Researchers at the University of South Dakota have released the federally endangered dragonflies that were raised in a laboratory over past few years. These federally endangered dragonflies were released at a forest preserve last week in Illinois.

The Hine's emerald dragonflies, which were thought to be extinct for past few decades, were carefully raised at the University of South Dakota for last four to five years after eggs were collected from a dragonfly in southwestern Wisconsin.

Researchers said in a statement that three out of 20 dragonflies that were fit for release have already been freed this week at a forest preserve near Chicago. Not more than 320 of the insects are left alive in Illinois, they said.

Project leader Daniel Soluk, a professor at the University of South Dakota, said, "We are trying to maximize their survivorship in captivity, and not very many dragonfly eggs survive to become adults". Bringing these endangered dragonflies to the laboratories increases their chance of survivor, they added.

The Hine's emerald dragonfly was first discovered in Ohio, but by the mid-1900s, scientists believed the insect was extinct. The belief changed when one adult specimen was collected in the Des Plaines River Valley, southwest of Chicago, in 1988.

The Hine's emerald was listed as a federally endangered species in 1995. It can now be found in Illinois, Michigan, Missouri and Wisconsin.

According to experts, the female Hine dragonflies lay their eggs by dropping the tip of their body into shallow water. Immature eggs known as nymphs, hatch from the eggs in the spring and live in water for nearly two to four years.

Later, they crawl out of the water, shed their skin for the last time and emerge as flying adults. The adults live for only four to five weeks between June and August.