54 



appendages were lost, and those of the thoracic region alone 

 remain as the three pairs of legs characteristic of the Insecta. 

 Wings then developed, as outgrowths from the dorsal region 

 of the second and third thoracic segments, one pair on each. 

 The Orthoptera most nearly represent this ancestral stage. 

 They are certainly more primitive than any of the higher 

 orders of Insects, and probably, along with their allies, the 

 Neuroptera, the Pseudo-Neuroptera, and the Dermatoptera 

 (forming altogether Packard's " Phyloptera "*), constitute a 

 branch which diverged about this point. 



Higher up, the main stem probably split into two 

 branches, one leading through the ancestral Hemiptera, 

 after considerable modification, to the Coleoptera,t while 

 the other forms the line from which the ancestral Diptera, 

 Lepidoptera, and Hymenoptera successively diverged. Some 

 of the Diptera have undergone considerable degeneration. 

 The Hymenoptera are the most perfectly differentiated 

 forms. They attain to a higher level of organisation than 

 any of the other Tracheata, and probably occupy the highest 

 point among Invertebrates. The Insecta is a very extensive 

 class, but, notwithstanding the very large number of species 

 it includes, and the high position attained by some of its 

 members, the range of organisation in the group is com- 

 paratively slight. I 



Keturning once more to the vermean axis, we must next 

 examine the upper third, the region above the origin of the 

 Arthropoda. Near this point, and distinctly below the Anne- 

 lides, is placed, on a side branch, the group Discophora or 

 Hirudinea. It is possible that this may not be the true 



* Ann. and Mag. Nat. Hist., v. xii, p. 146 (1883). 



f See Packard, Guide to Study of Insects, p. 105 (New York, 1876). 



+ In regard to the modifications of the larval condition, and the meaning 

 of the metamorphosis of insects, see Lubbock's Origin <ind Metamorphosis 

 of Inure la, Nature Series (l874j. 



