162 



THE AMERICAN NATURALIST 



[Vol. LIII 



1912, Lignau, 1911, Chamberlain, 1917, Heymons, and 

 others. The retention of the stumps of three pairs of 

 legs on the abdominal region (in addition to the three 

 pairs of thoracic legs) at first caused some zoologists to 

 doubt that the Protura are really insects (since the idea 

 that such forms with vestigial abdominal legs could not 

 be true ''hexapods" if they had more than six limbs 

 seemed to stand in the way of their realizing the true 

 insectan nature of the Protura), but the overwhelming 

 evidence of their structural organization has convinced 

 all recent entomologists that the Protura are true insects. 

 As pointed out in a recent paper (Crampton, 1916) the 

 Protura are quite closely related to such other Aptery- 

 gota as Tomocerus; and, with the Entomobryids and 

 Sminthurids, they constitute the non-styli-bearing divi- 

 sion of the Apterygota. 



Of the styli-bearing Apterygota, the next group to be 

 considered, which may be referred to as the Campodeoid 

 group, comprises the Ehabdura (e. g., Campodea), the 

 Dicellura (e. g., Projapyx, Japyx, etc.) and their imme- 

 diate relatives. Dicellura, such as Projapyx, Anajapyx, 

 etc., have segmented cerci, and occupy a position inter- 

 mediate between the Ehabdura, such as Campodea, and 

 the other Dicellura, although their closest affinities are 

 clearly Mitli tlie Dicellura. The Campodeoid group, 

 wluc^c iHctiiljcrs lia\ c •■iitoi^iiathous mouth parts and ves- 

 tigial abdominal le.us suggestive of the proturan struc- 

 tures, occupy a position intermediate between the lower 

 apterygotan Protura and the higher apterygotan forms, 

 such as Nicoletia, Lepisma, etc., which also belong to the 

 styli-bearing apterj^gotan subdivision which includes the 

 Campodeoid group as well (Crampton, 1916). The Cam- 

 podeoid group, while inheriting certain features from the 

 symphylan side of their common ancestry, have inherited 

 in addition certain other features more typical of the 

 crustacean side— which likewise reappear in the isopod- 

 amphipod descendants of their common ancestors. 



The Lepismoid group, composed of the lepismids, 



