The narrators mark time in the book via the arrival of the fish flies, an insect that lives a short life before dying and stinking up the Great Lakes. They first appear when Cecilia makes her first suicide attempt:
That was in June, fish-fly season, when each year our town is covered by the flotsam of those ephemeral insects. Rising in clouds from the algae in the polluted lake, they blacken windows, coat cars and streetlamps, plaster the municipal docks and festoon the rigging of sailboats, always in the same brown ubiquity of flying scum. (1.3)
Just as Cecilia's suicide spreads a dark blanket of sadness over the town, the fish flies settle on everything, making the neighborhood ugly. Cecilia foreshadows the connection between the fish flies and the suicides:
"They're dead," she said. "They only live twenty-four hours. They hatch, they reproduce, and then they croak. They don't even get to eat." (1.3)
Cecilia has zeroed in on the tragically short life of the flies, and her words are an uncanny foreshadowing of her own longevity. We see a girl fascinated by death.
Just after she dies, so do the fish flies:
No one understood what got into us what year, or why we hated so intensely the crust of dead bugs over our lives. [. . .] The collective action of digging the trench led to cooperative sweeping, bag-carting, patio-hosing. [. . .] We didn't stop with our own houses. Once our walls were clean, [. . .] soon we were all over at the Lisbon house, brushing walls and scraping away bug husks. (3.18-20)
Maybe because they can't erase Cecilia's death and its effects on the neighborhood, they focus their attention on the fish flies, something they can control.
One year later, around the anniversary of Cecilia's death and the time when the girls will follow her in suicidal sisterhood, the fish flies return (nothing supernatural; they're just seasonal creatures):
Around the aureolae of streetlights we noticed a dim swirling we didn't recognize at first because we knew it so well, a senseless pattern of ecstasy and madness: the massing of the first fish flies of the season. (4.126)