The cicadas are here! All you need to know about mega-brood of bugs in the eastern US

Cicadas, red-eyed bugs singing loud, sci-fi-sounding songs, can seem downright creepy. Especially since the trillions of them coming this year emerge from underground only every 17 years.

But they are not monsters nor a plague of locusts. Once you get to know them, scientists say you can appreciate the wonder of these unusual creatures. So here are some answers to the cicada questions that may be bugging you.

What are cicadas?

They are a family of insects called Magicicadas. They belong to a group of bugs that differ from other insects in that both nymphs and adults have a beak they use to drink plant fluids. Adults have two sets of wings.

There are more than 190 known varieties of cicadas in North America and 3,390 of them around the world.

Most cicada species come out every year. In the US, there are groups of cicadas that stay underground for either 13 years or 17 years. These are called periodical broods.

Except for one species in India and one in Fiji, only the US gets these periodic cicadas.

Cicadas are not locusts. They are not grasshoppers. Those are different species. But when Europeans first arrived in America, some started calling them locusts and even grasshoppers.

What are broods?

Different groups of cicadas come out in different years in different places. This year's group is called Brood X, as in the Roman numeral 10. There are 15 broods that still come out regularly. Others went extinct.

Some come out every 13 years. Some, including Brood X, come out every 17 years.

Why so many years?

Some entomologists think it is an evolutionary defence mechanism. They stay underground so long that most predators will have no memory or history to look for them.

Why so many cicadas?

That is another evolutionary defence mechanism. Lots of creatures – even ants – eat cicadas. When they first come out and try to moult their skin, they can get stuck and are particularly vulnerable to attack.

They come out in large numbers so that some of them will survive. The survivors make the next brood.

Where will they be this year?

Brood X can be seen in Delaware, Georgia, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia and Washington, DC.

Why do cicadas sing loudly?

It is all about sex. Cicadas sing by flexing small drum-like organs in their abdomens, and what you hear in the trees is called a chorus. It is so loud because they sometimes want to woo a female far away. Get thousands of them together and they can reach 105 decibels, which is louder than a lawnmower.

Where do they lay eggs?

Female cicadas make slits in small tree branches and usually lay 20 to 30 eggs in each slit. A female can lay 400 to 600 eggs in a lifetime. The eggs hatch in late July to early August. The hatched cicadas fall to the ground and immediately burrow underground.

What do they do underground?

Periodic cicadas spend most of their 13 or 17 years underground, where they feed off plant roots while their bodies grow and change.

When do they arrive?

Any day now. Early ones are already out. In Tennessee and other southern states they have started coming out in large numbers. They do not come out in mass until the soil temperature at  a depth of about 20 centimetres reaches 18°C.

Are cicadas edible?

Yes – there’s even a cookbook. Experts suggest eating the nymphs but warn anyone with a shellfish allergy to avoid them.

When do they leave?

Once the males mate, they die. After females lay their eggs, they die. So except for the eggs, they will be gone by about July 4. When a lot of them die in the same place, it can smell foul.