Dec, 1916.] Relationships of Apterygotan Insects. 271 



sphccra" described by Tomosvary as related to the Lepismatidse, 

 having subsequently been proved to be a larva of the beetle Cephen- 

 niiim. The antennae are usually well developed and consist of more 

 than six segments. No postantennal organ has been observed in 

 members of this group. 



The second super-section, or non-styli-bearing group (Astyligera) 

 usually lacks segmented cerci and forceps-like terminal abdominal 

 structures, in addition to the styli. The " cerci " of Tomocerus and 

 others of the group (to which Willem, Folsom, and others have called 

 attention) are not segmented, and there is some question as to 

 whether they are to be considered as strictly homologous with the 

 true cerci of the other forms, although this point has not been suffi- 

 ciently investigated to be definitely decided. The antennse may be 

 lacking in this group, but if present, are usually composed of not 

 more than six segments — although an " annulation," or " ringing " of 

 the terminal segments, sometimes occurs. The postantennal organ, 

 or its homologue, is frequently present; but may be wanting. 



All of the members of the non-styli-bearing group have mouth- 

 parts of the concealed type (i. e., entognathous or cryptognathous) 

 the mouthparts being retracted into the cavity of the head, as in 

 Eosentomon, Tomocerus, Entomohrya, etc. On the other hand, some 

 of the styli-bearing group have mouthparts of the concealed type (the 

 mouthparts being retracted into the cavity of the head, as in Cam- 

 podea, Japyx, etc.) while others of the group have mouthparts of the 

 exposed type (ectognathous or gymnognathous) as in Lepisma, 

 Machilis, etc. 



There are apparently three main lines of evolution represented in 

 the Apterygota. In other words, the Apterygotan insects group 

 themselves about three principal centers. These are represented by 

 the Protura {e. g., Eosentomon, etc.) the Rhabdura {e. g., Campodea, 

 etc.) and the Thysanura {Lepisma, etc.) and the insects forming 

 these groups may be regarded as representing the three principal sec- 

 tions of the sub-class Apterygota. 



The lowest or most primitive section of the Apterygota com- 

 prises those insects which group themselves about the Protura (such 

 as Eosentomon, Acerentomon, etc.) which have departed as little as 

 any from the probable ancestral condition of the group as a whole. 

 These insects form the section Proturadelphia or " Proturan-brother- 



