HEMIPTERA HOMOPTERA. 



699 



FIG. 444. Phylloxera vastalrix. a wingless root louse 

 seen from the back ; b from the ventral surface ; 

 c winged form. 



duce males as well as females, and then pair. The female gives rise to 

 a fertilized egg which survives the winter and in the spring a partheno- 

 genetic female emerges from this. The relation of the winged to the wing- 

 less forms is obscure. As 

 a rule many wingless genera- 

 tions succeed one another 

 and then a generation of 

 winged females suddenly 

 appears and undoubtedly 

 aids in the dispersal of the 

 species. The curious ab- 

 dominal processes are said 

 to open into the body 

 cavity. They excrete a 

 waxy fluid. The honey- 

 dew (so prized by ants, 

 who in some cases have 

 domesticated certain species 

 of Aphides) comes from the 

 alimentary canal. Some 

 genera e.g. Pemphigus, 

 Chermes (p. 640;, Schizon- 

 eura produce galls. Phoro- 



don includes the Hop plant louse, Nectarophora the green-pea plant louse, 

 Drepanosiphum the maple plant louse. Aphis is perhaps the best known 

 genus. 



Phylloxera vastatrix * is the well known enemy to vines and affords 

 a good example of the complicated life-history presented by the Aphi- 

 dae. The wingless root-dwelling forms radicolae are found with 

 their proboscis firmly fixed in the tissues of the young roots. They do not 

 move about, but lay little clumps of thirty to forty parthenogenetic eggs,. 

 which give rise in six to twelve days, according to the temperature, to 

 young larvae. These moult once or twice, creep about a little, and then 

 fix themselves by their probosces and lay parthenogenetic eggs like their 

 mother. In this way many agamic generations succeed one another, 

 and the rate of increase is so great that it has been calculated that the 

 descendants of a single insect which laid its eggs in March would number 

 twenty-five millions by October. 



As autumn comes on some of the eggs give rise to larvae which are 

 provided with the rudiments of wings ; before their last change of skin 

 they creep above ground, and then at the final moult a winged female 

 emerges and flies awa-y. This form serves to spread the vine disease from 

 one district to another. It is also parthenogenetic, but lays two kinds of 

 eggs. From the larger of these a female hatches out, whilst the 

 smaller produces in eight or ten days a male. This is the first and only 

 appearance of this sex in the life-history of the Phylloxera. The male is 

 devoid of mouth and alimentary canal ; it fertilizes the female, which soon 

 after lays a single fertilized egg, the so-called " winter egg." This is de- 

 posited in some crevice or crack in the bark of the vine. In the spring a 

 " stock-mother " hatches out of this egg and makes her way to the yotmg 

 buds of the vine, and inserts her proboscis into the upper surface of a 



* H. Stauffacher, Zeitschr. wiss Zool , Ixxxviii, 1907, p. 131. 



