Insects are the most abundant and diverse group of animals on Earth. They are critical to ecosystem function in terrestrial and aquatic systems, yet they are one of the most understudied groups of organisms. Only a small proportion of the more than five million insect species have been assessed by the IUCN Red List. For most of these species, there is not enough evidence to know what is happening to their populations. In fact, for most insect species globally, there is very little data available on where they live, how they live and what environmental conditions they need to persist in the long term. A number of threats affect insect biology and life cycles generally, including climate change, habitat clearing, invasive species, use of broad-spectrum pesticides, and pollution of soil and waterways. These threats should be addressed immediately to prevent further declines in insect populations. To understand insects better, greater investment in research and documentation of the world’s insect diversity is urgently needed.

The abnormal animal featured here is a tadpole with two heads. In order to explain how it got that way, the chapter desccribes how the axes of the body are established in the embryo. This leads to a discussion of axes in conjoined twins and symmetry planes in various parts of the body, plus a three-eyed frog.