How to tell apart 17-year cicadas and the dog-day cicadas that arrive every year

For the first time in 17 years, the Chicago area saw periodical cicadas emerge this spring. But those are long gone now.

Any cicadas you'll see these days are annual cicadas, or "dog-day" cicadas, the ones that come out every year and mark the end of summer with their buzzing at dusk in August and September.

It can be difficult to keep the two types straight.

"It is confusing," said Stephanie Adams, the Morton Arboretum’s plant health care leader. "Biology's messy."

Here's how to distinguish between the two types of cicadas:

Differences in appearance

Two of the most noticeable differences between the annual cicadas and periodical cicadas are their size and coloring. The periodical cicadas are smaller, around an inch and a quarter long at their largest, and the annual cicadas are around 1.5 times the size of that, according to Adams.

The 17-year cicadas also have bright red eyes and orange veins in their wings. The annual cicadas are far less colorful, with black eyes and black or greenish wings.

Along with the timing of their emergence, the physical appearance is a good indicator of which cicada you encounter, according to Jim Louderman, a collections assistant in the Field Museum’s Gantz Family Collection Center.

"They don't look anything alike," Louderman said. "The dog-days are much larger. There's no way you could mistake a dog-day for a periodical or vice versa."

Distinguishing between periodical and annual cicadas should be easy because of their different appearances. But within each type, there are several species of periodical cicada and annual cicada, and those distinctions are much harder to make just by looking at the insects.

"One thing to keep in mind is there are many different genus and species of cicadas," Adams said. "Unless you're a cicada enthusiast or cicada expert, you're probably not gonna notice that much of a difference."

Timing of cicadas’ emergence

The periodical cicadas' time in the sun is over for the next 17 years. This year marked a historic year for cicada emergence, as the breeding group that emerges every 17 years overlapped with the 13-year breeding group, seen in downstate Illinois.

The swarms of periodical cicadas emerge from underground in the late spring, around late April or early May depending on the weather and soil temperature, and stick around until July. But the annual cicadas don't come out until later in the summer, and there's not much overlap, if any, between them, Adams and Louderman said.

"They only overlap for maybe a week or two," Adams said. "Any cicada you're seeing right now is going to be annual."

If you're seeing cicadas now, they are almost definitely an annual cicada because of the timing. Encountering the two cicadas at the same time isn't necessarily impossible, but it's pretty unlikely, Louderman said.

"I guess it can happen, I've never seen it," Louderman said. "I've never encountered them at the same time. There's usually a week or so between them. It's pretty unusual, if [it happens] at all."

While the dog-day cicadas emerge around the same time every year, they still spend multiple years underground like the 17-year cicadas do. The emergences are staggered, though, so some will emerge every year in the late summer, Adams and Louderman said.

The trademark buzzing of cicadas also differs depending on the type. Annual cicadas will usually sing at dusk, while the periodical cicadas are loudest in the late morning and early afternoon, Adams said.