Why might clonal raider ants place young larvae on pupae, as they do in this photo taken by Rockefeller University researcher Daniel Kronauer, who is also a Howard Hughes Medical Institute investigator? Because there's something nutritious in it for them and it elicits parenting behavior.
In their Nature paper postdoctoral fellow Orli Snir, Kronauer and colleagues figured out that ant pupae secrete a 'social fluid' that alters the physiology and behavior of ants. The substance is nutrient-rich and contains hormones and psychoactive substances.
Here's a short video I did about their work: