Since the time of Linnaeus (1707-1778), dichotomous (“two-branching”) or pathway keys, developed and published by taxonomists, have provided a major support tool for species identification, including for biosecurity purposes. The typical form of a printed dichotomous key, shown in the box below, shows part of a key for determining the order to which an insect specimen belongs.

 Box: Section of a dichotomous or pathway key to insect orders     

The user follows a pathway through the key by comparing the features of the unidentified specimen with descriptions (and images) in the key. By choosing that component of a couplet that correctly describes the specimen, the user is led to the next relevant couplet. For instance, if components 1(a), 2(a) and 3(a) in the Box above, apply to a particular specimen, then it is identified as belonging to the order Diptera.

Pathway keys that are published on the web are likely to be easier to use than printed keys, particularly where images of features are included, and hyperlinks automatically bring up the next relevant couplet. For pathway keys that have a limited number of taxa, building and using the key can be relatively easy.  


Screen shot: The initial couplet of a dichotomous key to the genera of Ulidiidae (Diptera: Tephritoidea) of the United States and Canada. Wallace C (2021). For more complex dichotomous keys, software initially developed for matrix keys (see next section) has recently been modified to allow the construction of digital dichotomous keys. More importantly, it allows existing, published (hard copy) dichotomous keys to be imported via builder software, checked for inconsistencies, and incorporated in an online player. For example, a series of keys to insects and spiders found in rice in Southeast Asia and West Africa, initially published in books over 20 years ago, have now been incorporated in the software and are freely available online at IAPPS.