Larvae of I. verticalis reared under laboratory conditions have 4 different growing types, characterized by 9, 10, 11 and 12 instars respectively. This paper is concerned with the morphology of larval forms for individuals of the 10- -instar type, the most common type of development. Morphological characteristics such as antennae, labium, metathoracic legs, gonapophyses and caudal lamellae are principally used in the description of the different instars. Also included is a comparative study of the variability of major morphological structures for the four growing types.