bo 
NO 
EN TOMOLOGY 
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 antennze 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. 
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 afhnities; 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 larvee of other insects, is the natural key of the classifi- 
sation.” 
Orthoptera probably arose directly from the original thys- 
anuriform stem. 
