I am with Lilac. They're probably sneaking in by way of the vent. If your furnace is also in the basement you might want to check those vent seals too.
My Aunt always used pomegranates (the fruit) lying around in her basement and she said they repelled the spiders?
Ortho Home Defense Max works for us. We use it around the perimeter of the house in spring and fall.
Those spiders may be from an egg sac that hatched. Or they might be coming in through the dryer vent. Make sure everything is sealed around the vent so if they come in the vent hose they can't get out.
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