"of insects" is an adjective phrase because it's a prepositional phrase that acts as an adjective.  It modifies the word "group"; In other words, "of insects" is used to describe the word "group".
 
        
             
        
        
        
It's a hot summer afternoon near Soledad, California, sometime during the 1930's. Everyone is poor and scrambling around desperately for work,food, and money. We meet Lennie Small and George Milton: two guys among the poor and scrambling.