After you finish the two missions for one level, you don't have enough food to complete the challenge to unlock the next pair of missions. Currently, the only way to obtain more food is to replay the missions. While the higher difficulties do provide more of a food reward, Hard and Insane are particularly difficult, especially to someone on their first playthrough. Thus, playing the missions for the express purpose of obtaining more food can be something of a boring endeavor.
I suggest that there should be more ways to get food for the formicarium. Immediately, I see two ways to do this; you could add more missions per level, or you could add missions/miniature challenges in the formicarium itself. The latter method could be something like what we already have after completing the second challenge, in that multiple non-ant enemies would be dropped in to be fought, defeated, and harvested. I'm sure there are additional ways to provide a fun, challenging experience as well.
I suggest that there should be more ways to get food for the formicarium. Immediately, I see two ways to do this; you could add more missions per level, or you could add missions/miniature challenges in the formicarium itself. The latter method could be something like what we already have after completing the second challenge, in that multiple non-ant enemies would be dropped in to be fought, defeated, and harvested. I'm sure there are additional ways to provide a fun, challenging experience as well.