NOTES ON THE NATURAL CONTROL OF OYSTER-SHELL SCALE. 189 



The ability to feed upside-down with apparently as great ease as when in the more 

 normal position is due to the presence of long tarsal suckers, on the tips of which 

 the mites walk, and to the long caudal hairs that adhere tenaciously to surfaces as 

 slippery as polished glass. On glass slides it was found that Monieziella, which 

 had no tarsal suckers and much shorter caudal hairs, could not walk at all ; 

 Hemisarco]]tes however — perched up as it were on tip-toe — would quickly walk out 

 of the microscopic field quite as actively on the lower surface of this slide as on the 

 upper. In either case the long caudal hairs were invariably dragged along the surface 

 of the slide and were never carried in the air. This ability to walk on slippery 

 surfaces is probably of considerable assistance in climbing young apple and thorn 

 twigs. 



Scaly twigs warmed up in the laboratory were frequently found with some of the 

 adult mites migrating from one scale to another. When the mites have not had a 

 meal for some time they are flat enough to crawl through the very slight opening 

 usually found between the bark and the caudal end of the scale. In a few cases 

 mite-free scales have been encountered where the surrounding ones had all been 

 mite-infested. It seemed that these few scales were so closely attached to the bark 

 that the mites were deterred from entering. The eggs are evidently deposited by 

 these wandering mites in groups of one to six in each scale ; they are small, less than 

 half the diameter of the oyster-shell eggs, fairly white, and usually to be found in 

 the posterior end of the scale. Scales have been repeatedly found showing no trace 

 of mite work other than a tell-tale little group of minute eggs in the caudal extremity. 



In Canada, as in Iowa and in France, the mite is able to hibernate in any or all 

 of its stages. In material collected in the winter I have found eggs, six-legged forms, 

 and eight-legged adults, though the last have been found in far the greatest abundance. 

 The species maintains its activity at comparatively low temperatures, and this is 

 to a considerable extent responsible for its usefulness, as it enables the destruction 

 of scale eggs to be continued until the days, as well as the nights, become frosty. 

 At Fredeiicton the mites were found to be active on warm days even in December, 

 although most of the pre-winter feeding was accomplished during August and 

 September, as Shimer found in Ohio. Mites brought into the laboratory in January 

 warmed up sufficiently to walk in a very few minutes, so that the species can 

 evidently make use of any warm days during late autumn and early spring. 



The present observations seem to show that Hemisarcoptes is perhaps even more 

 useful than has been supposed. Two districts have come to notice in which the scale 

 is evidently on the verge of total extinction on account of the immense numbers of 

 these mites. One of these is Moncton ; at this place the infestation of the scale 

 was fairly heavy in 1916, but the mites were so abundant that by 16th October they 

 had killed nearly all the eggs ; an estimate of the ten collections itemized on the chart 

 (page 191) indicated an egg destruction of over 90 per cent., and mites were found 

 in all but 2 '7 per cent, of the 1,400 sample scales examined ; with the mites increasing 

 and the food supply decreasing it seemed certain that by hatching time in 1917 there 

 would be exceedingly few scale eggs left, and a field examination made in October 1917 

 showed this promise of enormous destruction to have been amply fulfilled (vide 

 fig. 4). Similar conditions were found in a group of places in Huntingdon County, 

 Quebec, including Havelock, Hemmingford, Covey Hill, and Barrington. The 



