About 98% of described species of Lepidoptera belong to Ditrysia. The group can be divided into the primitive but paraphyletic "micromoths" and the derived monophyleticApoditrysia, which includes mostly larger moths as well as the butterflies. Those with a dorsal heart vessel belong in section Cossina. Others, having a ventral heart vessel belong in section Tineina.
Kristensen, N. P. and Skalski, A.W. (1999). Phylogeny and paleontology. Pages 7-25 in: Lepidoptera: Moths and Butterflies. 1. Evolution, Systematics, and Biogeography. Handbook of Zoology Vol. IV, Part 35. N. P. Kristensen, ed. De Gruyter, Berlin and New York.
Capinera, John, editor (2008), Encyclopedia of Entomology, 2nd ed., Springer Verlag, New York.