180 



TRANSACTIONS OF THE ILLINOIS 



insect is, if placed head to tail, they would absolutely make a line 

 300,000,000 miles long, and, if placed in close compact over a superficial 

 area, cover 102 square miles. This will give some explanation of the 

 rapid spread of this disease in Europe. 



This gall form is but a transient form. Three or four years ago it was 

 most abundant, especially on the Clinton ; last year it was more on the 

 Taylor and the Delaware than on the Clinton. This year it has scarcely 

 been seen. As I said, it is only a transient form, the true being the root 

 form or radiciclosa. (See Figure 3.) 



Fig. 2. 



Explanation of Fig. 2. 

 — a, winged female ; b, her 

 foot or tarsus — after Signoret ; 

 c, an enlarged egg , d, the 

 newly hatched gall-inhabiting 

 form, ventral view ; e, same, 

 dorsal view ; f, a section of a 

 gall ; g, the tubercled root- 

 inhabiting form; //, the 

 mother gall -louse, at the 

 height of her fertility, ventral 

 view ; i, same, dorsal view — 

 all from nature ; j and k, dif- 

 ferently veined wings of the 

 oak phylloxera, of Europe. 

 All these figures are greatly 

 enlarged, and the natural size 

 is approximately shown by 

 hair lines. 



On the roots they collect in little companies, and cause, by their 

 punctures, not galls, but swellings of the roots, (as seen at b, figure 3.) 



The moment the li( e have done their work and begin to leave, these 

 swellings will shrink up, and the roots decay and fall off. So it is with 



