Adults of the damselfly Pyrrhosoma nymphula (Sulzer) were to a great extent found to have their midgut infected with a eugregarine Hoplorhynchus oligacanthus (Siebold). Heavy infestation seriously damaged the gut epithelium, and the viability of the host was estimated to be reduced. Marking experiments have revealed that individuals which attained great adult age were either lightly infected or non-infected, or they were thought to have been infected late in their adult life. The conclusion is that the eugregarine infestation apparently reduces the longevity of adults when other conditions are sub-optimum.
Åbro, A. (Institute of Anatomy, University of Bergen, N‐5000 Bergen, Norway). The mode of gregarine infection in Zygoptera (Odonata). Zool. Scr. 5 (6): 265–275, 1976.—The eugregarine Hoplorhynchus oligacanthus (Sieb.) is an intestinal parasite in imagines of the zygopterans Pyrrho‐soma nymphula (Sulz.) and Enallagma cyathigerum (Charp.). Gameto‐cysts, resulting from the union of a presumed male and a female gregarine gamont, leave the host with the faeces which drop to the woodland floor. Gametogony, zygote formation, and subsequent sporogony take place within the extrahost gametocysts. Infective sporozoites become enclosed in oocysts furnished with a highly resistent shell. When fully formed, the oocysts are released by simple rupture of the gametocyst envelopes. The oocysts, which probably disperse in the habitat by means of rain‐water, appear to enter a cryptobiotic state. Enzymic tests implied that liberation of sporozoites occurred more readily if oocysts were submitted to repeated freezing and thawing. Zygopterans consume lots of small insects; about three‐quarters of zygopteran food was found to consist of Chironomidae and Ceratopogonidae. Examination of various small insects available as prey for zygopterans revealed imagines of medium‐sized Chironomidae and Ceratopogonidae as vectors of Hoplorhynchus oocysts, which were found suspended on their tarsal bristles. An infective oocyst appears to contain 8 sporozoites. When introduced into a host, each of the sporozoites released from the oocyst will eventually develop into a gamont. Only one in a hundred chironomids and ceratopogonids was found to carry oocysts; none the less, this is considered sufficient to cause a high infection of the entire zygopteran populations. The occasional large number of non‐infective oocysts without sporozoites indicates that sporogony and formation of an oocyst capsule are independent events.
Water‐mite larvae of the subgenus Arrenurus (Acari, Hydrachnellae) act as habitual ectoparasites on zygopteran imagines, mostly attached to the soft membranous cuticle. The powerful larval pedipalp claws grasp the cuticle and the distal sabre‐like cheliceral segments tear it, thus obtaining the host's tissue fluids. The chelicerae and palps co‐operate to anchor the larva. The attached larva soon produces—in the host's subcuticular epidermis layer, and separated from the haemocoele by the thin sheet of subepidermal connective tissue—an elongated pouch, the stylostome, consisting of an acellular gelatinous substance, apparently secreted by the larva. Presumably, interaction with components in the host's tissue fluids solidifies the stylostome, which becomes firmly fixed to the host's body wall. The stylostome is thought to lengthen by repeated alternation between the suction of tissue fluids from the host and secretion of fresh soft substance forming additional stylostome segments. The resilient stylostome develops in a cleft between the host's cuticle and the subepidermal connective tissue in connection with partial decomposition of the host's epidermis, probably caused by cytotoxins leaking from the stylostome. In attempts to wall off the wound and thus to avoid influx of foreign substances, the host lays down a melanin sleeve around the stylostome at the perforation site. Lodgement of the functional stylostome separated from the haemocoele within the host's tissues might explain why the newly‐formed stylostome seems immune from cellular host reactions. However, cellular encapsulation and melanization appear to happen to outworn, non‐active stylostomes.
The zygopteran Calopteryx virgo from a habitat in western Norway was found to be infected by the eugregarine Hoplorhynchus oligacanthus. Females were most heavily infected, in contrast to other gregarine-infected zygopteran species, where no differences between males and females have been recognized. This disparity between males and females of C. virgo is attributed to their dissimilar behaviour and place of activity. The gregarine infection of C. virgo is considered in relation to infected coexisting zygopteran populations.
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