So at some point in the first series, (I'm not sure which book.  Sea of Monsters, maybe?  Tell me if you know) Percy noted that Annabeth's hair smells like lemon soap.

So I recently remembered that, for some reason my brain won't tell me, and realized something...

Lemons repel spiders.

Annabeth uses lemon soap to repel spiders.

What other lemon-scented things do Athena campers have?  Deodorant?  Furniture polish?  Tissues?  Some of that Febreze stuff?  Do they drink their water with lemon in it?  The list goes on. 

So I looked up what repels spiders, just to see what else the Athena campers might use.  Lavender, Peppermint, Cloves, and Cinnamon are some of the results that came up.

Cinnamon.

Somewhere else in the first series (Whenever Percy meets Calypso) he notes that her hair smells like cinnamon.

Cinnamon repels spiders.

It may just be a coincidence, but I think Calypso is afraid of spiders, too.