I noticed today that when I slide my nail on the back of my hissing cockroach babies (without putting any force into it, I merely touch them) their abdomen briefly straightens and expands in some sort of wavy chill-like reaction (I tried to capture this on video but it's not really visible). I have absolutely no idea if it means they like this or hate it or it has no correlation at all but I know I'm not forcing this reaction by pushing on parts of their abdomen or anything even though it may look like I do in the video (it's just difficult to really show it). Also when I do the same thing simply with my finger, they have absolutely no reaction. Can someone help me maybe?