4 INSECTS INJURIOUS TO VEGETATION. 



ance fall into many mistakes. Moreover, cultivation, in many- 

 cases, has destroyed the balance originally existing between the 

 different tribes of insects, nor can this be restored until we make 

 ourselves thoroughly acquainted with their natural history. In- 

 formation on this subject is to be obtained by observation alone ; 

 it can be communicated and rendered useful to others only by 

 means of correct descriptions of the insects themselves, accom- 

 panied by full accounts of their habits in every stage of their ex- 

 istence. The attention should not be confined to the history of 

 individual species or kinds, however hurtful or beneficial these 

 may be ; it should be extended to groups composed of different 

 species, whereby some general ideas of this branch of science 

 may be obtained, and the facts derived from scattered sources 

 may be systematically arranged, so as to aid us in our future re- 

 searches. For a general knowledge of the natural groups, or 

 families of insects, not only leads to higher and more philosophical 

 views of the whole class, but will often be practically useful when 

 we come, for the first time, to the study of any single species. 



As this treatise may fall into the hands of persons entirely un- 

 acquainted with this branch of natural history, it may be proper 

 to begin with some brief remarks on insects in general, in order 

 to show how they are formed, and wherein they differ essentially 

 from other animals. 



The word Insect,* which, in the Latin language, from whence 

 it was derived, means cut into or notched, was designed to ex- 

 press one of the chief characters of this group of animals, whose 

 body is marked by several cross-lines or incisions. The parts be- 

 tween these cross-lines are called segments or rings, and consist of 

 a number of jointed pieces, more or less movable on each other. 



Insects have a very small brain, and, instead of a spinal mar- 

 row, a kind of knotted cord, extending from the brain to the 

 hinder extremity ; and numerous small whitish threads, which are 

 the nerves, spread from the brain and knots, in various direc- 

 tions. Two long air-pipes, within their bodies, together with an 

 immense number of smaller pipes, supply the want of lungs, and 

 carry the air to every part. Insects do not breathe through their 



* Insectum is an abbreviation of infersecfum; and from the same source we 

 have the word intersect, to cut or divide. 



