Despite advances in computer technology and the increasing availability of multiple-access taxonomic keys, traditional dichotomous keys remain the most often used taxonomic identification tools. On the other hand, there seems to be a lack of an editor of dichotomous keys, which is both freely available and easy to use. The DKey software was developed in order to alleviate this problem. A taxonomic key in text format can be imported to the software in order to edit it. Various editing options are possible, including: moving couplets, removing couplets, combining keys and renumbering keys. The software can output the key either in the traditional text format, ready for publication in a scientific journal, or in hypertext linked format, which makes identification faster and easier, due to the fact that pointers can be clicked in order to move to the next couplet. The DKey software should be useful for both taxonomic experts creating keys and those who use them for identification. The DKey software is freely available and open source.

key, single-access key, software, taxonomic identification of species

The identification of species is often based on dichotomous keys (also called single-access keys or pathway keys) (Quicke 1993, Walter and Winterton 2007). They are a series of consecutively numbered couplets. Each couplet consists of two parts called leads. At the end of each lead, there is a reference, which can be either a number pointing to another couplet or a text indicating the name of a species or another taxonomic group. The two types of references are called “pointer” and “endpoint”, respectively. The identification based on a dichotomous key starts at couplet number one and it is stepwise. At each step, the user reads both leads of a couplet and chooses one that better fits the identified individual. If the chosen lead is associated with a pointer, then the user goes to another couplet, which is the next step of the identification. The identification is finished when the chosen couplet is associated with an endpoint.

Originally, dichotomous keys were constructed by taxonomic experts without any support form computer technology. However, when computers became more widespread, computer programs for the automated construction of taxonomic keys were developed (Pankhurst 1971, Dallwitz 1974, Quicke 1993). Computerized key construction software requires a data matrix consisting of a table with information about multiple characters of each taxon in the key. The key is constructed automatically by a computer algorithm; however, there is often the need for some intervention from the user to improve it. The data matrix can also be used for the construction of a free-access key (also called a multi-access key) (for review see Walter and Winterton 2007, Hagedorn et al. 2010, Cerretti et al. 2012). In this type of taxonomic key, there is no fixed sequence of identification steps. Instead, the user chooses characters from the data matrix that are easily available. This attitude can be preferred when some characters are not available in the identified specimen.

Multiple-access keys have many advantages (Dallwitz et al. 2000). In those keys not only qualitative but also quantitative characters can be used. Moreover, tolerance to errors is higher than in dichotomous keys. Despite their advantages multiple-access keys have not become overwhelmingly popular among taxonomist and manmade dichotomous keys remain the most often used identification tool (Quicke 1993, Walter and Winterton 2007). The preparation of the data matrix and, in particular, the fine tuning of the process of key construction requires some specialist knowledge. On the other hand, most taxonomists have limited interest in computer technology and sophisticated software can intimidate them. In consequence, many taxonomists design dichotomous keys without the use of any dedicated computer software, except a text editor. Creating a key in this way is time-consuming. A relatively simple task of numbering couplets is tedious and error prone. This task can be repeated many times when preparing the key, as any removal or addition of a couplet requires renumbering of some couplets and pointers. This problem is particularly acute in the case of large keys consisting of hundreds of couplets. There seems to be a lack of an editor of dichotomous keys, which is freely available and easy to use.

The aim of the project was to develop the DKey software, which would assist taxonomists in the preparation of dichotomous identification keys. A taxonomic key in text format can be easily imported to the software in order to edit it. The software can output the key either in traditional text format, ready for publication in a scientific journal, or hypertext linked format, which makes identification faster and easier. The DKey software is freely available and open source.

DKey was developed in the C++ programming language. It is based on the QT framework. At the moment, executables are available for the Windows operating system (http://drawwing.org/dkey). However, users of macOS and Linux can obtain the source code and build executables for their operating system, as QT is a cross-platform framework. In future access to DKey should be equally easy for all three operating systems. The source code of the software can be downloaded from the GitHub (https://github.com/DrawWing/DKey). DKey is open source and it is licensed under GNU General Public License, version 3.

