270 Journal New York Entomological Society, [^'"i- xxiv. 



Ceroplatus, etc., there is a sort of increase in the number of the 

 abdominal segments, by the interpolation of " intercalary " segments, 

 formed by the lengthening and demarcation of the intersegmental 

 region — although this is not strictly comparable to the type of true 

 segmental increase exhibited by the Protura. 



Although the Apterygota form a well-defined group, quite distinct 

 from the Pterygota, I do not know of any key which will enable one 

 who has no preconceived idea of the true nature of an insect (espe- 

 cially with regard to the larval and wingless forms) to place it cor- 

 rectly in every case. I would therefore offer the following key as a 

 purely tentative effort to supply this lack. 



1. Mouthparts either retracted in cavity of head, or practically wanting, or 



not mandibulate 2 



Mouthparts not retracted in cavity of head, but mandibulate 3 



2. Ventral region of the abdomen bearing either styli, vestigeal legs or ven- 



tral tube Apterygota. 



Abdomen without styli, vestigeal legs, or ventral tube Pterygota. 



3. Abdomen with more than one pair of styli Apterygota. 



Abdomen with at most one pair of styli, usually none Pterygota. 



The styli referred to in the preceding key, are paired, movable, 

 spine-like appendages, borne at the posterior margin of certain of 

 the abdominal sternites, and are not homologous wtih the entire 

 vestigeal legs, but are appendages of the basal segment of the legs, 

 and occur on the meso- and metathoracic coxae in such insects as 

 Machilis, and on the basal segment of the leg in certain "Myriopoda." 

 The vestigeal legs mentioned in the key are true leg-vestiges — whether 

 they are homologous with the so-called false legs (pseudopodia) of 

 larval Lepidoptera, etc., has been questioned. The ventral tube 

 (which represents the united vestigial legs of the first abdominal 

 segment) is an adhesive organ situated in the mid-ventral line of the 

 first abdominal segment, and may be large and columnar, or it may 

 be reduced to the merest vestige, requiring close scrutiny to detect it. 



The Apterygota may be divided into two supersections, the Eusty- 

 ligera, or styli-bearing Apterygota, and the Astyligera, or non-styli- 

 bearing Apterygota, according to the presence or absence of styli in 

 the two groups. 



The styli-bearing Apterygota (Eustyligera) are also cerci-bearing 

 in all cases thus far observed, the non-cerci-bearing form " Aniso- 



