But for an ecosystem that runs from the Canadian border to the Long Island Sound, maintaining ecological balance is crucial, said Rodger Gwiazdowski, the entomologist who is leading the project and who has been dreaming for nearly two decades about restoring the Puritan tiger beetle.

The beetles aren’t an obvious missing link in an ecological chain; no other creature is known to have died or flourished as their numbers diminished, as far as Dr. Gwiazdowski knows.

But “they’re part of a healthy river,” he said. “When we see a stretch of healthy river, we see beetles.” So, restoring them should help support the health of New England’s longest river, whose watershed is home to more than 2 million people. “It’s powerfully naïve to think it means nothing” to allow the Puritan tiger beetles to disappear, he said.

Which is why Dr. Gwiazdowski found himself motoring down the Connecticut River in October, remembering all he’d done to ensure that these immature beetles arrived safely at their destinations. If the boat should sink, he had a plan to float three tubs of beetle larvae to shore.