Here is my perspective on sitting with itchy noses, aching legs and backs, crawly spiders and such ...
Let me mention first that one day I was sitting at Sojiji (head temple of Soto Zen in Japan) in a 150 year old wooden building, with rafters nibbled by termites and with an overly heavy roof, when a big earthquake hit ... the roof is swaying. Everyone but me was Japanese. I looked around to see if folks would run for the door ... nobody moved from Zazen or even looked around but me! There is a great cultural tendency in Japanese culture to just "bear up" with pain and disturbances, and I have rarely if ever seen an experienced Japanese sitter move, shift legs or scratch during Zazen. In fact, my Japanese dentist tells me that there are two main difference between his Western patients and Japanese patients: (1) Westerners ask a lot more questions about the dental procedure and everything else; (2) Westerners moan and scream much more easily. :shock:
So, I have to balance my recommendations by taking into consideration what is just "Japanese culture", what is "Zen teachings", and what may be "BOTH"!
Generally, our Way of Shikantaza Zazen is about "just sitting" with whatever is, just how it is. As in life-in-general, not every moment is peaches and cream, fun and games. So, it is wonderful Practice for us to sit with discomforts, pain, annoyances. We drop all thought of the words "discomfort", "pain", "annoyance", "like" "dislike" "good" "bad" ... and just sit with what is.
On the other hand, the Buddha's way was never one of masochism or asceticism, complete denial of the body. In fact, ours is the Middle Way, the path of moderation in all things. Furthermore, we do not harm the body.
Thus, my advice is to just "sit with" the itches, discomforts and spiders (unless a highly poisonous spider!) . Note it, then move back to open, spacious sitting. In fact, you will find that the more your mind fixates on it, and thinks about it, the more of a problem it becomes. By not thinking about it, the so-called "problem" may even fade away on its own, or not be experienced as a problem.
But if you reach the point that there is truly the risk of harm to the body, then please give a small "Gassho" and discreetly and quietly change position (even do your Zazen standing or do Kinhin walking), or move the giant killer spider away.
Furthermore, our Way is not about ALWAYS having discomfort, so if your legs or back hurt all the time, you may have to change your sitting position or wait for your legs and back to adjust. We do not sit with leg pain every day or most days.
But, on the other-other hand, if you have a physical condition that means you ALWAYS have pain and there is absolutely no way to avoid that (for example, if you have arthritis that is always there), you just sit with that. You allow it, not thinking "good" or "bad".
You have to decide for yourself when is the time to sit with the ache or spider, when is the time not to.
Oh, and in case of earthquake ... I recommend "duck and cover" Zazen under a table, with one's Zafu placed over one's head! Notice the small prayer of "holding gratitude" at the end too.
PS- Wonderful true story about the Dali Lama and a mosquito, ...