CLASSIFICATION. 655 



are never tucked or invaginated into the body. This group 

 varies much in the degree to which metamorphosis is present ; 

 sometimes it is almost absent, and sometimes it is complete. 



The group includes the Orders (6) ORTHOPTERA, (7) PLECOPTERA 

 (PERLIDAE), (8) PSOCOPTERA (PSOCIDAE), (9) ISOPTERA (TER- 

 MITIDAE), (10) EMBIOPTERA (EMBIIDAE), (11) EPHEMOPTERA 



(EpHEMERIDAE), (12) P ARANEUROPTERA (OnONATA), (13) THY- 

 SANOPTERA, (14) HEMIPTERA.* 



IV. ENDOPTERYGOTA. 



Winged insects whose wings arise as invaginatioiis of the 

 hypodermis and for a time project inside the insect's body, but 

 are subsequently evaginated. Complete metamorphosis exists. 

 This group is by far the largest and perhaps contains at the 

 present day some ninety per cent, of the total number of in- 

 sects. It is probably descended from the Exopterygota, which 

 with one or two exceptions were the insects of the Palaeozoic 

 epoch. The group includes the Orders (15) NEUROPTERA, (16) 

 MECAPTERA (PANORPIDAE), (17) TRICHOPTERA, (18) LEPIDOPTERA, 



(19) COLEOPTERA, (20) S T REPSIPTERA, (21) DlPTERA and (22) 



HYMENOPTERA. 



Group I. APTERYGOTA. f 



Wingless insects believed to have descended from wingless 

 ancestors. Metamorphosis very incomplete. 



Order 1. COLLEMBOLA. J 



Minute wingless insects ; abdomen of no more than six visible 

 segments ; a protrusible papilla or blind tube borne on the first 

 abdominal segment ; a jumping mechanism often present. 



The Collembola are for the most part soft-bodied insects 

 covered with a hairy down or bloom. Their spring consists of 

 a median piece bearing a pair of processes. The whole arises 

 from the penultimate or ante-penultimate segment (Fig. 410,6). 

 In dead specimens the processes project backward, but during 



* Shipley, Zool. Anz., xxvii, 1904, p. 259. 



f J. Philiptschenko, Zeitschr. wiss. Zool., Ixxxviii, 1907, p. 99. 

 % Lubbock, Monograph of the Collembola and Thysanura, Ray 

 Society, 1873. 



