The Council of Science Editors has published a White Paper on Promoting Integrity in Scientific Journal Publications that gives a summary of research on authorship and attribution in scientific journals (see section 2.2 of the report). I recommend you begin by reading this material, to get an idea of the general principles for authorship and the required contributions. One of the things that the white paper is designed to address is the practice of "ghost authorship" where a contributor fails to receive authorship credit despite making a substantial contribution to a paper. It also recommends that if someone contributes to a paper, but not enough to receive authorship, then they should still be listed by name in the "Acknowledgements" section.
There is no universally agreed set of rules for the requirements for authorship, but the general view is that a person should be list as an author on a paper if they have made a "substantial contribution" to the paper. You can also find a set of recommended principles for authorship (in the context of medical journals) published by the ICMJE:
The ICMJE recommends that authorship be based on the following 4 criteria:
Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND
Drafting the work or revising it critically for important intellectual content; AND
Final approval of the version to be published; AND
Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
The above principles are written by a well-reputed body of medical journal editors. I do not have any source material from editors of biology journals, but hopefully there is a reasonable correspondence in approaches. In any case, whether or not your coding work constitutes a "significant contribution" to the work depends largely on the magnitude of the contribution relative to the overall paper, whether any customisation or innovation was required in the coding, and whether you made a contribution to the actual design of the study you were coding, or just coded a set of methods/procedures that were already determined by others. On its own, coding the analysis for a study would not usually constitute a "significant contribution", but it would usually constitute enough of a contribution to warrant acknowledgment. (Although it is for a different kind of contribution, you might find it helpful to read Parker and Berman (1998), which sets out recommendations for authorship credit for statisticians working on applied research. This is a different situation, but it may have parallels to someone doing coding work on a research project.)
The main problem you will encounter in this case is that you have not negotiated any credit on the paper prior to doing the work. In future, when you are asked to contribute in some way to an academic paper, it is a good idea to have an initial discussion with the principal author, to make agreement on what your contribution will be, and what acknowledgement (if any) you will receive. You can certainly try to negotiate this post hoc but you might find this more difficult, depending on the attitude of the main authors of the studies in question. In any case, if you feel that your contribution warrants a formal acknowledgement in a paper (or even co-authorship), I recommend you contact the principal author of the paper to start a discussion about that. The best way to do this would be to frame your request in the context of the authorship principles set out in the above documents (or other similar studies), so that you can point to an external objective source to back up your request. In the absence of some more direct document, the ICMJE principles should be applicable to the world of biology papers.
