English for Students

Arthropods

More than one million species of arthropods exist, making them the largest group in the animal kingdom. They live in almost all habitats, from mountain tops to the ocean depths. Arthropods are invertebrate’s animals without backbones. They come in many shapes and sizes, from tiny mites to large crabs. Their bodies are divided into segments and they have distinct heads with antennae or eyes. Rigid exoskeletons encase their bodies, but flexible leg joints allow them to more around and give them their name.


Types of Arthropod:

Arthropods vary in size, from minute creatures a fraction of millimeter long to outsized sea dwellers several kilograms in weight. There are four main types if arthropod: insects, arachnids, crustaceans and myriapods. Insects are the largest group, accounting for almost 90 percent of all arthropods.


Myriapods:

Myriapods include Millipedes and centipedes. They have more legs than other arthropods – as many as 200 in some species. Their bodies are long and tubular. They live in the soil or among leaf debris.


Arachnids:

Arachnids include spiders, scorpions and mites. They have eight legs. Spiders and scorpions are carnivores that live mainly on land. Spiders often kill their prey with poisonous fangs. Scorpions use their venom – filled sting.


Insects:

Insects are the most diverse group of arthropods. They live in all kinds of land and freshwater habitats. All adult insects have six legs and most have wings. They are the only arthropods that can fly.


Crustaceans:

Crustaceans include crabs, shrimps and lobsters. Most live in the sea or in freshwater and have five pairs of legs. Lobsters and crabs have very thick exoskeletons and some grow extremely large.


Exoskeletons:

The exoskeleton of an arthropod is a tough outer layer covering the entire body, including the eyes, antennae and legs. It protects and supports the muscles and soft organs within the body and helps to retain moisture.


Mounting and Growth:

Exoskeletons are fixed in size. In order to grow, an arthropod must shed or mould this rigid layer. Its body then rapidly expands before new exoskeletons harden in place of the old one. Moulting is part of a process called Incomplete Metamorphosis. This is where the young, called nymphs, emerge from eggs looking like tiny adults. They mould many times before reaching adult size. In Complete Metamorphosis the animal changes form as well as size.


Moulting:

An emerging adult grasshopper has cracked open its old exoskeleton and is starting to wriggle its body free, headfirst. Before this final moult, the nymph will already have been through four previous moults. The adult has pulled its legs and most of its body out of the old skin. It is already expanding in size, now that it is free from its confines. Moulting is now complete. The adult rests while its new exoskeleton hardens and its wings unfurl. Its old exoskeleton, now empty and brittle, still clings to the twig.


Reproduction:



Breeding habits are very diverse among arthropods. Fertilization may take place inside or outside the female’s body. Normally eggs are laid. Some are guarded. Others are hidden and left alone. The young of some arthropods, such as garden spiders, are tiny version of adults called nymphs. Others start life as larvae and look different from the adults.


Feeding:

Arthropods feed on all kinds of plant and animal matter, both living and dead. Some arthropods, such as praying mantises, have pincers to gather food. Others use their front legs. Many have cutting and chewing mouthparts; while those that feed on fluids, such as true bugs, have mouths modified for sucking. Small aquatic arthropods eat by filtering food particles form water


Herbivores:

Some arthropods, such as chafer beetles, eat only plant matter. Adults feed on stems, leaves and buds, while larvae eat plant roots.


Carnivores:

Many arthropods feed on other animals. Garden spiders, for example, feed mainly on insects. Some meat eaters also eat dead animals and are called scavengers. Sand crabs scavenge on dead birds and other debris found on the beach and seabed.


Defense:

As arthropods are generally small in size, they are the target for a great many predatory. Their hard exoskeleton which acts as a tiny suit of armor provides the first line of defense. Some arthropods, such as pill millipedes, take a passive form of defiance and roll up into a ball if danger threatens. Other arthropods have special protective weapons, including stings and pieces. Many ant species have glands on their abdomens from which they secrete formic acid to drive off enemies.


Sting and Pincers:

Some arthropods have pincers and stings which they use to defend themselves against attackers. Scorpions also use their large pincers to catch animals. They then use their venom - filled sting to paralyze their prey.