Question Dichotomous Keys

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Firstly I just wanted to say that I am enjoying the course - thanks.

I have been doing the pre-reading for the next lecture and just wanted to check my understanding of one small item and get some ID help. You may clarify the point in the lecture so sorry if I have jumped the gun.

The keys say that the Myrmeciinae family have a "waist consisting of two segments, with the post-petiole being much smaller than the first segment of the gaster". In the attached photo are the "two segments" the ones I have labelled B and C (or is C part of the gaster)?

Any suggestions as to the species? I think it may be Myrmecia tarsata or maybe M. simillima.

Many thanks

Phil