Three thoracic morphs are recognized: winged, wingless and intermediate (new term to identify flightless adults that develop from nymphs with wing pads). The wingless morph may be recognized in the fourth nymphal instar by the absence of wing pads. An individual of the fourth instar with wing pads must be observed in the fifth instar to identify the morph. In an intermediate morph of the fifth instar, the wing pads may reach to the second abdominal segment, remain short, become vestigial, or be absent. A winged morph of the fifth instar has long wing pads which extend to the third abdominal segment or beyond. Results of crossing studies gave little evidence to suggest that any one of the nine kinds of crosses (three morphs, sexes taken reciprocally) would yield significantly more of any one kind of morph. The occurrence of morphs (as nymphs) was related to field temperatures, called "the estimated mean air temperature" and derived from the climatologist’s "march of monthly mean temperatures." Fourth and fifth instars of the wingless morph were collected when the estimated mean air temperature ranged from a spring low of 16.7 C to a summer high of 28.3 C and to a fall low of 8.3 C. Fourth and fifth instars of the winged and intermediate morphs were found primarily, but irregularly, when estimates of the mean air temperature were 21.5 C or higher. Nymphs have one or two morph-determining points in development. The first, whether or not wing pads develop, occurs in all nymphs. The sensitive stage for the first critical point is unknown, but it is thought to occur prior to the late third instar. In the field, high temperatures and unknown factors during the first critical period are apparently necessary for a high frequency (11-45%) of nymphs with wing pads. In the laboratory, a high frequency of nymphs with wing pads was reared from a parental stock with a frequency of 11 to 45% and when the fluctuating temperatures averaged 25.6 C or higher. The second critical period occurs early to midway in the fourth stadium of nymphs with wing pads. A fourth instar with wing pads is more likely to become a winged morph in the field than in a laboratory rearing, but it is not known why.