210 AN INTRODUCTION TO ENTOMOLOGY 



his class Insecta into seven orders; these he named Coleoptera, 

 Hemiptera, Lepidoptera, Neuroptera, Hymenoptera, Diptera, and 

 Aptera, respectively. 



Since the time of Linnaeus many modifications of his classification 

 of insects have been proposed; and new ones are constantly appear- 

 ing. The result is that now there is a great lack of uniformity in the 

 classification used by different writers. 



The modifications of the Linna?an distribution of insects into 

 orders are based on the belief that in certain cases Linnaeus grouped 

 into a single order forms that really represent two or more distinct 

 orders. The result has been a great increase in the number of orders 

 recognized. 



Linnaeus included in his class Insecta, under the order Aptera, 

 not only wingless insects but also arachnids, crustaceans, centipedes, 

 and millipedes. The animals thus grouped by Linnaeus are now dis- 

 tributed into several classes; and to the class composed of the animals 

 now commonly known as insects, those characterized by the posses- 

 sion of only six legs, the term Hexapoda is commonly applied. Some 

 writers, however, apply the term Insecta to the class of insects as 

 now limited. 



Some of the more recently recognized orders of insects are repre- 

 sented among living insects by comparatively few species; but in 

 each case the structure of the insects included in the group is so differ- 

 ent from that of all other insects that we are led to believe that they 

 represent a division of the class Hexapoda that is of ordinal value. 



-^ There are given below the names of the orders of insects recognized 

 in this work. The sequence in which these orders are discussed is of 

 necessity a more or less arbitrary one. In general the plan adopted 

 here is to make the series an ascending one; that is, the more gen- 

 eralized or primitive insects are placed first and the more highly 

 specialized ones later in the series; but as the different orders of 

 insects have been specialized in very different ways, the relative de- 

 grees of their specialization cannot be shown by arranging them in a 

 single linear series, as must be done in a book. To indicate the 

 different ways in which the different members of a group have been 

 specialized and the relative rank of those specialized in a similar way, 

 use must be made of a diagram representing a genealogical tree. 



. Many such diagrams have been made, but no one of them has re- 

 ceived general acceptance; much remains to be learned before such 

 a diagram can be made that will inspire confidence in its accuracy. 

 In the course of the preparation of a special treatise on the wings 

 of insects (Comstock ' 1 8 a) , I wrote a table indicating the more strik- 

 ing of the methods of specialization of the wings characteristic of 

 each of the orders of winged insects; and in the discussion of the 

 different orders, I followed the sequence indicated by this table. In 

 doing this I did not advocate the basing of a classification of insects 

 upon the characters presented by the wings alone, but merely made 

 use of these characters for the purposes of that work. 



