Nick Davidson, of Green Island School, asks :-

How did insects first come to be?

Rob Cruickshank, an entomologist with the Bio-Protection and Ecology Division of Lincoln University, responded.

Entomologists (scientists that study insects) have been arguing about this for years. Although some of the details are still being worked out, we think we now have quite a good understanding of how insects first evolved.

A unique feature of insects is that their bodies are made up of three separate parts. At the front is the head, which contains the mouthparts and most of the sensory organs such as the eyes and antennae. In the middle is the thorax, which bears most of the structures the insect needs to move around, usually three pairs of legs and two pairs of wings. At the back is the abdomen, which contains the reproductive organs such as the ovipositor that female insects use to lay their eggs. Entomologists call these regions of the body tagmata, and in order to understand the origins of the insects we need to understand how these tagmata evolved.

It seems likely that the ancestor of all of the insects resembled something like a centipede or a millipede. These animals are made up of many identical segments, most of which bear one or two pairs of legs. Over a very long period of time (several million years), in one particular group of these animals, the segments towards the front of the body became fused together to form a head. A group of segments behind the head merged to form the thorax and those at the end of the body combined to become the abdomen. These three tagmata became specialised for different tasks: the head for eating and sensing the environment, the thorax for moving about, and the abdomen for reproducing and laying eggs. As they became more specialised the tagmata also became more different to each other in appearance, for example the legs were lost from segments that became the head and abdomen. The end result of this lengthy process was the first true insect with a fully differentiated head, thorax and abdomen.

This arrangement of three tagmata has been extremely successful for the insects. The tagmata have become modified in various ways in different groups of insects (for example the ovipositor has become a sting in some bees and wasps). This has led to the evolution of over a million different species of insects with a huge variety of diverse forms. But despite their apparent differences, all insects have the same underlying structure composed of three tagmata: head, thorax and abdomen.