Dec, 1916.] Relationships of Apterygotan Insects. 273 



cealed type (entognathous or cryptognathous), but a postantennal 

 organ is usually wanting. Eyes are absent in most of the members 

 of the group. The antennae are usually well developed and are 

 usually composed of many segments. Segmented uropods may occur 

 on the basal abdominal segment (as in Canipodea) although they are 

 usually greatly reduced or lacking. Eversible sacs, etc., are found 

 on certain of the abdominal sterna in some members of the group. 

 There is usually a " Y "-shaped suture in the meso- and metasternum 

 (of the thorax), and a longitudinal cranial suture, together with a 

 transverse occipital suture, usually occurs in the insects of this sec- 

 tion. The tarsi are usually apparently one-segmented, and the 

 abdomen is composed of ten or eleven segments. Maxillary palpi 

 apparently one- or two-segmented. Abdominal ganglia seven or eight 

 in number. The principal families of this section are the Cam- 

 podeidse, Projapygidse and Japygidae. The insects composing this 

 group might in some ways be regarded as occupying a position some- 

 what intermediate between the insects of the first section (Protura- 

 delphia) and the one next to be considered. 



The third section (which is also a division of the styli-bearing 

 Apterygota) comprises those insects which group themselves about 

 the Thysanura {Lepisma, etc.) forming the section Thysamiradelphia, 

 or " Thysanuran-brotherhood.'' Such forms as Machilis, or Prccma- 

 chilis, are more primitive, or have departed less from the ancestral 

 condition of the group, than the true Thysanura (such as Lepisma) 

 have, but the designation Thysanura, being the best known of the 

 terms applied to the members of the group, has been chosen to typify 

 the section as a whole. Styli are found at least on the terminal ab- 

 dominal segments of practically all members of the group, and in 

 certain forms, such as Machilis, Pramachilis, etc., styli are borne on 

 the coxae of the meso- and metathorax as well. Unlike the insects 

 of the preceding section, the members of this group usually possess 

 a median unpaired caudal filament, in addition to the paired caudal 

 filaments or cerci, although the cerci may be absent in rare in- 

 stances (Dasyleptus). All of the insects belonging to this section 

 have mouthparts of the exposed type (ectognathous or gymnog- 

 nathous). Segmented uropods are usually absent, and postantennal 

 organs are wanting in the members of the group. The eyes are 

 usually well developed, although absent in some cases, and the 



