INSECTICIDES AND GENERAL REMEDIES. 413 



surface, it would obviously take a very large proportion of the insecti- 

 cide on the leaf. But if the insect were literally of the piercing type, 

 the proboscis would pass through the smallest possible quantity of the 

 poison on the surface, and feed on the pure vegetable sap underneath. 

 It is therefore obvious that before any good results could accrue, we 

 should require to get the vegetable organism to incorporate the poison 

 with its own sap. How far this can be done is still to a large extent 

 a question for future scientists, but nature does give us some very 

 good suggestions in this direction. Take, for example, the case of 

 some scales with their piercing mouths— viz., Cryptococcus farji or 

 felt scale on beech and Mytilaspis pomorum or mussel scale on fruit- 

 trees. In the former case the copper beech often shows itself as prac- 

 tically immune alongside the badly infested trees of the common 

 beech, and in the latter certain varieties of fruit-trees are practically 

 immune while the surrounding trees of other varieties are literally 

 killed. Hence the practical inference is to either raise immune 

 varieties, as in fruit-trees, or if possible to so feed the vegetable 

 organism as to contain some ingredient so amalgamated with the sap 

 as to be inimical to the animal parasite. 



"With regard to the breathing-apparatus of an insect, it should be 

 remembered that breathing is not effected through the mouth, but 

 through slits or openings termed breathing-pores or spiracles. If these 

 openings are varni.shed over, the insect will be asphyxiated. Thus, 

 therefore, we get the suggestion of applying insecticides in the form 

 of an emulsion or thick spray. Nothing answers this purpose better 

 than soft-soap, because it adheres to the skin, and other ingredients 

 may be added within certain limits and according to the species of 

 insect and nature of injuries. Such ingredients are quassia, paraffin, 

 tobacco, sulphur, turpentine, caustic soda, caustic potash, &c. The 

 addition of any of these ingredients has the effect of corroding the 

 skin, and so intensifying the action of the emulsion. The skin of 

 insects is composed of a substance known as chitine. This substance 

 is more or less of a horny nature, and has the chemical formula of 

 CfiH^gNOg. It is practically unaffected by alcohol, ether, acetic acid, 

 alkalies, or even when boiled in caustic potash. It may, however, be 

 dissolved by concentrated mineral acids, but as the latter would in- 

 jure the vegetable organism, it follows that insecticides must be so 

 composed as to act on the insect without doing injury to the plant. 



The reproduction of insects may to some extent be checked, as for 



