Graduate Thesis Or Dissertation
 

The leafhopper genus Tiaja Oman (Homoptera:Cicadellidae), with a contribution to the biosystematics of the group

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  • The genus Tiaja Oman consists of eight known species of flightless leafhoppers of the subfamily Megophthalminae found along the western coast of North America between British Columbia and northern Mexico. Species occurring from the San Francisco Bay area northward are restricted to a narrow coastal zone within 1 km of the ocean; some of those species that occur farther south may be found inland as well as on the coast. Members of the genus occupy an unusual habitat in that most individuals are found on the undersides of their host plants or in the litter layer beneath them. Flightlessness and the position the insects occupy on their host plants are seen as adaptations to enhance survival in a windswept environment. Life history studies of T. friscana (Ball) and T. arenaria Oman show that nymphs normally pass through five nymphal ins tars although some individuals may have four or six nymphal ins tars. The final nymphal ins tar is of longer duration than any of the preceding ins tars but the length of the developmental period appears to depend on temperature conditions in the rearing chamber. The northernmost species, T. arenaria, is univoltine with obligatory diapause while T. friscana and T. montara Oman, two species of the central California coast, are multivoltine with no diapause under favorable conditions. Other species are presumed to be multivoltine. Host association studies show that members of the genus Tiaja are usually found associated with woody shrubs and that these shrubs may serve as feeding but not necessarily oviposition hosts. The oviposition host of T. arenaria is Fragaria chiloensis (L.) Duch. Infraspecific variation in disjunct populations of T. friscana is examined. These populations lie 550 km apart and the range of another species, T. montara, intervenes. Crossbreeding experiments show that while there is some genetic differentiation as a result of isolation, the populations are similar enough and hybridize to a sufficient extent to be considered members of the same species. Crossbreeding experiments between T. friscana and T. montara show that these allopatric species from the San Francisco Bay area hybridize to some extent when artificially placed together. However, fertility is considerably reduced compared to parental crosses and this is taken as evidence of the reliability of morphological features used for species differentiation in this genus. Factors influencing the success rates of these crosses are discussed. The presence of members of the genus on islands and in populations isolated from the remainder of a species despite the limited mobility of the group is discussed. It is speculated that dispersal of T. arenaria to Vancouver Island subsequent to Pleistocene glaciation, dispersal of T. insula Sawbridge to Santa Barbara Island following its submersion in the late Pleistocene, and the colonization of San Simeon by T. iris cana can be attributed to dispersal by drift of egg-containing host plant material aided by ocean currents and prevailing winds. Taxonomic information includes a discussion of the place of the Megophthalminae within the Cicadellidae, recharacterization of the genus Tiaja, a revised key to the eight known species, and species recharacterizations including morphological data, distributions, seasonal occurrence, and host and habitat information. Criteria for differentiation of nymphal ins tars through setal patterns and wing pad development are discussed and illustrated. It is concluded that further study of the habitats, life history, behavior, and cytogenetics of the Ulopinae and other members of the Megophthalminae will be necessary before current data on Tiaja can be used to help redefine relationships within and between these subfamilies.
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