Step 15: Use the line in the middle as a guide to draw the second leg that's on this side of the fly's body. Use thin segments as you follow the basic path of the guide line. The base of the leg is blocked a bit by the leg itself.
Step 16: Use the line on the left as a guide to draw the third leg that's on this side of the fly's body. Draw the individual segments as you follow the path of the guide. Make sure that all the legs are coming from the thorax, not the abdomen.
Step 17: Use the arc on top of the fly's body as a guide to draw the wings. Darken the shape and extend the line so that the base of the wing is inside the thorax. Add another line above the wing that's parallel to the shape for the fly's wing on the other side.
Step 18: Use the initial shapes as guides to draw the fly's thorax. Make the shape wavier as you darken the lines. The lines for the thorax shouldn't overlap the legs.
Step 19: Darken the outer portion of the initial guide lines on the left to create the shape of the fly's thorax.