You absolutely need to respond. It shows that 1. you care. 2. it will increase your review-rate. That is one of your host matrix and listing optimization. The less responsive, the less likely you be ranked higher in position.
I would reply like Ash953, that is a PRO reply. But i put it a little different spin...
"Thanks for your feedback. The amenities you listed were in the cupboard, and I'll make sure future guests know where they are located. [I sincerely apologize for] the spider. [I am sorry this happened to you and you should have bring to my attention immediately and situation could have been resolved to your satisfaction. Furthermore, I have taken steps to ensure this won't happen to you or any of our guest in the future. Once again, thanks for your feedback and staying with us. Safe travels
P.S. 100% Money Back Guarantee if you see a Massive Black spider on your pillow.