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 tip is not everted, the aculeus must be dissected to examine the tip. The aculeus consists of an elongate 8th tergite, a pair of slender, elongate 8th sternites, which are often difficult to see, and the cerci and intermediate parts which are fused to the 8th tergite. The aculeus tip is the apical part of the aculeus, distal to the apices of the 8th sternites and the cloacal opening. The lateral margins of the tip, and sometimes extending slightly proximal to the tip, in ventral view may be entire (nonserrate) or may bear serrations of various sizes to varying extents. The length of the serrate part is measured from the most basal serration to the extreme apex of the aculeus. In a few species the serrations, more often the proximal ones, curve onto the dorsal side of the aculeus, and these should be included in the length of the serrate part. The length of the aculeus tip is measured on the ventral side from the inner margin of the sclerotized area just distal to the cloacal opening to the extreme apex of the aculeus.
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