical, triploblastic, segmented and coelomate animals. The body of arthropods is covered by chitinous exoskeleton.

Arthropods exhibit a higher level of organization called organ-system level, meaning their bodies are composed of various systems (like digestive, respiratory, and circulatory systems) working together for survival. They are bilaterally symmetrical, meaning their left and right halves mirror each other when divided down the middle. As triploblastic organisms, they have three germ layers: ectoderm, mesoderm, and endoderm. Segmentation indicates that their bodies are divided into segments, which allows for specialized functions. Their exoskeleton, made primarily of chitin, provides protection and support.

Imagine a segmented toy where each part can move independently. Similarly, the segmented body of arthropods allows different sections to perform specialized tasks. For instance, the head is designed for sensory reception, while the abdomen may focus on reproduction.

The body consists of head, thorax and abdomen. They have jointed appendages. Respiratory organs are gills, book gills, book lungs or tracheal system.

Arthropods have a distinctive body structure divided into three main parts: the head (where sensory organs are located), the thorax (often where the legs and wings attach), and the abdomen (which can contain reproductive organs). They are characterized by jointed appendages, such as legs and antennae, which enhance their ability to move and interact with their environment. Their respiratory systems are quite varied: some use gills for underwater breathing, while others, like insects, have a tracheal system that allows direct oxygen transport to tissues.

Consider the way a plane is built. The different sections—the cockpit (head), body (thorax), and tail (abdomen)—work together for flight. Similarly, in arthropods, each body part has its unique function that contributes to the organism's overall survival and adaptability.

Circulatory system is of open type. Excretion takes place through malpighian tubules. They are mostly dioecious.

Arthropods have an open circulatory system, meaning that instead of blood being enclosed in vessels, it circulates freely in a body cavity. This allows for efficient flow to various body parts. Their excretory system utilizes specialized structures called malpighian tubules, which remove waste products from the hemolymph (the fluid equivalent to blood) and excrete them. Most arthropods are dioecious, meaning they have distinct male and female individuals for reproduction.

Think of an open circulatory system as a water park where the water flows freely around without fixed pipes. In this park, the waste from the visitors, like wrappers and leftover snacks, is managed by designated cleaning staff (malpighian tubules), who pick up trash and keep the park clean.

Fertilisation is usually internal. They are mostly oviparous. Development may be direct or indirect.

In arthropods, the process of fertilization usually occurs internally, meaning that the male and female gametes meet inside the female’s body. Most arthropods lay eggs (oviparous), with the young hatching out of the eggs either resembling the adults (direct development) or going through various larval stages before reaching adulthood (indirect development).

Consider gardening, where you plant seeds (like eggs) in a soil bed. Each seed grows into a plant (the young arthropod) which may initially look different from its mature form, showing how indirect development works, similar to how caterpillars transform into butterflies.

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