METAZOA INSECTA. 389 



INSECTA. 



Sections 15 and 16. 

 Order i. THYSANURA. 



No ancestral form of insect has been discovered among 

 the pre-Cambrian or Cambrian rocks so primitive in struc- 

 ture as the living Campodea of the order Thysanura. 

 This genus is therefore one of the best living representa- 

 tives of that extinct ancestral type that gave rise to the 

 great group of Insecta. The embryology of Campodea 

 exhibits no such vestiges of a more specialized condition 

 as have been seen in many genera, but the characters are 

 primitive and the changes in structure are progressive. 



The larva develops in a primitive way without a meta- 

 morphosis or marked change of any kind. It sheds its 

 skin only once (quoted by Packard after Grassi) and has 

 no resting period during life. The external appearance 

 of the larva is preserved in the adult, so slight is the 

 specialization attending growth. 



The very small adult (PI. 945, fig. i, greatly enlarged) 1 

 has a long, somewhat flattened (Meinert) body divided 

 into segments that are uniform in breadth. A slight 

 differentiation of these segments has brought about a 

 grouping into three regions, which is characteristic of 

 insects, but not found among Myriapods. The anterior 

 region is the head (fig. i ; PL 946, fig. i, seen from below), 

 which is generally considered to be made up of an uncer- 

 tain number of consolidated segments. According to the 



1 On account of the minute size of many insects, or because they 

 are rare and difficult to obtain it has been necessary to have a large 

 number of drawings made. These drawings as well as nearly all 

 in the Synoptic Collection have been made by Miss L. R. Martin, 

 assistant in the Museum. 



