HAGERSTOWN, Md. -- This spring's crop of 17-year cicadas will seem like junk food to dogs and cats, and like junk food, they can make pets sick, the Humane Society of the United States warned Tuesday.

"Imagine a yard full of chicken nuggets. That's sort of what it's going to be like," Randall Lockwood, a Humane Society vice president and animal behaviorist, said Tuesday.

He said the insects are protein-rich but their exoskeletons are indigestible, so eating too many can cause vomiting or constipation.

Millions of the large, red-eyed insects are expected to emerge from the ground across the eastern United States within days or weeks for a once-every-17-years mating dance.

Experts say the nymphs will climb into trees, shed their shells to reveal wings, and then go about their business, including loud buzzing by males to attract mates.

The Washington-based Humane Society advises keeping pets indoors, securing screens and holding tight to dog leashes outdoors.

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The 1½-inch-long bugs "combine all the stuff that particularly dogs like to chase," Lockwood said. "They're kind of flying pet toys, they are loud, slow-moving, often low-flying."

For most pets, it's a once-in-a-lifetime experience and almost irresistible, he said.

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"They're just so abundant that this is kind of the canine equivalent of a bag of potato chips," he said.

The bugs have a nutty flavor, David George Gordon, author of "The Eat-A-Bug Cookbook," told The Washington Post.

Lockwood said cicadas are meaty, and eating a few won't hurt your pet. But too many can overload an animal's digestive tract with chitin, the hard substance of which insect shells are made. If a pet has more than two vomiting episodes or appears to be in pain, a trip to the veterinarian may needed, he said.

A little cicada chitin may be beneficial, according to the makers of Skin-eze, a product marketed by South Carolina-based AllergicPet.com to relieve skin itching in dogs and cats. The small amount of cicada chitin in Skin-eze "decreases nervous spasms," according to the company's Web site.