CLASSIFICATION. 151 



is converted into the imago by the liberation of its wings at 

 the last moult. 



The insects which undergo metamorphosis are consequently 

 divided into metabolic insects, or insects with complete meta- 

 morphosis, in which the pupa is quiescent and does not feed, 

 and in which the greatest weight and bulk is attained at the 

 end of the larval stage; and into hemi-metabolic insects, in 

 which there is an active nymph and the imago is the bulkiest 

 and heaviest form. Metabolic insects form 95 per cent, of the 

 whole class. 



Insects may be grouped either according to the structure of 

 their bodies, or their mode of life, and Entomology uses the 

 former of these characters in their classification, but in Forest 

 Protection it is of greater convenience to study the latter. 



Much difference of opinion has existed regarding the classifi- 

 cation of insects. The simplest method, based on the systems 

 of Linnaeus and Fabricius recognises seven Orders. The 

 former naturalist relied principally on the structure of wings 

 in distinguishing the different orders of insects, and the latter 

 on the parts forming the insect's mouth. The Orders may 

 also be arranged, according to the degree of completeness of 

 their metamorphoses, in two groups containing the metabolic 

 and ametabolic insects respectively. Though the degree of 

 metamorphosis is of the first importance as a guide to the 

 systematic position of an insect, it is not, when taken alone, 

 of the highest value in classification, as it cannot be decided 

 by mere observation of any particular insect, without study of 

 its life-history. 



Certain writers who have attached great weight to structural 

 differences, particularly of the wings, have increased the 

 number of Orders to thirteen (Westwood) or sixteen (Packard) . 

 The groups which have been raised to the rank of additional 

 Orders are of minor importance, and contain as a rule a small 

 number of aberrant forms. In the present work, the broadest 

 and most generally-received classification will be followed, in 

 which the Insects are divided into seven Orders, characterised 

 as follows : 



1. Orthoptera .- with biting mouth-parts, a free prothorax, 

 and incomplete metamorphosis. Cockroaches, crickets, locusts, 



