Biosystematics and morphology of symphyta—III external genitalia of Euura (Hymenoptera: Tenthredinidae): sclerites, sensilla, musculature, development and oviposition behavior

Abstract The insect ovipositor is composed of 2 pairs of gonapophyses: laterally flattened appendage rudiments (endites?) of abdominal segments VIII and IX. The gonostyli (telopodites) of IX may form a 3rd set of blades in some orders. In Hymenoptera, they are sensory and protective devices, 2-segmented in pupal Euura , 1 in adult. Gonapophyses IX in this order form a dorsally fused track for the serrated, independently movable VIII below. Articulation of the hymenopterous gonocoxites VIII has shifted caudally from tergite VIII to the proximoventral margin of tergite IX. The tergogonocoxal (tractor) muscles between it and tergite VIII are atrophied. Articulation of gonocoxite IX in Hymenoptera has moved proximally to the lower edge of gonocoxite VIII. In Symphyta and Parasitica, enlarged paired tergogonocoxal muscles inserting at either end of the L-shaped coxopodite IX constitute the ovipositor tractors. As the 4 contract alternately on each side, gonocoxites IX pivot, rotating VIII back and forth in a substantially greater arc against tergite IX. The swinging of gonocoxites VIII slide gonapophyses VIII back and forth under IX. Muscles between sternite VII and the lower ramus of gonapophyses VIII assist. Gonapophyses IX remain largely stationary by compensatory flexion in their rami, and through tension exerted by gonocoxapophyseal muscles. These are a levator and a depressor on the base of each gonapophysis IX, and which originate on the proximal and ventral rims of gonocoxite IX. Coxosternites VIII and IX lie between the respective gonocoxites, and bear remnants of tergosternal and sternopleural musculature. Outer gonapophyseal faces are sclerotized, and teeth (ctenidia, serrulae) project from the outer and proximal margins of each annulus. Flexible cuticle interconnects the pseudosegments. Mesal surfaces are an expansible corium covered with distallypointing combs (pectines). Folds in the corium (ptyches) form dorsal and ventral walls to the external egg canal between the lateral gonapophyses. Eggs and cecidogenic fluid are forced through the canal by thrusts of gonapophyses VIII and projecting pectines. Patterned campaniform sensilla on the gonapophyses provide orientation. In Euura , the host is entered by both vibratory wedging aside of tissue and actual sawing. Internally, there is a central axon bundle emanating from the terminal ganglion (segments VII-XI), and a tracheal trunk. Both send sequential branches into each pseudosegment. Gonapophyseal cuticle of Euura exhibits a plywood effect, and there may be a resilin component in parts of the cutting edges and between pseudosegments. Male Hymenoptera gonocoxites IX are connected with each other through a basal composite fragmentum (gonobase). The gonobase is attached to sternite IX (hypandrium) by 3 muscle pairs in Euura . The 180° terminalia inversion upon emergence of male imagines twists muscles, nerves and trachea. Gonocoxites and apical gonostyli act as primary claspers, gripping the female gonocoxites. Fragmenta (volsellae) on the inner face of either gonocoxite serve as accessory mesal pincers, grasping female genital membranes. Gonapophyses IX are secondarily separated sclerotically, but remain dorsally connected by muscles and fascia. Since they are independent of the gonocoxites and each other, the 5 muscles from the parent gonocoxite on each permit individual motion. Gonapophyses IX have lost most of their lower rami (rhachies), but portions remain with relict gonapophyseal nerve trunks. No annulation is evident, and the portion not occupied by muscle insertions is filled with fatbody. Campaniform sensilla resembling those of the female stud some areas. The inner faces are a corium bearing pectines and dorsal ptyches. A discussion is included on techniques employed in light microscopy (phase contrast systems, fluorescence), electron microscopy (transmission, scanning), and histological preparations.