The simple and quite versatile study would fit quite nicely here, at least that is what I would have chosen if I were to write a biology essay on butterfly metamorphosis.
From OED v4:
10. a. To examine in detail, seek to become minutely acquainted with or to understand (a phenomenon, a state of circumstances, a
series of events, a person's character, etc.); to investigate (a
b. To scrutinize (a visible object) in order to ascertain its nature or to be familiar with or interpret its appearance; loosely, to
look at as if examining minutely.
This is certainly a less precise choice of word than monitor, it is after all the tenth of fifteen meanings listed in total, but I feel monitor can have some unwanted connotations. Usually the word is used when you are controlling a process. That is, keeping an eye on it in case something undesired were to happen, and in that case possibly rectify it or take some other appropriate action. It does not fit that well the act of observing and dutifully logging a natural process with the intention of acquiring knowledge.