Invertebrates

Invertebrates are the most successful living things in the world, accounting for more than three million different species — more than all other living things put together.

Invertebrates are the oldest known animals on earth. Their fossil record goes back at least 600 million years. Insects, the most common and successful invertebrates, originated about 400 million years ago.

Characteristics

Invertebrates are so named because they are animals that have evolved without backbones. They do, however, have a few biological traits similar in function to those of vertebrates (animals with backbones). These traits include:

  • Digestive systems and other organs
  • Hearts
  • Mouths
  • Muscles
  • Nervous systems

Size, shape and colour

Most invertebrates are smaller than a marble, but some can be much larger. The giant squid, for example, is larger than some whales.

Invertebrates occur in a tremendous variety of shapes and colors. These are adaptations to specific habitats.

For example, burrowers, such as earthworms, usually have long tubular bodies and are darkly coloured, similar to the soil in which they live. Immature forms of mayflies and stoneflies occur in fast-running streams. They have a flat shape, which reduces their resistance to water currents, and are a dull color, matching the stream bottom.

Invertebrates that are active during the day, such as butterflies, are often brightly coloured. Others have colours and patterns which serve to hide them.

Natural History

Habitat

Invertebrates are found everywhere, from mountain tops to ocean bottoms, and from barren deserts to the darkest corners of caves. The habitat requirements of many invertebrates change as they pass through different stages of their lives. Requirements of growth stages often differ from those of the adult.

Food

Invertebrates feed in many ways: some species eat living plants or animals; others feed on dead and decaying organisms.

As with habitat requirements, the food requirements of invertebrates in their growth stages often differ from those at the adult stage. Butterfly caterpillars eat the leaves of plants, whereas adult butterflies sip the nectar from flowers. Dragonfly and damselfly nymphs live in lakes and ponds and eat other aquatic insects and even fish. However, the adult damselflies eat flying insects.

Social organization

Although some invertebrates live alone, others live in colonies that can reach sizes of more than a mil lion individuals. Our most familiar examples of social or colony-dwelling invertebrates are bees, wasps and ants. The lives of individuals are spent working for the colony.

Reproduction and Growth

Though some invertebrates lay only a few eggs, others produce millions.

Invertebrates grow from egg to adult in three basic ways:

  • Some develop in a continuous process without interruption — for example, snails and clams. In this case the young look like the adults they will become as they undergo a steady increase in size.
  • Others undergo a process involving a series of rapid growth periods which occur after each moulting or skin shedding — for example, grasshoppers and spiders.
  • A third group, which includes flies, butterflies and beetles, undergoes a growth process involving four completely different stages:
    • Adult
    • Egg
    • Larva
    • Pupa

Most invertebrates live only a few weeks to a year or two, but some may reach 100 years or more.

Migration

Many invertebrates scatter or disperse by travelling with the wind. Mosquitoes and blackflies can travel 30 kilometres (20 miles) per day in this manner. Spiders may cover a distance of more than 50 kilometres (30 miles) per day by creating a balloon-like sail from the silk they use to spin their webs. Invertebrates that live in streams are often carried long distances by water currents.

Alberta's Invertebrate Species

In this website, only a very few groupings of invertebrate species commonly found in Alberta are covered. These groupings include:

Related Links

To learn more about invertebrates and their role in the natural world, see:

 

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Updated: Jul 28, 2015