The first longitudinal wing vein is

The first longitudinal wing vein is the Costal vein.
The wings are strengthened by several longitudinal veins, which often have cross-connections that form closed "cells" in the membrane.
The veins (and their branches) are named according to a system devised by John Comstock and George Needham the Comstock Needham system.
The costa (C) is the leading marginal vein in most insects. The costa rarely ever branches because it is at the leading edge, which is associated at its base with the humeral plate.