Distinguishing between arachnids is often very difficult, even when consulting several guides and using a microscope. There is often a considerable amount of natural variability within a genus or species. Most spiders are sexually dimorphic; being able to recognize one sex may not help with the other. Even when conserved, the features used to differentiate between species are often very tiny. Many key features are less than 1mm in length. Finally, the ethanol solution used to kill and preserve spiders alters their appearence. Sometimes the ethanol causes colors to fade, while other times it causes colors to become darker. Without ethanol, however, spiders quickly shrivel, becoming almost impossible to identify.
The visual key makes it possible to identify arachnids on this site using features that are visible using a small (10x power) hand lens. While convenient, this method of identification is only possible in a small guide with a limited number of species.