This is the story of a woman born into an impoverished rural family in the early 1900's, starting the day she is born and ending on the day she becomes a grandmother. The title refers to the way an insect's behavior is determined simply by the need for survival, acting by instinct alone and unaffected by concepts such as love or morality. In the same way, this woman goes through life rarely giving a thought to anything other than her own self interest. For example, she eventually drifts into prostitution and betrays her madam in order to take her place, only to be betrayed herself by a girl she mistreated.
Yet we can't really blame her too much. People with the same lack of moral standards surrounded her childhood and her life was beset by tragedies and exploitation by others which seemed to conspire to make it impossible for her to escape the kind of life she led. At the end of the movie her grandchild is about to be born to the same circumstances as she was, thus the cycle perpetuates itself to another generation.
Overall the movie is thought provoking and worthwhile. The sympathetic yet unflattering portrayal of the seamier side of Japanese society reminded me of Threepenny Opera. However, there seemed to be too much going on to fit into a small amount of time. This left a lot of gaps in the story, which made it a bit hard to follow, and left some important characters undefined since they appeared in only a few scenes.