Blooming palo verde trees are one sign that spring has arrived.

DAVID SANDERS / ARIZONA DAILY STAR 2006

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The well-muscled but lean woman in the long-brimmed shade hat, khaki shorts and hiking boots whom we met at the foot of Tumamoc Hill on Sunday had news:

"We had our first rattlesnake sighting today," she announced.

"Where?" we asked, of course.

"About 50 yards past the first gate, just off the road on the left," she said. "It's a good-sized one."

That, we would say, depends upon your definition of "good." We were glad that the snake had apparently moved along by the time we hiked past the spot.

Hikers on the Cactus Forest Trail in Saguaro National Park were startled Sunday by a sign warning that aggressive mountain lions had been sighted in the last 36 hours. As the Star's Enric Volante reported Sunday, the big cats are a part of urban life here.

All this, and more, signal that the Sonoran Desert spring is full upon us, with all its ambivalent blessings.

Above their razor-sharp spines, the hedgehog and prickly pear cactus are abloom. The orange blossoms at the tips of ocotillo are luring hummingbirds and bees.

Reptiles, some of them venomous like the rattlesnake and gila monster, are emerging from hibernation to bask in the warm sunlight and hunt their prey. Scorpions roam again.

Whether you are allergic or not, the palo verdes are draped in yellow flowers and the mesquite and ironwood soon will erupt into color as well.

And the insects — stinging, biting, sucking, creeping and swarming — are celebrating new lives as the weather verges toward summer.

"In May, the cicadas will come out to sing," rhapsodized University of Arizona entomologist Carl Olson. "And then, when it gets over 70 at night, the kissing bugs start to fly."

These last are, in Olson's words, "the really neat blue feeding insects that live with pack rats." Some people go into anaphylactic shock when bitten, "but it's rare," Olson says.

All kinds of native bees are busy again. Look around the flowering palo verde for huge carpenter bees — soon the males will be yellow and the females flat black.

"They look aggressive, but they're just curious. They fly up into your face and fly away," says Olson.

Olson also wants you to understand that the ubiquitous clouds of tiny teeming gnats are "usually nonbiting, but they do like to get a drink of water in your nose, or eyes or ears.

"What can I say?" he adds. "You're an oasis."

As the Star noted in a photo essay recently about desert critters, bobcats, coyotes, rabbits and many other animals deliver their young in spring. You will need to be watchful for protective mothers and keep a respectful distance — if possible.

Don't forget to be alert for the mountain lions that are prowling our valley and its foothills.

But take comfort: You probably won't risk stumbling upon a baby rattlesnake until summer.

On the floral front, the news of spring is more benign. Though the poppies, lupine and other wildflowers are past their prime, a wet spring is producing sago lilies that do not bloom in dry years.

"It looks like a tulip ... a very deep orange with a blackish center," Michael Chamberland, curator of horticulture at Tucson Botanical Gardens told us. "The Tucson Mountains are good place to go to see them. There are also larkspur blooming, usually alongside the sago lilies, either in very deep navy blue or sky blue."

Though spring will end as it must and summer will follow, do not despair. "Next the saguaro will bloom. Then we'll have the monsoon, and all sorts of things will bloom," Chamberland says.

The desert is a rare and fragile blessing, as we all know. And, as we are constantly reminded, its dangers are as bountiful as its beauty.

Read the Star's story tracking mountain lions in Tucson and see an interactive map of local mountain lion encounters at azstarnet.com/special/mtlions. For facts about native critters, visit azstarnet.com/special/critters.