Nema and the Xenos is the story about  Nema (a Steinernema nematode and the hero in our story), the relationship she has with the Xenos (Xenorhabdus bacteria), and how they work together to come to the rescue of a tree whose roots are under attack from a beetle larvae and gain something in return.  The story illustrates for us the importance of chemistry, with Nema using chemical cues from the tree to guide her and her friends through the dangerous soil environment to where the beetle grub is hiding.
 
As with all the stories in this series, they illustrate how nature is all about working together and sacrifice, and how even the smallest of creatures has an invaluable role to play