The field of zoological classification is concerned with recording the diversity of animal organisms and ordering them in a "natural" system. This involves grouping together animals with similar, related characteristics and integrating them into a hierarchical system that is intended to reflect the evolutionary relationships between various groups of organisms.

The term taxonomy denotes the practical method by which organisms are arranged into systematic categories (see box below). The precise use of taxonomy is important for clinical toxinology, as

• in many cases clinical case reports and studies can only be interpreted correctly if the animal that caused the accident has been accurately identified both taxonomically and geographically;
• the quality of the diagnosis and treatment of envenoming may depend on the accuracy of the taxonomical identification.

The basic element of taxonomy is the species. Under natural circumstances, individuals of the same species are able to breed and to produce fertile offspring. Subspecies and geographical variants are smaller units within a species. In principle, individuals of two different subspecies of the same species are also able to produce fertile offspring, but generally they are more or less geographically or ecologically separated from each other. In this way mixing of the gene pool between different subspecies is reduced or even prevented completely. Consequently, this leads to a different genotypic and phenotypic trend between different subspecies.

This fact is of significance for clinical toxinology, as the venom composition and thus the range of venom effects may vary considerably between two different subspecies. A medically important example is Russell's viper in Asia. The subspecies found in Sri Lanka, Daboia russelli pulchella, is known to cause neurological effects that do not occur following bites from the other subspecies. Considerable differences in the symptoms of envenoming have also been described for geographical variants of a single subspecies, for example the North American rattlesnake Crotalus scutulatus scutulatus.

These examples make it clear that for medically significant venomous snakes in particular, identification only at the species level is, in certain circumstances, not sufficient in order to predict the effects of envenoming and possibly also to choose the appropriate antivenom. On the other hand, for various other venomous or poisonous animals, identification at the genus, family or even order level is sufficient in order to treat a patient appropriately, as the expected venom effects hardly vary beyond a given range. Among the poisonous fish, for example, members of various families may contain the same toxins that cause ciguatera. Envenoming caused by bee, wasp or ant stings generally involve only autopharmacological effects. Thus, treatment is the same for the entire order of hymenopterans, regardless of which species caused the accident.

Taxonomy is subject to constant change. The technique of DNA barcoding is more and more becoming the main tool for taxonomists to catalogue life. Small sequences of mitochondrial DNA are cloned and sequenced. Such unique genetical fingerprintas are then compared among different organisms to estimate relationships and to construct classification systems. This approach sometimes leads to taxonomic separation between organisms, where morphological differences are barely visible and is not accepted by all taxonomists.

New subspecies are described, individual subspecies are raised to the status of species (eg Asiatic cobras) or existing genera are divided up (eg the former genus Trimeresurus). Even in the higher systematic categories reorganisation is going on. For example it seems clear now, that the australian Elapids are more closely related to the subfamily of seasnakes, than to the other elapids like cobras, mambas or coral snakes. In the last decades, experts have made substantial taxonomical changes with regard to terrestrial snakes and scorpions in particular but also within other groups of venomous/poisonous animals. The taxonomy used in this Guide is based on the recent literature. Older terms are also noted if they represent commonly used names.

Systematic categories using the example of Russell's viper in Sri Lanka