i62 INJURIOUS AND USEFUL INSECTS 



3. Wingless males and females, without mouth or alimen- 

 tary canal. The female, after fertilisation, lays a single large 

 winter egg, from which in the following spring is hatched : 



4. The stock-mother, which produces parthenogenetically 

 either gall-forming generations, which are succeeded by root- 

 infesting generations, or the latter only. 



The structure of Phylloxera shows its near relationship to 

 the aphids, from which it differs in such points as the number 

 of joints in the antenna, and the absence of the abdominal 

 tubules (which are not always found in aphids). Both are 

 polymorphic, exhibiting distinct forms, which are not solely 

 due either to differences of age or of sex, and both are capable 

 of precocious and parthenogenetic reproduction. Phylloxera 

 only in one particular case reproduces after that viviparous 

 fashion, which is normal in aphids. The resemblance of 

 Phylloxera to coccids (scale-insects) is less important, and 

 seems to be due to similarity in the conditions of life, rather 

 than to inheritance from common ancestors. 



Phylloxera has proved a most intractable pest, partly on 

 account of its extraordinary powers of dispersal and multi- 

 plication, and partly because it can always be reproduced 

 from subterranean forms, which cannot easily be attacked 

 without grave injury to the vines. Submergence of the soil 

 in which the vines are rooted has proved serviceable in 

 particular cases, but the insects are not drowned in less than 

 about six weeks, and the situation of most vineyards renders 

 such a remedy quite impossible. Of insecticides carbon 

 disulphide, poured into holes about the vines, or injected by 

 hand pumps, is now largely used in France ; tobacco infusions 

 have also been tried with success. Vines grown in sandy 

 soil resist the Phylloxera better than others. 



32. THE MUSSEL-SCALE OF THE APPLE TREE 

 (Mytilaspis pomorutn) 



Trees and shrubs of many kinds are apt to be infested by 

 scale-insects, which are commonly small and inconspicuous, 

 sticking close to the bark, absolutely motionless, and, unless 

 they multiply to a great degree, passing for casual discolora- 

 tions of the bark. They are not always inconspicuous, 



