Phylum Arthropoda
Arthropods are an incredibly diverse group of animals, including insects (like butterflies and ants), crustaceans (like crabs and lobsters), spiders, scorpions, and centipedes. They are found in almost every type of habitat on Earth, from land and water to even in the air. Some of these animals, especially the insects, have the ability to fly. Let’s break down the main characteristics and systems of arthropods in a simple and detailed way.
Body Structure
- Triploblastic, Bilateral Symmetry, and Coelomates:
- Triploblastic means that arthropods have three primary germ layers during their development: ectoderm, mesoderm, and endoderm. These layers give rise to all the different tissues and organs in the body.
- Bilateral Symmetry means that their bodies are symmetrical when divided into two equal halves along the midline.
- Protostome Coelomates refers to their developmental process. The coelom (body cavity) forms early in their development, and in arthropods, this cavity is reduced and mainly surrounds the reproductive and excretory systems.
- Jointed Appendages: Arthropods are known for having jointed appendages that can perform different functions depending on the species:
- Running (insects like ants or grasshoppers).
- Crawling (spiders and centipedes).
- Swimming (crustaceans like crabs and lobsters).
- Capturing Prey (spiders use their legs and fangs to catch and immobilize their prey).
- Respiration (some appendages are modified to help with breathing, like gills in aquatic species).
- Reproduction (some appendages are used for mating, like in scorpions).
- Tagmata: The body of arthropods is segmented, but many of the segments are fused together to form specialized regions called tagmata. These include:
- Head: Contains sensory organs like antennae and compound eyes.
- Thorax: Typically the middle section, where legs and wings are attached.
- Abdomen: The rear section, often containing reproductive and digestive organs.
Exoskeleton (Cuticle)
- Made of Chitin: Arthropods have an exoskeleton or cuticle that covers their bodies. This exoskeleton is made mostly of a substance called chitin, which is tough and protects the animal from external damage.
- Ecdysis or Molting: As arthropods grow, their exoskeleton doesn’t expand. Instead, they go through a process called ecdysis or molting, where they shed their old exoskeleton and grow a new, larger one. This allows them to grow quickly after shedding, but they are vulnerable during this period.
Circulatory System
- Open Circulatory System: Most arthropods have an open circulatory system, meaning that their blood (called hemolymph) does not flow through confined blood vessels like in humans. Instead, the blood fills the hemocoel (the body cavity), bathing the organs directly.
- Hemolymph: Hemolymph is colorless because it lacks hemoglobin (the molecule responsible for carrying oxygen in the blood). Instead of circulating oxygen in the blood, arthropods have other ways to breathe (like gills or tracheae).
Respiratory System
- Tracheal System: Many terrestrial arthropods (like insects) breathe through a system of air tubes called trachea, which open to the outside through small holes on their bodies called spiracles. This system directly delivers oxygen to the tissues.
- Gills in Aquatic Arthropods: Aquatic arthropods, such as crabs and lobsters, breathe through gills, which extract oxygen from water.
Digestive System
Arthropods have a tube-like digestive system, which is divided into several parts, each specialized for different functions. For example, food enters through the mouth, goes down the esophagus to the stomach, and then moves into the intestines where nutrients are absorbed.
Excretory System
- Malpighian Tubules: The excretory system of arthropods consists of Malpighian tubules, which are narrow tubes that extend from the digestive system. They help remove waste products, mainly nitrogenous wastes, by converting them into solid uric acid crystals, which are excreted from the body.
Nervous System
- Central Nervous System: Arthropods have a well-developed central nervous system. This includes:
- A brain made up of three fused pairs of cerebral ganglia (nerve clusters) located in the head.
- A ventral nerve cord running along the underside of the body, with nerve clusters (ganglia) in each segment.
These ganglia control the movements of the arthropod’s body and help it respond to environmental stimuli.
Sense Organs
- Compound Eyes and Antennae: Arthropods have compound eyes, which are made up of many smaller individual lenses, allowing them to detect movement and light very efficiently. Insects, for example, can see ultraviolet light that humans cannot. They also have antennae, which are used to detect chemicals (smell) and touch.
Locomotion
Arthropods can move in various ways depending on their environment:
- Swimming: Crabs and lobsters use their legs and other appendages to swim.
- Crawling: Centipedes and spiders use their many legs to move across land.
- Flying: Many insects, like bees and butterflies, have wings and can fly.
Reproduction
Most arthropods are unisexual, meaning that there are distinct male and female individuals. However, in some species, like certain types of insects, the males and females look very different from each other. Reproduction typically involves internal fertilization, and many arthropods lay eggs.
Mnemonic to Remember Key Features of Arthropods:
To make it easier to remember the key features of arthropods, you can use the mnemonic “CUTE CRABS HOP FAST”:
- C = Coelom reduced (arthropods have a reduced body cavity around reproductive/excretory systems).
- U = Unisexual (most arthropods are male or female).
- T = Tracheal system (arthropods have a respiratory system with trachea for breathing).
- E = Exoskeleton (their body is covered with a tough outer shell).
- C = Circulatory system (open circulatory system with hemolymph).
- R = Reproduction (most are unisexual, lay eggs).
- A = Antennae and compound eyes (important sensory organs).
- B = Body segmentation (body divided into segments with fused sections called tagmata).
- S = Specialized appendages (jointed appendages for various functions).
- H = Hemolymph (blood is called hemolymph and is colorless).
- O = Open circulatory system (blood is not confined to vessels).
- P = Photosensitivity (some arthropods can detect ultraviolet light).
- F = Flight (some arthropods like insects can fly).
This mnemonic summarizes the key features of arthropods, making it easier to remember the essential characteristics that define this phylum. Each letter corresponds to an important aspect of their biology, helping to reinforce the key ideas for easy recall.