THE MUSSEL-SCALE OF THE APPLE TREE 163 



however. Some are clothed with white filaments which 

 catch the eye at once. A large species, which infests the 

 elm in Canada, and has a fluffy white investment, can 

 easily be recognised across a wide street. There is some 

 resemblance between scale - insects and aphids. Both are 

 provided with piercing mouth-parts of similar construction ; 

 both feed in the same way upon the juices of the food-plant. 

 When wings are acquired they are formed in both scale-insects 

 and aphids, not from internal rudiments, like the wings of 

 insects which undergo complete transformation, but as purely 

 external growths, at first small and simple, but gradually 

 enlarging, and becoming at length suitable for flight. In both 

 the wings are membranous and of large size, relatively to the 

 size of the body. On such grounds both scale-insects and 

 aphids are placed in the same order, Hemiptera, and in the 

 same sub-order, Homoptera (with membranous wings only). 

 Noteworthy differences can be pointed out between the two 

 families. Aphids, though often sluggish and sedentary, are not 

 permanently fixed to the food-plant, but most scale-insects are 

 fixed either for life or for the whole of the feeding-stage. 

 Aphids are capable of producing young at frequent intervals 

 from unfertilised eggs, which develop within the body of the 

 mother ; scale-insects are not often viviparous, but produce a 

 multitude of fertilised eggs once a year. Wings are not a 

 mark of sex in aphids ; in scale-insects they are possessed 

 only by the male. 



We shall take as an example for description one of the 

 commonest species, the mussel-scale of the apple-tree. This 

 scale-insect is believed to infest many other trees besides the 

 apple, among the rest the lime, horse-chestnut, sycamore, 

 elm, willow, and poplar; it also attacks broom, heather, and 

 bilberry. The mussel-scale thrives best in warm, sheltered 

 situations ; on standard trees it is chiefly confined to the trunk 

 and main branches, while on a tree trained against a south 

 wall it will cover every branch and twig, and even overrun the 

 fruit and leaves (Newstead). An infected tree, if examined in 

 winter, shows on the younger branches a great many small 

 raised blotches; these are from 2-2.5 "^t"- lo"g (iV-Tir inch), 

 of grey or violet colour, and shaped like a single valve of the 

 common mussel. The hollow side is turned towards the 

 bark. If one of the scales is lifted with a needle, there will 



