Carl's intrusive memory of his grandparents' Texas house and its fireflies, surfacing mid-charge.
As Shrapnel from the Poison-Infused Rocks pings against his shields, Carl thinks the fragments look like fireflies — and his mind goes to his grandparents' house in Texas. They had fireflies there, and Washington, where he grew up, did not.
He immediately catches himself: why would he think of that now? It is a rare glimpse of Carl's pre-The Crawl life, an unbidden childhood image arriving in the middle of the worst battle of his life while Casualty Notification plinks stab through his chest. He shakes it off and keeps running.
A brief flash of Carl's Earth childhood in the middle of the assault.