Instar number inMelanoplus femurrubrum(DeGeer) is variable. Virginia populations were found to be developmentally polymorphic, having from five to eight nymphal instars. Grasshoppers with five instars were rare. In the field, fewer instars were associated with years with low May–June temperatures. Variation in instar number among populations, between years, and within seasons is due to variation in early season temperatures. A laboratory population had six to eight instars, with fewer instars occurring at lower rearing temperatures. Females went through more instars than males. Antennal segment number was correlated with instar number. Virginia populations ofM. femurrubrumare developmentally adapted to a relatively long growing season.