Entomology definition of Arculus: Odonata; a small cross vein between radius and
cubitus near the base, leaving an elongate triangle between them:
Trichoptera; a point, often hyaline, on the forewing where the
cubitus (or post cubitus) runs into the margin: in Homoptera; a
cross-veinlet nearly reaching posterior margin at same point as
in Trichoptera: in other orders applied to a cross-vein in
similar position, apparently giving rise to the median.