arugula prickles

Arugula started bolting in the garden. I found the top of several, looks like a mass of dense prickles. At first I thought an insect was making a nest or cocoon in there, but it appears to be flower head forming. Like a thistle. Another plant habit I'm unfamiliar with. Not ready to see what it does yet- I cut their heads off so we can keep eating the greens.