Soft-bodied relatives of the arthropods, as well as trace fossils that were made by some arthropod-like organisms, appear in the Vendian. However, arthropods underwent rapid evolution in the Cambrian Period; Cambrian localities like the world-famous Burgess Shale in British Columbia are rich in unusual athropods, many of which are not obviously related to the traditional classes of living arthropods. Trilobites were a dominant marine group in the early Paleozoic. The earliest arachnids turn up in the Silurian, and move onto land shortly before the insects appeared in the middle Devonian Period, about 385 million years ago; over the next hundred or so million years both groups radiated into several diverse lineages.
The above image is the head and mouthparts of a fossilized larva of the water bug Schistomerus, found silicified in Miocene deposits (about 15 million years old) near Barstow, California. The actual size of the head is about one millimeter. This photograph was taken with UCMP's own Environmental Scanning Electron Microscope.