HEMIPTERA. 



16- 



quired wings, they rise in the air and spread to new vineyards, where 

 they lay their eggs, usually in the down of the under sides of the 

 leaves. Each individual of this generation lays from three to five, 

 and sometimes as many as eight eggs. These eggs are of two sizes; 

 the smaller, which produce males, are about three-fourths the size 

 of the larger, which produce females. From these eggs are hatched, 

 in the course of a fortnight, the third, or wingless, sexual form. It is 

 a very remarkable fact that this form emerges from the egg not as 

 larvae but as fully developed individuals. These sexual individuals 

 are born for no other purpose than the reproduction of their kind, and 

 are without means of flight, or of taking food. After pairing, the body 

 of the female enlarges somewhat and she is soon delivered of a solitary 

 egg. This impregnated egg gives birth to a young louse, which devel- 

 ops into the first or sedentary, agamic, wingless form; and thus re- 

 commences the cycle of changes through which the insect passes. 



It has been discovered that sometimes the first form, during the 

 latter part of the season, lays a few eggs, which are of two sizes, like 

 those of the second form, and like those also produce males and 

 females. These males and females are precisely like those born of 

 the winged form, and like them produce the solitary impregnated 

 egg. Thus the interesting fact is established that even the winged 

 form is not essential to the perpetuation of the species. 



Fig. 137. — Phylloxera, gall-inhabiting form, a, b, newly hatched nymph, ventral and dorsal view ; . 

 epR- ; d, section of gall ; e, swelling of tendril ; f, ,<>, k. mother gall-louse, lateral, dorsal, and ven- 

 tral views ; /, her antenna ; /, her two-jointed tarsus. Natural sizes indicated at sides. (From Riley. 1 



If to the above account we add that occasionally individuals 

 abandon their normal underground habit, and form galls upon the 

 leaves of certain varieties of grape-vine (Fig. 137), we have, in a 

 general way, the whole natural history of the species. 



