38. Invertebrates

How are animals classified?

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The individuals that belong to a same species of living organism are usually quite similar in appearance: they have the same structure and coloring. For example, tortoiseshell butterflies are easily distinguishable from brimstone butterflies. Individuals belonging to the same species can recognize one another and reproduce with one another.

A group of animals can contain many species. For example, there are thousands of different butterfly species in the world. These groups can then be combined to form even larger groups, based on structural and genetic similarities. By doing so, we can classify both butterflies and beetles as insects, and both insects and spiders as invertebrates.

In this manner, each species can be classified as belonging to larger and smaller groups. For example, a tortoiseshell butterfly is:

an animal
an invertebrate
an arthropod
an insect
a butterfly
a tortoiseshell butterfly

What are invertebrates?

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Inverterbate animals do not have internal supporting structures or skeletons.

Instead, invertebrates can have durable, external supporting structures. This is the case with all insects, crabs and mussels.

Water-dwelling invertebrates do not need these kinds of external supporting structures, because the water provides enough support to their bodies. For example, the octopus (pictured on the right) does not need an internal skeleton or an external shell to maintain its structure in the water.

Invertebrates do not have highly developed senses. Their circulatory and respiratory systems are likewise poorly developed.

Worms

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Worms are primitive invertebrates. They have no heads, eyes or brains.

Because worms do not have feet and their bodies are soft, they are an easy source of food for many organisms, especially birds. This is why worms mainly live underground.

The animals that we think of as worms actually belong to many different groups of invertebrates. For example, earthworms are decomposers that live in underground tunnels. They belong to the group of annelids, or segmented worms.

On the other hand, flatworms live as parasites inside other organisms. Most worms thrive in moist environments.

Molluscs

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Molluscs are more highly developed invertebrates than worms. They have eyes and a protective shell. They eat plant parts.

Snails, gastropods and mussels are all examples of molluscs. The image on the right displays a group of blue mussels. It lives under the water.

Octopuses also belong to the group of molluscs.

Spiders

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Spiders and insects are both arthropods. They have jointed legs. The group of arthropods also includes crustaceans, such as crabs.

Spiders have eight legs, whereas insects have six legs.

Spiders are predators. They mainly eat insects. Some spider species weave webs in order to catch flying insects.

All spiders are capable of biting. Spiders paralyze their prey with poison.

Insects

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Insects are a large group of organisms. They can be found almost everywhere. Insects favor hot and humid environments, which is why tropical rainforests are full of them.

All insects share the following characteristics:

six legs
three body parts: a head, a thorax and an abdomen.
multiple pairs of eyes
wings
hard, external supporting structure
metamorphosis: egg - larva - (pupa) - adult
cold-blooded or poikilothermic

The most common groups of insects include:

butterflies
dragonflies
beetles
flies
hymenopterans (e.g. bees and ants)

Different kinds of insects

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: The apollo butterfly is a day butterfly. It uses its long sucker to drink nectar from flowers.
: The swallowtail butterfly.
: A moth larva. Moths are butterflies that live during the night. Butterfly larvae eat plant leaves.
: Ants live in colonies. Different ants of the colony have different roles: the queen reproduces, the workers search for food, and the male butterflies reproduce with the queen.
: Beetles have hard shells. The picture shows a green tiger beetle, which belongs to the group of ground beetles.
: The hoverfly looks like a bee. However, it does not have a stinger.
: Dragonflies have inhabited planet Earth for 300 million years. They are the oldest group of insects in the world.

Terminology

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INVERTEBRATEs
Term	Explanation
species	A group of organisms that contains individuals that look similar, have similar structures and can reproduce with one another.
invertebrate	An animal that does not have an internal supporting structure or a skeleton.