434 



AN INTRODUCTION TO ENTOMOLOGY 



Fig. 504. — Leaf of grapewith galls of Phylloxera. 

 (From Riley.) 



appear upon the lower surface of the leaves galls, which are more or 

 less wrinkled and hairy (Fig. 504), which open upon the upper surface 



of the leaf, and each of 

 which contains a wingless, 

 agamic plant-louse and her 

 eggs; second, when the fi- 

 brous roots of a sickly vine 

 are examined, we find, if the 

 disease is due to this insect, 

 that the minute fibers have 

 become swollen and knotty; 

 or, if the disease is far ad- 

 vanced, they may be en- 

 tirely decayed (Fig. 505, c). 

 Upon these root-swellings 

 there may be found wing- 

 less, agamic, egg-laying 

 plant-lice, the authors of 

 the mischief. 



The life-histor>' of this 

 species is a complicated one, 

 due to the fact that parallel 

 series of generations with 

 different life-cycles may be developed at the same time. While a 

 fertilized winter egg may be considered a part of the typical life-cycle, 

 some of the agamic females hibernate on the roots of the vine and 

 form a part of a series of agamic generations that apparently may 

 continue indefinitely year after year. 



The typical life-cycle, that one in which males and sexually 

 perfect females form a part, extends over two years and includes 

 four forms as follows : 



The gallicolcB. — From an over-wintering fertilized egg, there hatches 

 in the spring a wingless agamic stem-mother, which passes to a leaf 

 and by her attack causes the growth of a gall, within which she passes 

 the remainder of her life. She reaches maturity in about fifteen 

 days, fills the gall with eggs, and soon dies. The young that hatch 

 from the eggs laid by the stem-mother resemble her in being wingless 

 agamic females ; they escape from the gall, spread over the leaves, and 

 in turn cause the growth of galls. Six or seven generations of this 

 form (Fig. 506) are developed during the summer. They are termed 

 the gallicolcB. 



The radicicoloB or colonici. — On the appearance of cold weather, 

 young hatched from eggs laid by the gall-inhabiting form pass down 

 the vines to the roots, where they hibernate. This completes the 

 first 3^ear of the two-year cycle. In the following spring these colonici, 

 that is, settlers in a new region, attack the fibrous roots, and cause 

 the growth of knotty swellings on them (Fig. 505, b, c) and ultimately 

 their destruction. This is the most serious injury to the vine caused 

 by this species. There is a series of generations of the root-inhabiting 



