Topic 5.4: The insects

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Content

Topic Introduction

The insects and their kin (the Hexapoda) constitute a subphylum within the phylum Arthropoda. They are the most diverse group of animals on Earth. They are the only invertebrates that can fly, and the only ones to undergo indirect development or complete metamorphosis. They comprise the class Insecta, as well as three small, closely-related wingless groups: the Collembola, Protura and Diplura. All hexapods have a three-part bodyplan, with a head, a 3-segmented thorax and an 11-segmented abdomen. They have three pairs of thoracic legs, with one pair of legs per segment.

Hexapods evolved on land, and have occupied almost every known habitat. Some have moved into aquatic habitats, including the marine environment. There are an unknown number of species, with some estimates being over 100 million species. To date, approximately a million species have been described, with many new species (5000–7000) being described every year. That compares to a total of about 6500 mammals species described to date.

There are so many different types of insects that it is impossible to undertake a thorough review in a course such as this. Instead, we provide an introduction that summarizes the different groups and provides an illustration of each, and then we examine three particular groups: the Odonata (dragonflies and damselflies), Coleoptera (beetles), Hymenoptera (ants, bees and wasps) and Lepidoptera (moths and butterflies). With this last group (Lepidoptera), we focus on the butterflies, which are much better known than the moths. This was a hard choice. There are many other important groups (such as the Orthoptera (locusts, katydids, crickets and grasshoppers), Hemiptera (true bugs), Thysanoptera (thrips), and Diptera (true flies, mosquitoes and gnats), as well as some fascinating groups such as the Mantodea (mantises), Phasmida (stick and leaf insects), and Neuroptera (lacewings, ant lions, mantisflies and owlflies). In our examination of the Hymenoptera, we have included the termites, even though they belong to a different order of insects (Blattodea).

By the end of this topic, you should be able to:

  • Describe the main orders of insects.
  • Assess different explanations for the diversity of the insects.
  • Explain why the insects are important for the ecology of tropical forests.
  • Distinguish between dragonflies and damselflies.
  • Explain why some regions have richer odonate faunas than others.
  • Describe some of the most important beetle families found in tropical forests.
  • Demonstrate their ecological roles in forests.
  • Give examples of different types of ants and their life histories.
  • Contrast the roles that different types of bees play in pollination.
  • Evaluate the ecological significance of ant gardens and ant plants.
  • Explain the difference between butterflies and moths.
  • Give examples of each of the five butterfly families.
  • Contrast the life histories of butterflies in tropical moist and seasonally dry forests.

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