17-year cicadas are waking up. They have red eyes and are as noisy as a lawnmower. There's nothing to fear.

A "brood" of big, red-eyed cicadas is slowly emerging in the eastern U.S. this month after patiently waiting 17 years underground.

"Brood IX cicadas have emerged in North Carolina, Virginia, and West Virginia," according to CicadaMania, which bills itself as a website "dedicated to cicadas, the most amazing insects in the world."

The bugs are harmless to humans, other than the "ick" factor if you're not a fan of large, noisy insects. 

As many as 1.5 million of the bugs are emerging each acre, entomologists at Virginia Tech say, which can be quite noisy. 

“Communities and farms with large numbers of cicadas emerging at once may have a substantial noise issue,” predicts entomologist Eric Day of Virginia Tech. “Hopefully, any annoyance at the disturbance is tempered by just how infrequent – and amazing – this event is.”

The cicadas' loud noise is made by males to attract females. The males contract ridged membranes on their abdomens to make the sound, which is amplified by their almost-hollow abdomens.

Each species has its own sound, and the chorus can reach 90 decibels – as loud as a lawnmower, CicadaMania said.

Although cicadas are harmless, they're big enough to startle humans. The largest ones can have 3-inch wingspans. If enough of them emerge, the crunchy critters can wreak havoc outdoors as they scuttle across parks, patios, decks and sidewalks.

Young, small trees can be damaged when females deposit their eggs inside branches, said John Cooley, a research scientist from the University of Connecticut.

Here they come: Cicadas will emerge in North Carolina, Virginia and West Virginia starting this month

An adult cicada usually lives two to four weeks, which isn't very long after waiting 17 years underground.

There are at least 15 separate cycles, or "broods," of periodic cicadas in the USA. Some emerge every 17 years, while others come out every 13 years. Some species show up every summer.

Different broods of cicadas appear in various parts of the eastern half of the country in varying years, according to CicadaMania.

More than one type of brood may emerge in some areas at the same time because of staggered development, according to the United States Department of Agriculture. 

The timing of a 13- or 17-year cycle is one of the great mysteries of the insect world, according to Virginia Tech. Research and mathematical modeling suggest that the length of these brood cycles could be attributed to predatory avoidance.

This article originally appeared on USA TODAY: Cicadas: 17-year brood insects are emerge in the US. Are they harmful?