In fly tying, there are many ways to form an abdomen/thorax area. In the abdomen region (rear 1/2/ to 2/3’s of hook shank/fly) you can dub fur, hair, synthetics; wrap wool yarns, threads, quills, tinsels, silk and chenilles.

One technique, I like, is to tie in strands of a material, twist it into a fairly tight noodle and wrap it forward and tie off. What you get is a nice segmented body. I pointed to this technique awhile back it is depicted below with strands of Krystal Flash twisted and then wrapped.

green bead head pupa SBThe Twisted Sister (my quaint name to work in the word ‘twisted’…but you can use it too) is a bead headed Pupa Pattern with the twisted abdomen, a dubbed thorax region to provide some flash and leggy material on a size 14 pupa hook. This sub surface pattern is dredged and tumbled along the seams near rapids, riffles and pocket water. It is a touch too big here to be a dropper pattern, but this size can be reduced down: smaller bead, smaller hook, fewer strands of twisted fibers for a more slender abdomen.

nymph parts mffs
http://mrffs.com/atlanticsalmontrinsects.html