INSECTS AND VEKMIN. 



157 



Insects are the most extensive class of animals. They 

 are destitute of an internal skeleton, but possess a sort of 

 external one, serving both for skin and bones, and divided 

 into numerous segments connected together by slender 

 points of attachment. They all have six or more articu- 

 lated legs, and are generally oviparous, or produced from 

 eggs. They possess sight, hearing, smell, and touch at 

 least, — senses in common with those of the superior animals. 

 They do not breathe through the mouth or nostrils, but 

 through vessels, for the reception of air, called spiracula, 

 placed along each side of the body. 



Nearly all insects have four stages of existence. First, 

 eggs which hatch into larvae ; these change into pupce, 

 where they remain dormant for a longer or shorter period, 

 and from which they emerge at last as perfect insects. 

 Some insects, however, bring forth their young alive, as 

 well as deposit eggs. In others, as the Orthoptera, or 

 grasshopper family, the young has nearly the form of a per- 

 fect insect. Some insects are injurious to plants only in 

 one stage of their existence, others at all times, when not 

 in a dormant state. 



A knowledge of the habits and transformations of in- 

 sects is necessary to detect how and at what period of 

 their existence they can best be destroyed, or in what man 

 ner vegetation can best be shielded from their attacks. 



By many insects plants are at once destroyed ; by others 

 wounds are inflicted that end in a diseased condition of 

 the parts affected, which is communicated to the whole 

 plant. Plants in a weak or diseased state are far more 

 liable to bo attacked by insects than those which are 

 healthy and vigorous. 



Various remedies are proposed when plants are attacked 

 by insects, among which those most generally applicable 

 are dusting the leaves with quicklime, sulphur, snuff, 

 soot, dust impregnated with the oil of turpentine. Also 

 sprinkling or washing the plants with water heated to 



