To better understand insect evolution, fossils – mainly known by their wings – must be used as terminals in phylogenetic analyses. Such analyses are, however, rarely performed because of a lack of consensus on the homology of venation in insects. Authors do not agree with current concept on the exact number and the identity of the main veins. Here, we confirm the presence, questioned since the early twentieth century, of an independent main postcubital vein (PCu) between the cubital and anal veins (29 fossil and extant examined orders, >85% of observed insects). The vein PCu corresponds to the so-called vein 1A or first anal vein. It is easily identified by the unique shape of its bulla. It may have several branches, and be partially fused with the cubital and anal veins. Once the PCu vein identified, we reconsidered as an example the particular case of the Phasmatodea, showing that extant stick insects have a unique venation among insects, with a reduced median vein, and a simple cubital vein, adjacent or fused to the PCu. This study is a new approach towards resolving wing vein homology issues, crucial for future large-scale phylogenetic analyses in insects combining extant and extinct taxa.