THE BEHAVIOUR OF FREE-LIVING MITES AS AFFECTED BY HUMIDITY(Acarina; Oribatoidea)

BY D. S. MADGEDepartment of Entomology, Rothamsted Experimental Station, Harpenden, Herts.

Oribatid mites may be divided into groups of"indicator species" (ecotypes) as followsxerophyl, mesophyl or hygrophyl . The experi-ments outlined here were made to study theeffects of humidity on mites from these groups .

Mites were exposed in a linear humiditychamber made of glass with a sintered glass floor .Most experiments were done at 15°C . in the darkand records were taken half-hourly or hourlyfor 3-4 days .

Humerobates rostrolamellatus (a xerophyl)much preferred the lowest humidity but thisinitial dry reaction was reversed to a moist oneover 4 days. For instance, 34 per cent . and 9 percent . of them chose 10 per cent. and 95 per cent .R.H. respectively for 1-12 hours but from 91-96hours 16 per cent. and 20 per cent. had selectedthe same humidities . Both adult and juvenileBelba geniculosa (a mesophyl) chose the highestR.H. region after a day . Platynothrus peltifer (amesophyl or a hygrophyl) also chose the moistzone. Adult Fuscozetes fuscipes (a hygrophyl) didnot react to linear humidities .

To eliminate end-effects experiments were alsodone in a "bifacial humidity gradient" and theseconfirmed the above results. Desiccated miteschose their preferred humidities quicker thannormal mites. For example, 73 per cent . desic-cated adult B. geniculosa chose 95 per cent . R.H .during 0-3+ hours whereas only 36 per cent .normal mites selected the same region . The

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initial dry reaction of H. rostrolamellatus was,however, not immediately reversed even after 4days desiccation . Experiments at 5°C . or 25°C.and tests on starving mites did not change thepreferred humidities of the mites .An alternative humidity chamber was made

from paraffin wax cells with perspex sides and asintered glass floor . Results confirmed theresults given above, but gave some additionalinformation. Batches of H. rostrolamellatus re-acted differently to a choice of 30 or 95 per cent .R.H ., depending upon their initial distribution .B. geniculosa andP. peltifer showed greater sen-sitivity in the higher range of humidities ratherthan in the lower range . Results were again in-conclusive with F. fuscipes and they appearedindifferent to any alternative humidity butpossibly sometimes chose the moister humidity .With all the mites, the greater the differences ofR.H. offered, the stronger is the humidity react-ion. It was tentatively concluded that the hygro-receptors of H. rostrolamellatus function at allhumidities, those of B. geniculosa and P. peltiferwork at high humidities rather than at lowhumidities . Results with F. fuscipes suggest thatthis mite may have no humidity receptors .

A film showing the behaviour of individual B .geniculosa in a humidity gradient suggested thatthe hygroreceptors might be located on theforelegs. This was supported by laboratoryexperiments .