Host Relations and Distribution of Australian Species of Trombella (Acarina, Trombellidae) Parasitizing Grasshoppers

The geographical distribution and host relations of the five species of Trombella parasitising adult 'short- horned' grasshoppers in Australia are described on the basis of data derived from a screening of tens of thousands of grasshoppers in the Australian National Insection Collection and covering 133 localities and 335 individual hosts in 79 species. Most of the data refer to T. cucumifera Southcott, which occupies a large region across northern Australia, where it is sympatric with T. fusiformis Southcott, and extending south into New South Wales. T. rugosa Southcott and T. sternutor Southcott are sympatric in the south- west of Western Australia. T. calabyi Southcott is represented by a single specimen from the arid east-Pilbara area of Western Australia. Host species are listed for each species of mite. Eumastacidae and Tetrigidae appear not to be attacked, but within the other two Australian families (Pyrgomorphidae and Acrididae) representatives of all of the five Australian subfamilies serve as hosts. For T. cucumifera, attack rates of 20-67% of the captured samples of some species of those families were recorded at some localities. There is a suggestion that the most abundant of the geophilous grasshoppers present at any locality may be favoured and that the species of Trombella do not differ in their utilisation of the grasshopper species accessible to them. There is no evidence of sex bias in host selection. The most common number of mites found on a single host individual is one in the three mite species with adequate data; it ranges up to 11, but numbers above five are rare. All species are attached almost exclusively to the membrane between prosternum and mesosternum of the host, or the membranous insertion of coxa 111.