The biology of the weta Zealandosandrus gracilis : (Orthoptera: Stenopelmatidae) from the Cass region
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Aspects of the biology of Z. gracilis in Middle Bush, Cass, and from Craigieburn Cutting Bush were investigated from April 1979 to January 1981. Three recognisable stages in egg development are described and the time spent in each stage given. Length of stage 1 was variable with development possibly being triggered by cold temperature. Size frequency distributions were constructed using measurements of nine body structures from 175 specimens. Results of these plots indicate that male Z. graailis have seven larval instars whereas females have nine; growth increments at moults were significantly larger in males than females. Development of male 'and female genitalia is described and illustrated. Collections of wetas made monthly for 27 months indicated that Z. gracilis has a non-seasonal life history with virtually all instars and eggs present at all times. Aspects of adult biology were also investigated. Sub-adult females had a slightly higher egg count than adults with the difference being attributed to nutrition and competition for space by the ovarioles. No premating courtship behaviour was observed and sexual recognition appears to be by touch alone. The method of copulation and oviposition is described and corresponds to methods used by other Stenopelmatidae. The observed sex ratio of Z. graacilis in Middle Bush varied, depending on the method of collection. However, the 1:1 ratio obtained by hand collecting was considered to be the least biased. Regeneration of appendages including two cases in which an entire limb was regenerated from a coxite, are described and illustrated. Cannibalism occurred within the laboratory population but there is no evidence to suggest that it occurs under natural conditions. The diet of Z. gracilis was investigated by analysing crop contents of 67 individuals collected throughout the year and including representatives of all instars. Wetas were carnivorous feeding mainly on small invertebrates inhabiting the forest litter. Population density was examined by long term pitfall trapping, followed by destructive sampling within three enclosures constructed in Middle Bush. Densities ranged from 0.71 to 1.63/m² , with highest numbers being associated with the presence of Nothofagus seedlings (1 - 3 m tall). Mean weta densities in destructive sampling 0.25 m² areas of moss were 0.74/m² . A laboratory study showed that pitfall traps captured Z. gracilis very inefficiently, as only a small percentage (32.7%) of the animals which contacted a trap actually fell in. Finally, Z. gracilis was examined for internal and external parasites. Mites, fungi, and gregarines were recorded as well as two undescribed and unrecorded hymenopteran egg parasites [an undescribed genus of Scelionidae and an undescribed species of Platypatasson (Mymaridae)].