"And she uncovered his feet and lay down" (Ruth 3:7). "And it came to pass at midnight," etc. (ibid. 3:8) — that he had arisen to occupy himself with Torah. "The man trembled and turned" (ibid.). He came to cry out; immediately she seized him. And this expression is nothing but an expression of grasping, as it is said: "And Samson grasped" the two middle pillars (Judges 16:29). What is the meaning of "and he trembled"? He thought: perhaps it is a spirit or a harmful demon (mazziq). He placed his hand upon her head and found her to be a woman, for the demons (shedim) have no hair.
Why Boaz Felt Ruth's Hair on the Threshing Floor
Midrash Tanchuma Buber, Bo 16:4
ותגל מרגלותיו ותשכב (שם), ויהי בחצי הלילה וגו' (שם שם ח), שעמד לעסוק בתורה, ויחרד האיש וילפת. (שם), בא לצעוק מיד אחזה בו, ואין הלשון הזה אלא לשון תפיסה, שנאמר וילפת שמשון (שופטים יז כט), מהו ויחרד, סבר שמא רוח הוא או מזיק, נתן ידו על ראשה ומצאה אשה, כי השדים אין להם שיער.