Fig. 1. Schematic representation of the larval cardiac tube. Only cardiac cells are shown in this scheme. The cardiac tube is formed by two rows of cardiac cells which are both epithelial and muscular cells. There are six pairs of cardiac cells per segment from T3 to A7 and only two pairs in T2 and A8. The boundary between aorta and heart is situated within segment A5. The cells shaded grey express tin, ß-3 Tub and D-sur. The cells colored yellow express svp and Tb66F. It has been proposed by Molina and Cripps (Molina and Cripps, 2001) that the svp-expressing cells in the heart form the larval ostiae. This scheme has been drawn according to the actual knowledge on cardiac tube differentiation. cvv, cardiovascular valve; am, alary muscles.