GALLS INDUCED BY PLANT-LICE 89 



scale in some species and a white powdery covering in 

 others. They are mite-like at first ; the female loses the 

 power of locomotion later. The generations are usually 

 similar ; viviparous reproduction and parthenogenesis is the 

 exception rather than the rule. The sexes are usually very 

 different ; the male is very minute. The Coccidae are very 

 remarkable in the great differences exhibited in the post- 

 embryonic development of the two sexes in the few forms in 

 which it has been at all closely studied. 



"When hatched from the egg the young Coccids are 

 all similar, male and female being indistinguishable. A 

 difference soon appears, with the result that the male, after 

 passing through more than one pupal condition, appears as 

 a winged insect. The female never becomes winged, but, if 

 we may judge from the incomplete accounts we at present 

 possess, her development varies much according to species. 

 In some she retains the legs, antennae, and mouth-organs ; 

 in others she loses these parts, though retaining the original 

 form in a general manner ; while in a third (Margarodes) she 

 becomes encysted, and apparently suffers an almost com- 

 plete histolysis, reappearing after a long period (it is said it 

 may be as much as seven years) in a considerably altered 

 form " (Sharp). In Australia certain Coccids cause enormous 

 galls on Eucalyptus, sometimes a foot in length. The galls 

 caused by British species are all obscure. Perhaps the best 

 known are the galls-pits in the bark of Oak twigs caused by 

 the presence of AsUrodiaspis quercicola. Growth takes places 

 around the female, which remains fixed to one spot, causing 

 pits about 2 mm. wide and i mm. deep. They may be 

 found not uncommonly on scrub Oaks in summer. 



Mytilaspis pomorum, aspecies of wide distribution in Europe, 

 is said to cause tufts of little abnormal branches on a slightly 

 swollen part of the stem of the Common Ling. Douglas 

 mentions, amongst other habitats for this insect in Britain, 

 " stem of Heather (Calluna)," but does not allude to the gall. 

 It can scarcely be doubted, however, that M. pomorum does 

 give rise to these galls on Heather in Britain as well as on 



