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The Water Mite Hydrachna Conjecta Koenike, 1895 (Acari, Hydrachnellae), Bionomics and Relation To Species of Corixidae (Hemiptera)

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image of Netherlands Journal of Zoology
For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

1. The larvae of Hydrachna conjecta and H. skorikowi are described and a survey is given of the larvae of European species of the genus Hydrachna together with their hosts. Corixidae are hosts of H. conjecta and H. skorikowi. Species of the genera Callicorixa, Corixa, Cymatia and Sigara are known as hosts of H. conjecta; hosts of H. skorikowi are species of the genera Callicorixa and Sigara. H. conjecta, H. cruenta, H. globosa globosa and H. globosa uniscutata may be regarded as common species in The Netherlands. The shape and dimensions of the epimera, as well as the arrangement of the bristles, constitute excellent specific characters. A key to the larvae is presented, based on these easily recognizable characters. The form described as Hydrachna georgei is most likely identical with the nymph of H. conjecta. 2. The food of both nymph and adult of H. conjecta consists of corixid eggs. In contrast with Sigara eggs, those of Cymatia, which are stalked, are not taken. Similar to the other Hydrachna species, H. conjecta oviposits in the air cavities of waterplants, in this case Ceratophyllum demersum. The number of eggs per cavity is dependant on its size. The biological importance of oviposition in waterplants is discussed. The maximum number of eggs deposited by one female, over a period of four weeks, was 1475. The time of egg development varies from 17-45 days, dependant on the temperature. At room temperature the hatching larvae must find a host within four days: free larvae are dead after a week. The parasitic phase in the summer is estimated at approximately 17 days. H. conjecta hibernates as parasite or as egg. Eggs may hatch as late as March. In the second half of April the nymphs emerge from the bugs and 3-8 days later enter an inactive stage (tritonymphal stase), which lasts 5-9 days. It is presumed that the life span of the adult animals is approximately one month. There are 2-3 generations per year depending on climatic circumstances and the moment of oviposition. First generation females oviposit from the beginning of May till the beginning of June. In comparison with H. conjecta the egg production of Eylais discreta is much larger, namely between 10,000 and 13,000 per female over a period of 11 2 months. This large egg production is necessary to compensate for the extensive loss of larvae compared with that of H. conjecta. Due to the sampling technique males are often more easily captured than females. Breeding results, however, showed that there actually are more male than female H conjecta, so the surplus of males is real. 3. A description of the gnathosoma is given. The skin-fold, which surrounds its distal part, is characteristic. The chelicerae are composed of two parts, a cylindrical part and a much wider basal part. The basal parts are distally linked by dovetailing grooves and ridges, which play a role in the process of perforating the skin of the host. H. conjecta larvae attach themselves to the corium on the underside of the hemelytra of corixids. They pierce into the blood sinus in the corium. The preference for the corium is discussed. The stylostome, a ramified tubular, structure, develops during the parasitic phase from the mouth opening of Hydrachna larvae in the blood sinus of the host's wing. The development of the stylostome results from a reaction of the haemolymph to the saliva of the mite larva. The stylostome becomes larger and the channels widen during the parasitic phase. In consequence of the longer parasitic phase, the stylostomes are larger in winter, than in summer. Sigara falleni is "immune" to the parasite. Presumably here, the haemolymph reacts so strongly that eventually the sinus in the wing is completely filled with stylostome-like material. The mite larvae die and shrivel. 4. The incidence and intensity of infections was regularly determined for Cymatia coleoptrata, Sigara striata and S. falleni ("immune" to the mite larvae) over a period of a few years. The length of the parasite (H. conjecta larvae) was used as a measure for the growth. The left hemelytron is less often infected than the right one. This phenomenon is most pronounced in Sigara species: here the right hemelytron overlaps the left one more than in Cymatia species. H. conjecta larvae prefer the space between the overlapping parts of the hemelytra to that between the left hemelytron and the abdomen. The parasite grows more quickly and becomes larger on S. striata than on C. coleoptrata. In spring (April) nymphs of H. conjecta emerge approximately ten days earlier from S. striata than from C. coleoptrata. The cause of the growth reduction inC. coleoptrata is discussed. When there are two or more parasites per host, they influence each other only at the end of the parasitic phase. Growth stops during winter. During the parasitic phase on C. coleoptrata, H. conjecta larvae increase 300 à 400 fold in volume and on S. striata 500 à 600 fold. The volume of a full grown larva is 31 % of that of C. coleoptrata and 13 % of that of S. striata. Both the incidence and intensity of infection are lower in C. coleoptrata than in Sigara species. The incidence is usually between 10 and 30% but it is higher in June for both host species. On the basis of the incidence throughout the year, conclusions are drawn concerning host populations and the occurrence of new infections. The distribution pattern of the parasite on various host species can be described by a negative binomial distribution. Comparing these distributions among and within the three host species, we conclude that heavily infected hosts disappear from the population. This is especially true for C. coleoptrata. Factors influencing such distrubition patterns are discussed.

Affiliations: 1: Zoological Laboratory, University of Amsterdam, The Netherlands


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