STRUCTURE, SENSORY EQUIPMENT, AND SECRETIONS OF THE OVIPOSITOR IN A GIANT SPECIES OF HYMENOPTERA: MEGARHYSSA ATRATA F. (ICHNEUMONIDAE, PIMPLINAE)

Abstract

AbstractMegarhyssa atrata F., the largest hymenopteran parasitoid known, is found in the holarctic. The stylus of its ovipositor can reach a length of 142 mm, the longest of all known genitalia in Arthropoda. This pimpline parasitizes its host, Tremex columba L., at a great depth in wood. Valvulae 3, which are always shorter than the stylus, are characterized on their outer side by abundant trichoid sensilla of three different types, and on their inner side by a dense layer of cuticular spines probably used in cleaning and shielding the stylus while at rest. The joint region between stylus and abdominal segments is richly equipped with mechanoreceptive trichoid and campaniform sensilla. The ovipositor canal is located between the two valvulae 1 which are separate for their entire length and supported proximally by the two valvulae 2 which are joined to each other from the base. The distal end of the stylus, shaped like an indented lancet, has secretory pores, the first such to be observed in Hymenoptera, from which accessory gland secretions flow. These secretions reach the end of the stylus by intravalvular canals in valvulae 1 and 2. These substances, which may cause cell lysis and also act as lubricants, can accumulate among the tens of thousands of cuticular formations distributed along the stylus which are thought to be surface storage cavities for the secretions. The sensory equipment of the stylus consists of six rows of approximately 200 mechanoreceptive campaniform sensilla. The ovipositor of M. atrata is thus characterized by almost exclusively mechanoreceptive sensory equipment; chemical stimuli appear to be monitored by two types of trichoid sensilla on valvulae 3. The finding that secretory pores occupy exactly the same sites as chemoreceptors in Ichneumonidae, Chalcidoidea, and Proctotrupoidea demonstrates the importance of studying the ovipositor of this parasitoid adapted to withstanding extreme pressures as it penetrates deep into wood.