<p>The number of larval instars of insect pests is very important from a practical perspective. <i>Silba adipata</i> McAlpine (Diptera: Lonchaeidae), is the most destructive insect pest of fig (<i>Ficus carica</i> L.) around the world but the number of larval instars is not well defined. In this study, the number of larval instars and the larval growth ratio of this insect were determined using morphometry and the frequency distribution of four variables measured: length (A) and width of the larval body (B), length of the dorsal cornua of the pharyngeal sclerite (C), and the length of the cephalopharyngeal skeleton (D). The frequency distribution of each measured variable did not clearly express the differences between the larval instars. Therefore, two proportion factors were proposed to locate individuals in the corresponding instar: the first was obtained by multiplying the variables A and B (= proportion factor 1) and the second by multiplying the variables C and D (= proportion factor 2). Frequency distribution and a discriminant analysis for each proportion factor determined that <i>S. adipata</i> has three larval instars. The average growth ratios of larvae of this species were very similar for both proportion factors (1.50 and 1.58 for proportion factor 1 and proportion factor 2, respectively), and they are consistent with the growth constant of Dyar’s rule. We characterized the larval instars of <i>S. adipata</i>, which is a prerequisite for developing appropriate management strategies for this pest.</p>