The red-legged grasshopper, Melanoplus femurrubrum femurrubrum (De Geer), was found to be developmentally polymorphic. Local field populations in Virginia had five, six, seven, and eight instars, determined by antennal segment counts. Adults with five instars were uncommon, and six and seven-instar adults were the most abundant morphs. Laboratory studies showed that decreasing temperature decreased both the number of instars, and developmental rate. Grasshoppers with fewer instars had the differential number of instars deleted from between the third and penultimate instars, thus, temperature must affect instar number early in the life cycle, i.e., before the third instar. The laboratory study produced no five-instar grasshoppers. Most individuals had six or· seven instars, depending on temperatures. Few eight-instar individuals were produced in the laboratory. Grasshoppers with more instars had longer developmental times and larger body sizes. Females were larger than males and tended to go through more instars, however, males and females which developed with the same number of instars did not differ in developmental time. In field populations, instar number was positively correlated with accumulated heat units the month of hatch, but body size was negatively correlated.

Mean population in Melanoplus spp. grasshoppers is positively correlated with mean species body size. In the laboratory, ovariole number in M. femurrubrum was related to the size of the egg from which the female hatched, and in field populations mean population ovariole number in was determined by mean length of the growing season, and rainfall in the maternal generation. In field populations number of pods laid was less than two, and was related to rainfall in September, while pod size was related to body size and rainfall in October. Numbers of pods laid and total eggs were negatively correlated with the tegmen/femur ratio.