CHAPTER II. 

 The Sizes, Shapes, and Colours of Galls. 



The chief interest in galls lies in the formations themselves, 

 and not in the creatures which cause them. Yet the galls and 

 their producers are so inseparably associated, that a study of 

 one carries with it a knowledge of the other. This fact is very 

 clearly illustrated when considering the sizes of various galls. 

 It would naturally be expected that the more diminutive the 

 creature the smaller the gall, but this is not always so. 



The ? imago of Pliyllocoptes accricola is 120 yit. long and 

 46 /i. broad (/it. = roVrith part of a millimetre). It is one of 

 the smallest of the mites. They live upon the leaves of Acer 

 pseudo-platamis. A few of them will cause the growth of and 

 live within a globular-shaped gall not more than 3 mm. in girth. 

 Eriophyes tilice iypiciis is a much larger mite. The ? is 200 fi. 

 long, 35 /i. broad, yet it causes and lives within a hollow conical 

 gall 8 mm. high and 6 mm. in girth at the base. E. orientalis 

 lives on leaves of Cydonia vulgaris, in hollow scabious pustules 

 of the same size and character as those caused by E. piri 

 on pear leaves. The ? of E. orientalis is 270 /i. long and 

 55 /A. broad. It is next to the largest of all the gall-mites. 

 But what these creatures lack in size they supply numerically, 

 and when thousands are living upon any particular portion 

 of a plant, the galled area is proportionate. Examples of such 

 are furnished by E. inacrorhynchus, E. piri, and E. lu-vis, the 

 multiplicity of whose galls oftentimes absorbs an entire leaf. 



Among the Diptera a similar disparity is manifested between 



the size of the gall and the imago issuing therefrom. The 



action of three or four larvse of Urop/wra cardui in the stem 



of Cardiius arvensis will produce a more or less globular, 



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