The aculeus is the elongate, sclerotized, apical part of the ovipositor. Its base is attached to the eversible membrane and these can be retracted into the oviscape. If the aculeus is not everted or visible through the oviscape as is sometimes possible in specimens in alcohol, the terminalia must be dissected to examine the aculeus. Its length is measured from its base, where it connects to the eversible membrane, to its extreme apex.

[range for species with limited data estimated 20% of mean length]