Thyreus, STING MECHANISM

ENGEL, MICHAEL S., 2007, A Lateral Gynandromorph in the Bee Genus Thyreus and the Sting Mechanism in the Melectini (Hymenoptera: Apidae), American Museum Novitates 3553 (1), pp. 1-12 : 3-6

THYREUS STING MECHANISM

The sting and its associated structures in Thyreus (including, in a wider sense, the sixth sternum, the hemitergites of the seventh and eighth segments, and the gonangula and second valvifers) are typical of those of the parasitic Apinae . The sting is enlarged, and the metasoma has correspondingly undergone modification in two complementary directions to house the retracted sting (which may extend basad beyond the apical margin of the second sternum; cf. fig. 1 ) and to accommodate the extended range of movements necessary for its full exsertion.

The sixth sternum, which forms the floor of the sting atrium, is large, elongate, and trough-shaped. Postmedially, it carries a pair of angular lateral processes that are indirectly connected (through loose intermediate connections with the hemitergites of the seventh segment ventrally) with the apodemes of the hemitergites of the eighth segment basally to provide an axis about which the sting and its more intimately associated structures (the hemitergites of the eighth segment and the gonangula and second valvifers) may rotate on exsertion or retraction. Apically it is narrowed, and its sides are more strongly reflexed and incurved to form a semi-tubular sting guide.

The other structures associated with the sting have already been described (vide supra).

THE RETRACTED STING

When the sting is retracted ( fig. 2 ), the hemitergites of the seventh metasomal segment are inverted, and the hemitergites of the eighth segment, with the gonangula and second valvifers, are inverted and displaced basad, as the result of rotation (in an anticlockwise sense as seen from the right side) about the lateral processes of the sixth sternum. The costal processes of the second valvifers and the gonostyli are accordingly displaced ventrad and directed apicad and are located with the stylet in the trough of the sixth sternum. The rami of the first and second valvulae are located dorsally and directed basad and ventrad to a point above the membranous anterior margin of the sixth sternum (between the basal extremities of its apodemes), where they are abruptly flexed apicad immediately before the origin of the sting. The furcula is directed dorsad and apicad, with its appendix located in the conjunctiva between the bases of the valvifers and gonangula.

THE EXSERTED STING

When the sting is exserted ( fig. 3 ), by means of forces generated in the anterior part of the metasoma, the hemitergites of the seventh and eighth segments rotate clockwise around the lateral processes of the sixth sternum—the former through about 140 °, when their spiracles regain a normal, dorsal position; the latter through about 160 °, bringing their points of articulation with the gonangula into a ventral position near the base of the sting guide. The gonangula and second valvifers are now erect (with the costal processes of the latter and the gonostyli separated from the sting and directed dorsad and basad), the rami of the first and second valvulae are directed apicad and located in the sting guide, and the base of the sting lies near the apex of the guide. Concurrently with the rotation of the hemitergites, the furcula hinges clockwise on its articulations with the base of the sting and, as the sting travels back through the sting guide, describes an angle of about 150 °, coming to rest in a plane a little above that of the rami of the valvulae, where it functions as a strut between the base of the sting (above the insertion of the rami of the valvulae) and the region of the inter-valvifer articulations. These movements may be readily demonstrated in preparations from dried material.

Simultaneously with these movements within the sting atrium, the sixth sternum (which externally is largely enveloped by the lateroventral lobes of the sixth tergum) is itself partially exserted. This movement, and the upward probing or stinging movements of the apical segments of the metasoma repeatedly observed in living material, are presumably enabled by the great development of the sternal apodemes (cf. fig. 1 ). While in dried material the sting, when exserted, is not usually exposed beyond about the middle of the bulb (and then issues from the apex of the sting guide), in a few instances it may be found fully exserted and directed upward through the dorsal commissure of the guide. The gonoplacs are not exserted but remain within the sting atrium: having no capacity for movement independent of that of the second valvifers, they become (as mentioned) separated from the sting with the rotation of the gonangula and valvifers. Although the apices may just protrude from the opening of the sting guide when the sting is retracted or only slightly exserted, and consequently the apices may be presumed to retain a limited sensory function, it is apparent that their usefulness as ‘‘sting palpi’’, to revert to an older term, has become sensibly diminished.

Figures

Fig. 1. Thyreus ramosus (Lepeletier de Saint Fargeau), dorsal view of female metasoma depicting genitalia in situ. Scale bar 5 1 mm.

Figs. 2, 3. Thyreus ramosus (Lepeletier de Saint Fargeau), female, lateral view of genitalia, setae omitted; seventh hemitergite not shaded or stippled; eighth hemitergite densely shaded; gonangulum solid black; second valvifer stippled; furcula, sting, and sixth sternum (except anterior weakly sclerotized area) densely stippled. 2. Position of sclerites with sting retracted. 3. Position of sclerites with sting exserted (the sting is shown drawn away from the sting guide to make the relative positions of the furcula and the rami of the valvulae clearer). Scale bars 5 1 mm.