Membranous wings are characterized by having thin, unscleritized (meaning not leathery or hard) membranes between the veins of the wings.  For the purposes of this question on the key, the insects FRONT WINGS must be membranous.  Below are a few more examples of membranous wings.  Butterflies do indeed have membranous wings which are covered with scales (which come off onto your skin if you touch them).  The fly (far right) also has membranous forwings while its hind wings have been modified into halteres, knobs instead of wings.