86 INVERTEBRATE ANIMALS. 



Class INSECTA = HEXAPODA. 



Head, thorax, and abdomen distinct ; antennae a single pair : 

 three pairs of legs borne on the thorax. 



The species included in this wondrous class have been 

 estimated at 160,000 in number (190,000, Packard), 

 which is about equivalent to the sum of the species in 

 the whole of the rest of the Invertebrate Animals, 

 together with the Fishes, Keptiles and Amphibians, 

 Birds and Mammalia, with all the known species of 

 plants in addition. In other words. Insects constitute 

 about one-half of the species included in the Animal 

 and Vegetable Kingdoms. (Mr. Bentham's Presidential 

 Address to the Members of the Linn/ean Society, 1871.) 



The chief orders of the class have long been estab- 

 lished on characters mainly derived from the organs of 

 flight ; but all attempts to collect the orders into higher 

 divisions have hitherto been unsatisfactory. By far the 

 most important scheme divides the class into three sub- 

 classes. I. Ametabola ; Transformations absent. II. 

 Hemimetabola ; Transformations incomplete. III. 

 Metabola; Transformations complete. But the 

 advantage of this classification has in the present 

 instance been relinquished as incompatible to an 

 indefinite extent with the integrity of the Orders. 

 Objectionable on the same ground are the older divi- 

 sions : I. Haustellata ; Insects with sucking mouths. 

 II. Mandibulata ; Insects with biting jaws. Groups 

 197-222 chiefly illustrate the former, groups 223-240 

 the latter of these divisions. 



The order of the groups corresponds nearly with 

 the arrangement proposed in vol. 1, page 29, of the 

 "Introduction to the ISIodeni Classification of Insects,'" 



