so... where to stop?

This has happened to me before... once you start looking at things, you keep finding more, and more, and more structures, characters and character states, and then you really don't know where to stop... maybe it has to do with my obsessive-compulsive behavior.

At this point for me, there are two main causes of a need-to-stop decision: one has to do with the taxonomy, the other with the anatomy.

As you may know, I've been looking at the morphology of the Hydrophilidae and lately have been focused on head and mouthparts.

On the taxonomic side, a need-to-stop comes from getting too excited in your task to find the origin of your characters. There are some structures that are consistent across an entire family, and when you compare to works in other related groups, the same structure does not look at all as what you see in your group of interest. It could be a synapomorphy, but how to be sure?. Then you keep going up the tree to end up looking at other related families. If the same character state is present there, the next logical step would be to keep going up the tree. And then you end up looking at a very large group that not necessarily answers what you were asking, but leaves you with even more questions. So where should you stop? because at some point this can become an avalanche of characters and character states that can easily end up in a never ending story. Fortunately, advisors exist and get you back to Earth, limiting the outgroup sampling so you don't go (too) crazy [they -usually- know better].

An example of this would be the hypopharyngeal-premental sclerites which are conserved across hydrophilids, in contrast with what has been described by Weide et. al (2014) for Aleocharine staphilinids, and also VERY different from what you see in Scarabaeinae (see for example Tarasov & Génier 2015). It is a very tridimensional structure formed by fusions of the hypopharyngeal suspensoria (see Snodgrass 1935). Question is, where did this particular shape originated?.

Mentum in lateral view: staphilinid,  dung beetle and Globulosis

As for the anatomic side of a need-to-stop situation, there is this fine example: the maxillae and the mentum were studied by Williams in 1938 across Coleoptera. She described and illustrated these structures for Tropisternus glaber. The problem is that when you compare her drawings to a real sample, it looks like this:

Maxilla (top), Mentum (bottom): Tropisternus glaber (by Williams 1938); Globulosis hemisphericus.

As you can see, there are way more characters to look at, and it gets "worse" when you start looking at other genera, as more character states become more apparent.

Maxilla, different groups of hydrophilids.

So again, where do you stop among so many details? you can easily spend a lifetime just looking at each varying component of these structures (and I'm not even considering musculature, for example, and I'm only using one sample preparation technique). Then I guess it is a matter of judgement, practicality, informational value, and being conscious that you don't have a lifetime to complete your dissertation!

References

Tarasov, S. & Génier, F. (2015) Innovative bayesian and parsimony phylogeny of dung beetles (Coleoptera, Scarabaeidae, Scarabaeinae) enhanced by ontology-based partitioning of morphological characters. PLoS ONE 10(3): e0116671.

Weide, D., Thayer, M. K., & Betz, O. (2014). Comparative morphology of the tentorium and hypopharyngeal–premental sclerites in sporophagous and non‐sporophagous adult Aleocharinae (Coleoptera: Staphylinidae). Acta Zoologica, 95(1), 84-110.

Williams, I. W. (1938). The comparative morphology of the mouthparts of the order Coleoptera treated from the stand-point of phylogeny. Journal of the New York Entomological Society, 46(3), 245-289.