THE LOWG SCALE OF THE ORANGE. 19 



ble, and well serve to protect the helpless organism which they sur- 

 round, both from the action of the weather and the attacks of exter- 

 nal enemies. The molted skins consist of chitine, a substance which 

 forms the hard external parts of nearly all insects, and the excreted 

 portion of the scale is composed of a sort of hardened wax, having the 

 toughness of horn. 



The upper scale is therefore impervious to most liquids, and is not 

 soluble in acid or alkaline solutions strong enough to injure the plant. 

 It resists the action of oils and of bisulphide of carbon, an almost uni- 

 versal solvent. Many insecticides are therefore inoperative, and all in- 

 soluble substances, such as sulphur, etc., are clearly useless, as they do 

 not reach the eggs or mature insects. The thinner, ventral scale is not 

 impervious to the more volatile oils or to alcoholic solutions, some of 

 which reach and kill the insect by penetration through the bark. 



As the scale, like the shell of the snail, is formed by successive addi- 

 tions, and keeps pace in its growth with that of the body of the insect 

 within, its vulnerable point is the growing end, and there are times dur- 

 ing its formation when the posterior extremity of the insect projects 

 slightly beyond it and becomes exposed to the action of penetrating 

 liquids. This is particularly the case at the critical periods when the 

 Coccid sheds its skin. But when the scale is fully completed and tightly 

 sealed at all points, no insect is more difficult to reach and to destroy. 



LONG SCALE. 



(Mytilaspis gloverii, Packard.) 







[Plate III, fig. 2; 1Y; and Fig. 3.] 



Growth of the Scale. In the Long Scale the increase in size takes place 

 chiefly in one direction, producing a linear body, which may be either 

 straight or curved in the form of a cornucopia. The first molt or cast 

 skin of the wandering larva forms its extreme tip, but the delicate film 

 of wax which at first covered this skin disappears, or leaves traces only 

 in the form of two minute projections, and its surface exhibits more or 

 less plainly marked indications of the body -joints of the young louse. 

 Beyond, and partly underlying the shield-shaped first larva skin, is that 

 of the second molt, but this skin is overlaid and imbedded in the sub- 

 stance of the scale, so that its oval outline is faintly visible beneath 

 the coating of horn. The scale increases in width during the first half 

 of its growth, after which the width remains the same and the sides are 

 parallel. ' 



The plate which forms the underside is firmly united to the upper por- 

 tion of the scale, and projects beyond it on the sides, forming thin 

 flanges, that greatly increase the tenacity of its hold upon the bark. 

 The ventral plate does not entirely cover the under surface, but is di- 

 vided in the middle, leaving a long, narrow slit, through which the body 

 of the insect comes in contact with the bark. 



