Not all larvae grow at the same rate, meaning that there is variation in larval length, even among a group of larvae of the same age. Patterns of variation are represented in graphical displays and changes in these patterns are studied. Individual variation in development, for example, in the timing of molting, can be studied using other representations.

Initially, students note that larvae of the same age differ in size. This thinking is refined as they represent variation quantitatively, for example, by making ordered lists of larva lengths. They create and interpret displays, such as frequency displays, that show patterns of variation within the population of larvae. They can also compare variation at different times in the larval life span.