The 'prelims' are likely the last school-type exam you'll have: where teacher(s) are asking you questions about any topic that is not (necessarily) your specific research expertise. After this, you will be basically only asked about your research (and its implications of course). So after prelims, you'll want to be broadly informed to generate good research and participate in academic discussions, but you will probably never again be asked a question where the person is asking is sure of the answer and just wants to see if you do, too (or at least rarely ;-) ).
What this means is that the committee will want to make sure that you have all the basic biology knowledge they expect from someone with a PhD in biology (or EEB, or Insect Science, etc.).
Since this may be quite a lot, it is imperative that you ask each committee member in an individual in-person meeting (or, if need be, by email) what they want you to do to prepare, or which areas they think you need to work on. I would recommend doing this, and starting to prepare for the orals, about 6 months before they take place - not because you will need 6 months of full-time studying, but because you likely have many other obligations during this time (already started research, teaching, etc.).
I personally want each of you to have read 'An Introduction to Behavioural Ecology' by Davies et al. (older editions are Krebs & Davies, the newest one is Davies, Krebs, and West). I will never quiz you on a particular example study, but I want you to know the broad concepts treated in this book as well as some examples for them (i.e. you can pick the example, but have one).
I also expect you to have read any recent reviews that came out of our lab, such as these:
Dornhaus A, Powell S, Bengston S 2012 ‘Group size and its effects on collective organization’, Annual Review of Entomology 57: 123-141.
Dornhaus A, Powell S 2010 ‘Foraging and defence strategies’ In: ‘Ant Ecology’, Eds. L Lach, C Parr, K Abbott, Ant Ecology, Oxford University Press
It is also a good idea to be aware of most of the main results that have come from this lab (ask me, scan the publication list, ask around what others are doing) as well as other new areas of research that are broadly in our field (e.g. right now: behavioral syndromes, networks, behavioral genetics, kin and group selection).
Finally, I expect you to have a good grasp of the biology of your study organism. Unless you are studying honey bees, you should have read most of the publications on your particular species, and be aware of other topics of research on the genus. What is the worldwide distribution of the species, genus, family? How many species does the genus contain? How do they vary in natural history? What is the development of an individual, the annual life cycle, etc? Some basic physiology and morphology. You get the idea.