INBREEDING, rAETHENOGENESlS. ASEXUAL REPRODUCTION 249 



concerned with the retention of amphimixis, and how little mere 

 multiplication has to do with this. This is especiall\- striking in the 

 case of the bark-liee ; for instance, in their notorious representative, the 

 vine-pest, FIcylloxera vabtatrLv. 



As in all plant-lice, the advantage for the sake of which sexual 

 reproduction was given up depends upon the fact that a practically 

 unlimited food supply is at the disposal of these parasites of the vine, 

 which can be made full use of during the proper season, and which, 

 since evei-y animal is female and produces eggs, results in an 



Fig. 127. Life-cycle of the Viue-pest ^PJiylloxera vastatrix), after Leuckart 

 and Nitsche, and Hitter and Eiibsamen. A, the fertilized ovum. B, the 

 resulting apterous and parthenogenetic Phylloxera. C. its eggs, from which, as 

 the uppermost arrow indicates, there may arise similar apterous, partheno- 

 genetic forms, or, as the horizontal arrow indicates, winged forms (D), which 

 produce 'female' and 'male' ova (£' andX^" ; from these the sexual generation 

 arises, the female (JF^) and the male ^P-" ; the former lays the fertilized 

 ovum (J^. 



enormous increase in the number of individuals, and thus secures 

 the continuance of the species. These insects emerge in spring from 

 small fertilized eggs, which have lain dormant throughout the winter 

 (Fig. i2j,A). and they develop rapidly into wingle.ss females {B), 

 which, sucking the juice of the -s-ine, multiply by producing large 

 numbers of little white eggs (C). These develop without fertilization 

 into similar wingless females. Several generations of females succeed 

 each other, but then, usually from Augtist onwards, diflerently fonned 

 winged females [D} make their appearance, and these, flying from 

 plant to plant, etfect the distribution of the species. But these, too. 



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