Abstract: 【Aim】 To determine the instars of Sitobion avenae (Fabricius) rapidly based on morphological characteristics.【Methods】 The body length, body width, head width, antenna length, cornicle length, and hind tibia length were measured in the winged and wingless morphs of S. avenae, based on the comparisons of external morphological characteristics. 【Results】 The results indicated that the body length, body width, head width, antenna length, cornicle length, and hind tibia length are significantly different among different instars of wingless and winged morphs. The body length, body width, head width, and antenna length were observed to have high overlaps between adjacent instars. However, the hind tibia length showed low or no overlap between adjacent instars of winged and wingless morphs. Except for an overlap of 13.93% between the 4th instar nymphs and adults of the winged morph, the cornicle length displayed low or no overlap between adjacent instars of two wing morphs, indicating that the hind tibia length and cornicle length could be regarded as remarkable criteria to identify instars of two wing morphs. Other external morphological differences were also found in the antennae, wing pads, and cauda among instars. Wing pads and wing length of the 3rd-4th instar nymphs and adults are larger in the winged morph than in the wingless morph, indicating that the external morphologies of wings are useful for the determination of 3rd-4th instar nymphs and adults or wing morphs. There are five antennal segments in 1st and 2nd instar nymphs and six antennal segments in 3rd-4th instar nymphs and adults, respectively. The cauda is welldeveloped in adults while incomplete in nymphs. The number of antennal segments and development degree of cauda could be used as the convenient features to divide the instars. 【Conclusion】 Practice has shown that the instar stages of wing morphs could be quickly identified using the cornicle length and hind tibia length as the remarkable criteria, based on the convenient features such as the external morphologies of the wings, number of the antennal segments, and development degree of the cauda.

Key words: Sitobion avenae, wing morph, winged morph, wingless morph, instar, morphological characteristics