Got plenty of sun over the three-day holiday, did you?

Headed for more?

Now I know this announcement will hardly clear the beaches, but the fact remains that scientists now believe that the vast majority of skin cancers are caused by exposure to sun.

There are, of course, varying forms of skin cancer, ranging from the easy-to-treat and rarely life-threatening basal cell cancers through a series of slightly more serious ones, to the potentially fatal malignant melanoma.

Even the latter, once virtually a death sentence, can be treated better these days, but prevention is still best. And in most cases, exposure to the ultraviolet rays of the sun is the culprit.

Some dermatologists are saying flat out that blue-eyed blonds should keep to the shadows. Keep in mind that even the most golden of tans is the result of injury to the skin.

And if that isn't enough to keep you out of the sunbeams, at least use a heavy-duty cream or lotion. Remember the higher the number on the label, the more effective the screen or block, although it doesn't do everything. Some prescription drugs can make skin sensitive to other parts of the spectrum not covered by the sun-blocking products.

And don't forget, any kind of a skin lesion, spot, mole, wart or whatever that either doesn't heal, or changes shape or color, should be seen by a doctor at once. Creepy Alert

Nancy Furey of Vienna, Va., woke up one night last month because her leg hurt. She found a sore spot, red and hot, about the size of a 50-cent piece.

The next day she started running a fever and after another day or so her husband insisted on taking her to the emergency room where a doctor suggested she'd been bitten by a spider. She was treated--a tetanus shot, a shot of antibiotics and a prescription for antibiotics--and was sent home with instructions to keep the leg elevated for a couple of days and see her family doctor.

By the time she got to her own doctor she was a little better, but still had a swollen, discolored ankle, abdominal spasms and other symptoms implicating the bite of, perhaps, said her doctor, a black widow spider.

Well, maybe, says Dr. Toby Litovitz of the National Capital Poison Control Center.

Actually there are a lot of spiders with plenty of venom around, but only two whose fangs are strong enough to get through human skin--the black widow and the brown recluse.

Even so, such bites are relatively rare and the only really serious danger is to the very young and the very old. The black widow spider bite can be controlled by an anti-venom in children and old people and calcium does very well to control the spasms in anyone else, says Litovitz.

Brown recluse spider bites can produce ulcers at the bite site and flu-like symptoms. Nasty, but there are other noxious creatures around who do more damage, says Litovitz.

For example, there is the Saddleback Caterpillar, "a quite beautiful creature, really," says Litovitz, "with a venomous disposition. Usually what happens is that someone sees the green caterpillar with the black, brown and white saddle-like markings on the back, crawling on someone else's shirt. So they brush it off. And they get the poisonous spines."

She suggests that the first order of business is to get the spines out. A bit of tape applied and then stripped off often does a good job.

The saddleback can cause reactions ranging from simple dermatitis to severe swelling of the extremities. There are usually about a dozen reported cases each summer.

Some people are also allergic to the hairs of the gypsy moth caterpillar, as if its leaf ravages weren't enough.

Questions about bites, noxious berries and other nasty things? Phone the poison-control center, 24-hours a day: 625-3333. Crawly Alert

And speaking of the spider family, the early onset of the hot and the humid has provided us with early-onset wood ticks.

We pulled a dilly out of our yellow Lab on Sunday, so it seems an auspicious moment for a reminder that Rocky Mountain spotted fever is a genuine, albeit relatively rare, threat.

RMSF can be fatal--especially in adults--if it is left untreated.

Its symptoms usually appear about three days after a tick bite, but this can vary widely: from one to 12 days.

The illness usually starts with headache and fever, followed, on about the third day, with a characteristic rash beginning on wrists and ankles and spreading to the rest of the body.

There may be muscle pain and gastrointestinal symptoms. Eventually there is bleeding into the skin followed by other complications that can lead to death.

However, Dr. Chad Helmick of the U.S. Centers for Disease Control emphasizes that symptoms, especially in adults, can vary. It makes good sense to report any illness following a tick bite to a doctor. (A Maryland resident wrote us that he became ill last summer and insisted to a rather skeptical doctor that his symptoms matched those described above. At his continued insistence, his doctor treated him for RMSF and, indeed, when the analysis came in, that was what he had.) Because the lab reports take so long, they are useful only in a retrospective diagnosis. However, says Dr. Helmick, work is being done to make a rapid diagnosis possible via skin biopsies of the rash. The illness is treated with antibiotics: tetracyclines and chloramphenicol.

According to CDC, there were 1,170 cases of RMSF last summer, 66 in Maryland, 105 in Virginia and 1 in the District.

Dispose of embedded ticks with tweezers and alcohol, applied before and after, pulling on it with a steady, smooth motion. Try not to touch the tick itself. The disease-causing organisms may be transmitted through cuts in human skin.