Discovery Science: Earth – Invertebrates – Insects

Earth Science: Invertebrates – Insects

Insects (phylum Arthropoda) are by far the most diverse group of animals. More than a million species have been described to date, and there are probably many more unknown insects still to be discovered.

All insects are relatively small-even the largest species only reaches a length of about 12 inches (30 cm). They have conquered almost all terrestrial and freshwater habitats. All have a segmented hard outer protective layer, or exoskeleton, made primarily of chitin.

Adult insects have three sections: a head, a thorax, and an abdomen. The head has a pair of antennae for feeling and smelling, as well as eyes and mouthparts, which may differ consider- ably between species and be adapted for chewing or piercing-sucking. Three pairs of legs and sometimes wings are attached to the thorax.

The abdomen contains the major part of the digestive and blood circulatory systems as well as reproductive organs. Insects have a life cycle with several larval stages until the animal reaches reproductive age. Many species, including butterflies and beetles, change their entire appearance (holometabolism or complete metamorphosis) during this process.

A pupal stage, during which the insect does not grow, comes before the final developmental step. If young or juvenile insects do not
change considerably in appearance when they mature, the metamorphosis is considered incomplete (hemi-metabolism). These larvae mature into adults increasing gradually in size without a pupal stage.

Insects are the only invertebrates able to actively fly, contributing to their vast spread across various habitats. Their wings consist of an extremely thin membrane that is supported by a network of veins. Winged insects usually have two pairs of wings, but several species, especially parasites, have reduced wings.

The latter are “secondarily wingless,” as they have evolved from winged insects. Some insects never had any wings; they are “primarily wingless,” as for example springtails.


Some insect species, mainly ants, termites, and some bees and wasps, are social insects and live in large colonies to care for the next generation. In such insect colonies, the labor is strictly governed by social rules.

Sterile workers take over several tasks, for example, gathering food, protecting the nest, or caring for the young. The queen alone is responsible for reproduction.


THE BOMBARDIER BEETLE sprays an acidic liquid when attacked.

Two highly reactive chemicals are stored separately and then mixed inside the beetle’s body.