You see a bully take an insect and dissolve it in some acid. Bullies are messed up, mean people. They do awful things that normal kids and people wouldn't do... like dissolve a living creature.
The only bullies heartless enough to dissolve insects like this are youths (ゆう). Do you really think an adult would melt a harmless critter? No. Only inexperienced youths could be so cruel.