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Lots of good advice here and I have enjoyed reading the thoughts.
If the doco's are anything to go by, you can get real close so lens length won't necessarily be a problem. It will be IQ that you will need to think about more than anything else.
Perhaps a lens with no CA and excellent sharpness across the frame? I would very seriously consider packing an extremely high quality macro. Given the friendliness of the locals, a manual would be no worries to.
So what macro lens are you taking? May I suggest the Lester Dine 105/2.8???? If AF is required then the Sigma 105/2.8. The Pentax 100/2.8 would also be awesome - weather sealed too - but the 105 is possible a little better for such an awesome trip.
---------- Post added 12-28-15 at 09:16 AM ----------
Of course, my absolute preference would be the FA*200/4 macro or the A*200/4 macro
---------- Post added 12-28-15 at 09:21 AM ----------
Oh and as an after thought, what is everyones thoughts on ND and polarising filters? The galapagos is a very well lit place (equator) and as such much of the photos will be exposed to plenty of scattered light and closed down aperture. If you want to get better separation between subject and fore/background then ND filters would help immensely I suspect.
Just a thought.