DKey allows for reticulation, which means that a couplet can be reached over more than one path and a single taxon can be associated with more than one endpoint. The only restrictions are that the couplet numbers need to be unique, they need to start with 1, they need to be consecutive, each couplet need to be reached from couplet 1 and it needs to refer either to an existent other couplet or an endpoint. The software does not create the key automatically; it is a responsibility of the taxonomic expert to create couplets and to arrange them in an optimal way. In order to test the software, I have used the existing key to the genera of Agathidinae (Hymenoptera, Braconidae) (Sharkey et al. 2009).

The DKey software fills the gap for an easy-to-use and free editor of conventional dichotomous taxonomic keys. The main advantage of the DKey software is its wide availability. It can be downloaded free of charge and used by both scientists as well as a wider audience. The price of commercial software, for example Lucid Phoenix (Anonymous 2017), can be prohibitive for some enthusiasts of taxonomy. Not only is the DKey software free of change, but its source code is also widely available. Therefore, it can be developed in the future by a larger group of programmers who can correct and improve it. In consequence, the software can survive longer. Many projects related to biodiversity informatics vanished or stopped to be developed when the funding has finished or when the original developer lost interest in the further improvement of the software. In the case of open source software, there is a chance that programmers, other than the author, will continue the work on it.

There were other attempts to develop free and open source software for taxonomic keys. Open Key Editor (Martellos et al. 2010, van Spronsen et al. 2010) can be used to create user-friendly taxonomic keys available online or on mobile devices. This software is open source; however, the license under which it is distributed is relatively restrictive (Hagedorn et al. 2011). The installation of Open Key Editor can be difficult for most taxonomists, as it requires a web server, the creation of an SQL database and a manual modification of the configuration files.

There is also a wide range of freely available software, which can be used for the creation of online identification keys, including: ActKey (Brach and Song 2005), ARPHA Writing Tool (Smith et al. 2013), KeyBase (Thiele and Klazenga 2016), WEBiKEY (Attigala et al. 2016). The purpose of this software is to make identification easier. Taxonomic keys published in scientific journals and books are formatted for efficient printing and page layout. This formatting can create problems for the key users, as images are usually on a different page then the couplets referring to them. In order to minimize those problems, dichotomous keys can be implemented as a webpage or a computer program. In this form, the key is more user-friendly and the identification time can be markedly reduced because the user does not need to flip pages to find the next couplet or an image (Wright et al. 1995, Farr 2006, Burkmar 2014). However, the process of preparing the computer-based keys can be time-consuming because it requires the preparation of a data matrix or rewriting the key. On the other hand, DKey allows a relatively easy import of existing keys and images. A user of a traditional printed key can scan it, convert it to text and import it to the DKey software in order to make identification faster and more flexible. Moreover, taxonomic experts who use the software to create a key for scientific publication do not need to invest any more time or effort to create a computer-based version of the key.

In contrast to other free software, DKey is focused not only on the creation of computer based keys but also provides many editing options, which are not available elsewhere, including: import from text, export to text, combining keys, moving and copying couplets. The DKey software is particularly useful in the case of large taxonomic keys consisting of hundreds or thousands of couplets. Nowadays, the identification of large taxonomic groups involves using many small keys. There are separate keys for families, genera, species groups and species. The smaller keys are easier to manage by the taxonomic experts who created them because their renumbering is less time-consuming. On the other hand, this solution is less user-friendly because the user needs to find the key to the next taxonomic level. DKey can be easily used to combine many smaller keys into one very large one.

Taxonomists are encouraged to publish their datasets and allow others to re-use them in the future (Penev et al. 2008, 2009). The current recommendations (Penev et al. 2009) are related mainly to interactive keys based on data matrices and there is no standard format for dichotomous keys. The later keys are usually presented in two main formats: bracketed and indented (Quicke 1993). Within each of those formats, there are many variants with various punctuation marks around couplet numbers. This variation hinders the process of parsing a key in text format by a computer software. The use of standard formatting would make the exchange of information easier. The format in which the key is saved by the DKey software can become such a standard.

The DKey software is an easy-to-use and freely available dichotomous taxonomic key editor. It can be used for importing existing keys, editing them and exporting them in various formats. Some of the formats are suitable for publication in scientific journals; others make identification easier and faster. The DKey software should be useful for both taxonomic experts creating keys and those who use them for identification.

I would like to thank Shaun Winterton and Lyubomir Penev for the helpful comments on an earlier version of the manuscript. This study was supported by the Ministry of Science and Higher Education grant number DS-3500.