22 ENTOMOLOGY 



then, probably had three simple and equal thoracic segments 

 differing but slightly from the ten abdominal segments; three 

 pairs of legs and no wings; three pairs of exposed biting 

 mouth parts ; a pair of long many-jointed antennae and a pair 

 of cerci of the same description; a thin naked integument; a 

 simple straight alimentary canal distinctly divided into three 

 primary regions; a ganglion and a pair of spiracles for each 

 of the three thoracic and the first eight abdominal segments, 

 if not all the latter; no metamorphosis; functional abdominal 

 legs and active terrestrial habits. 



The existing form that best meets these requirements is 

 Scolopendrella, which is not an insect, however, but belongs 

 among or near the diplopods. The most primitive of known 

 insects are Anajapyx and Campodea, through which other 

 insects trace their origin to the stock from which Symphyla 

 and Diplopoda arose, i 



Collembola, though specialized in several important ways, 

 all have the same peculiar kind of entognathous mouth parts 

 as Campodea and Japyx, for which reason and many others it 

 is believed that Collembola are an offshoot from the thysanu- 

 ran stem. Collembola, however, are not nearly so primitive 

 as Thysanura, for the former have fewer abdominal segments 

 than the latter, exhibit much greater concentration of the ner- 

 vous system, and are uniquely specialized in several respects, 

 notably as regards the ventral tube and the furcula, or spring- 

 ing organ. 



Returning to Thysanura the genera Machilis and Lepisma 

 show decided orthopteran affinities; thus their eyes are com- 

 pound and their mouth parts strongly orthopteran ; indeed, the 

 likeness of Lepisma to a young cockroach is striking, as is also 

 that of Japyx to a young forficulid. 



In short, as Hyatt and Arms express it, " The generalized 

 form of Thysanura, and the manner in which it reappears in 

 the larvae of other insects, is the natural key of the classifi- 

 cation." 



Orthoptera probably arose directly from the original thys- 

 anuriform stem. 



