208 AN INTRODUCTION TO ENTOMOLOGY 



A renewed study of the relationships of the different orders to each 

 other, in which an effort has been made to correlate other characters 

 with those presented by the wings, has not indicated the desirability 

 of changes in the sequence of the orders indicated in that table, ex- 

 cept in the allocation of those orders in which wings are wanting. 



The importance of the wings of insects for taxonomic purposes 

 was early recognized by entomologists, as is well shown by the fact 

 that the names of the Linnsean orders are all drawn from the nature 

 of the wings, except one, Aptera, and that from the absence of wings. 



The different methods of specialization of the wings arose very 

 early in that part of the geological history of insects that is known 

 to us. And as most of the fossil remains of the older insects consist 

 of wings, we are forced to depend very largely on the characters 

 presented by these organs for data regarding the separation of the 

 primitive insects into the groups from which the orders of recent 

 insects have been developed. But in characterizing the orders as they 

 now exist all the results of the study of the structure of insects and 

 of their transformations are available. 



Aside from the structure of the wings, the characters most used 

 in characterizing the orders of insects are those presented by the 

 structure of the mouth-parts and the nature of the post-embryonic 

 development. While these characters are of value in defining the 

 orders, but little use has been made of them, as yet, in working out 

 the lines of descent of the various orders from the primitive insects. 



The primitive insects had chewing mouth-parts and this type has 

 been retained in the greater mmiber of the orders. But although 

 many detailed accounts of the structure of the mouth-parts of chew- 

 ing insects have been published, no one has worked out the various 

 ways in which they have been specialized in such a manner as to in- 

 dicate the phylogeny of the orders. 



Several different types of sucking mouth-parts exist among living 

 insects; but these are apparently of comparatively late origin, and 

 while they are of great value in defining the orders in which they 

 occur, they do not afford characters for determining the primitive 

 divisions of the Pterygota. 



The nature of the post-embryonic development of insects, like 

 the structure of the mouth-parts, affords characters for defining the 

 orders of recent insects, but is of little value in determining the 

 phylogeny of the orders. 



The primitive insects doubtless developed without any marked 

 metamorphosis as do the Thysanura and Collembola of today. With 

 the development of wings, there arose that type of development 

 known as gradual metamorphosis, and this type is retained by eight 

 of the orders recognized in this work. Incomplete metamorphosis 

 is the result of a sidewise development of the immature instars of the 

 insects exhibiting it, in order to fit them for life in the water, and it 

 doubtless arose independently in each of the three orders in which 

 it occurs; it is therefore an ordinal characteristic in each case and not 

 one indicating a natural group of orders. This is also true of com- 



