Dec, 1916.] Relationships of Apterygotan Insects. 277 



pedetidoe), described by Joseph, may constitute an order or suborder; 

 but, aside from the fact that they are strongly aberrant, blind forms, 

 with four-segmented maxillary palpi, the known details of their 

 anatomy are too meager to determine this point. 



The relationships of the different groups are brought out in the 

 following descriptions, in which are discussed the more important 

 characters of the different groups, which could not be treated in 

 detail in the preceding key. 



Non-Styli-Bearing Apterygota (Astyligera). 



I. Section Proturadelphia. 



This section might be divided into two subsections, on the basis of 

 the presence or absence of the ventral tube (or its homologue), but 

 a further division is apparently unnecessary. 



1. Eosentomoides (Protura). — Styli absent. Cerci, or forceps- 

 like terminal abdominal appendages, ventral tube, and springing 

 apparatus wanting. Mouthparts retracted in cavity of head. Labial 

 palpi present. Mandibles and labium somewhat similar to those of 

 Tomoceriis, as is true of the ventral groove of the head region. 

 Postantennal organ (the " pseudoculus ") usually present. Eyes 

 wanting. The antennee described and figured for " Protapteron" 

 (Eosentomon) by Schopotieff probably belong to some other insect, 

 so that all members of the group thus far studied lack antennae, the 

 forelegs being modified to serve as tactile organs in the place of the 

 lost antennae. Abdominal segments of the adult insect usually twelve 

 in number, although at first only nine — a postembryonic increase in 

 the number of abdominal segments being peculiar to these insects. 

 The basal two or three abdominal segments usually bear vestigeal 

 legs. Abdominal ganglia usually six in number (Acerentulus). The 

 principal families contained in the group are the Acerentomidse and 

 Eosentomidse. 



2. Entomobryoides (Euarthrura). — Styli absent. Segmented 

 cerci, or forceps-like terminal abdominal appendages wanting. The 

 uropods (or abdominal legs) of the first abdominal segment are more 

 or less grown together and modified to form the ventral tube 

 (columella) whose vesicles are usually short and sac-like. The 



