Why are there swarms of dragonflies near Lake Michigan in Milwaukee?

You may have seen a few (hundred) insects flying around the Milwaukee area Wednesday night. For some, it was hard to miss the swarms of hundreds that flooded the area.

For those unfamiliar, those were dragonflies, the four-winged insects with long, thin bodies. They are fast and they are organized, but they are harmless. Yet, for the second year in a row, they made a noticeable stop in the area.

For those looking to know more about why, here's what you need to know.

Why were so many dragonflies in the Milwaukee area at once?

This time of year, dragonflies begin their migration south in search of warmer climates for winter, according to Dan Jackson, treasurer of the Wisconsin Dragonfly Society and record reviewer for the Wisconsin Odonata Survey.

More: A look at some dragonflies and the places that help them

The migrant species of dragonflies in Wisconsin typically arrive locally in the spring. That's when females lay their eggs in ponds, lakes and sometimes streams and rivers. The eggs hatch as aquatic nymphs and they spend the summer growing until mid to late summer when they emerge as flying adults. After feeding for a while, they migrate south, Jackson said.

Okay, but why are they making a pitstop in Milwaukee?

Dragonflies are known to travel in swarms during migration. According to Jackson, they congregate a lot on the shores of Lake Michigan and other natural features that funnel them together. For the lake specifically, they choose to hit the lake shore and head south instead of crossing the water. This often results in higher densities of dragonflies. If the dragonflies are passing by, they are actively migrating. If the swarm is stationary, then they are likely feeding in preparation for migration.

However, Jackson said that large swarms can be found anywhere in the state. "I have been seeing a few hundred over a small, feral hay field in rural Vernon County lately," he said.

What kind of dragonflies are these?

While not all species of dragonflies migrate for winter, 10 species in Wisconsin do. Jackson said that these are likely Common Green Darners, which are the most widespread of the migrating dragonflies in the state. He added that there are often other species mixed into these swarms like Black, Red, or Carolina Saddlebags, and Wandering or Spot-winged Gliders.

You can learn more about the Common Green Darners on the Wisconsin Odonata Survey website.

Are dragonflies or damselflies harmful?

They are completely harmless to humans. In fact, Jackson said they are beneficial to ecosystems because they are capable predators both under water as nymphs and as flying adults. The flying adults specialize in eating flying insects and are important predators for mosquitos.

How many species of dragonflies are there in Wisconsin?

There are 167 species of dragonflies and damselflies in Wisconsin, according to Jackson. That is broken down by 119 dragonflies and 48 damselflies. Most of these do not migrate for winter.

Where do the dragonflies go from here?

Jackson said they continue flying south to the gulf states and into Mexico. There, the dragonflies will mate and females will lay their eggs before the end of their life. The nymphs that hatch there will develop over the winter and emerge to fly north for the spring. The Common Green Darners will typically be the first seen in the spring when the first warm winds come from the south.

Are these the only insects that do this?

They are not. One of the most well-known migrating insects is the monarch butterfly, which is an endangered species that embark on a months-long migration that passes through Wisconsin. In Milwaukee County, a group named the Friends of the Monarch Trail have worked to create safe havens for the migrating butterflies.

More: Wauwatosa event celebrates and spreads awareness for the endangered monarch butterfly

Drew Dawson can be reached at ddawson@jrn.com or 262-289-1324.

This article originally appeared on Milwaukee Journal Sentinel: Dragonflies swarm near Lake Michigan in Milwaukee to migrate south