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 ( Margarodcs ) 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 Astevodiaspis quercicola. Growth takes places 
around the female, which remains fixed to one spot, causing 
pits about 2 mm. wide and 1 mm. deep. They may be 
found not uncommonly on scrub Oaks in summer. 
MytilaspispomoYum, a species 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. pomovum does 
give rise to these galls on Heather in Britain as well as on 
