== Presence of an HPA axis in animals == 

 In the intro you write "A wide variety of species, from single cellular to multi-cellular organisms including humans, share components of the HPA axis." 
 Somebody has correctly put the "citation needed" because they doubt that this is correct. I also think that the APH-Axis and corticosteroids are common to vertebrates only. While it is correct to say that insects and other invertebrates have similar systems with similar functions in stress response,they do not have this axis and do not synthesise steroids. See:  
 I suggest therefore: 

 While steroids are produced only by vertebrates, the physiological role of the HPA axis and corticosteroids in stress response is so fundamental that analogous systems can be found in invertebrates and monocellular organisms as well.