324 Agricultural Experiment Station, Ithaca, N. Y. 



into a cavUy witiiiu tiie substance of the leaf (Figs. 3 and 4, o) and in 

 this cavity I'eside the creatures that cause the disease. 



Appearance of the Inhabitants of the Galls. — The minute creatures 

 that attack the tissues of the pear leaf in such a manner as to cause the 



Fig. 4 — Section of leaf showing structure of ?ail in autumn; g, gall; n, uninjured part of leaf; 



o, opening of gall. 



abnormal growths — the galls — which form their homes, are what are 

 popularly known as mites. They are exceedingly small, being practi- 

 cally invisible to the unaided eye; and even with a good lens they 

 a])pear as minute whitish specks. In fact, the best of microscopes is 

 necessary in order to study their structure satisfactorily. So small are 

 they that it woidd take 150 of them placed end to end and 600 placed 

 side by side to measui-e an inch. They are usually whitish in color, 

 sometimes with a slight red or brown tinge. The body is cylindrical 

 in form, tapering slightly towards each end (Fig. 5). It is ringed 

 throughout the greater part of its length with about 100 very fine 

 rings. Its four legs are placed near the head end of its body, so that 

 when the mite walks it drags its body after it. The head is in the 

 form of a conical snout, within which ai"e two sword like jaws. The 

 body and legs are furnished with a few hairs which are constant in 

 number and position. 



Classijicatioji of the Pest. — The cause of this Pear Leaf Blister is 

 not a true insect, but a mite. (The mites are more closely allied to 



..^.,mjl0^f?9!X'<'.W^~->:^ *. 



( 





Fio. 5.— The adult noite, greatly magnified. 



the Spiders and Scorpions than to the true insects.) The southern 

 Cattle-tick, the Itch-mite and the Red Spider are well-known members 

 of the same order {Acarina) of animals to which this pear pest 

 belongs. As a rule, newly-hatched mites have three pairs of logs and 

 a fourth pair is added during growth. The members of the genus 



