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Meanwhile I found out that you did not cite from a paper of "Matile & B-B 1969" (http://www.online...n_n_sp.pdf). The linked paper deals with discrimination between the genera Keroplatus and Cerotelion, especially the larvae. The right paper where the quotes and drawings come from is this: http://www.online...arctic.pdf. It is a paper from 1986 by L. Matile alone.

I am no Keroplatidae expert and as a photographer do not deal with preparation of genitalia. My knowledge comes from the literature and Diptera.info :D (And I know Russian ;))

I understand from Kjærandsen et al. (2007) that all the shapes of the middle ventral outgrowths depicted in Zaitzev (for K. testaceus, K. tuvensis and K. dispar) can in fact be found in K. testaceus samples together with a forth form. So K. testaceus alone can have outgrowths that are
- narrow triangular
- broad triangular
- narrow and bicuspidate (with triangular cleft at tip)
- broad and short bicuspidate.
Given this variability, I would not especially look for "concave sides" - I know what you mean, but I think all these descriptions concern only the upper part of the outgrowth, and Zaitzev's drawings do not show much concavity there...

That is all I can contribute to our discussion - not much, I am afraid. Perhaps you could ask the authors of this paper https://www.resea...and_Serbia about their opinion; Jukka Salmela did the morphological examinations and is also a Diptera.info member (though rarely seen here). Perhaps you should write him an e-mail.

Regards, Sundew