Head and neck region. A, B Relative width of the apical field (dashed lines) A as wide as the trunk (S. tremuloida) B wider than the trunk (S. longipes) C–F Elevation of the apical field (top line) relative to the auricles (bottom line) C level (S. grimpei) D slightly elevated (S. tremula) E intermediate (S. oblonga) F strongly elevated (S. grandis) G–I Elevation of the dorsolateral styles (thick lines) G not or very slightly raised (S. tremula) H intermediate (S. gyrina) I strongly raised (S. baltica) J–M Auricle size (thick lines) and orientation (arrows) J not clearly demarcated from rotatory organ, directed laterally (S. grimpei) K small, directed laterally (S. tremula) L medium, directed semi-caudally (S. oblonga) M large, directed caudally (S. grandis) N–Q Separation of the head and trunk region N gradual transition, the head is not distinctly offset from the trunk (S. tremula) O head and trunk are demarcated by the narrower neck (S. tremuloida) P demarcation by a sharp constriction in the neck region (S. pachypoda) Q by distinct transversal folds (S. oblonga) R Presence of saccate appendages (thick lines) caudal to auricles (S. bicornis). Drawings modified from: P Kutikova and Vassiljeva (1982) R Koste (1981). Scale bars: 50 µm.