CLASSIFICATION 2 1 



ancestral type of arthropod. This primordial type, 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- join ted 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 meta- 

 morphosis; functional abdominal legs and active terrestrial habits. 



The existing form that best meets these requirements is Scolopen- 

 drella, which is not an insect, however, but belongs in the class Sym- 

 phyla. The most primitive of known insects are Anajapyx and Campo- 

 dea, through which other insects trace their origin to the stock from 

 which Symphyla and Diplopoda arose. 



There is not the slightest evidence to support the assumption by 

 Handlirsch that Thysanura and Collembola are degenerate descendants 

 of winged ancestors. They are primitively wingless insects (Aptery- 

 gota) ; in other words, they originated before insects acquired wings. 



Among Thysanura, the genera Machilis and Lepisma show decided 

 orthopteran affinities; thus their eyes are compound and their mouth 

 parts strongly orthopteran; indeed, the likeness of Lepisma to a young 

 cockroach is striking. According to Crampton, Lepisma leads to 

 Plecoptera and Ephemerida; while Machilis has suggestions of affinities 

 with Crustacea. 



"The generalized form of Thysanura, and the manner in which 

 it reappears in the larvae of other insects, is the natural key of the clas- 

 sification" (Hyatt and Arms). 



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 Col- 

 lembola are an offshoot from the thysanuran stem. Collembola are 

 not nearly so primitive as Thysanura, however, for they have fewer 

 abdominal segments than the latter, exhibit much greater concentra- 

 tion of the nervous system, and are uniquely specialized in several 

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

 ing organ. 



Collembola are no longer regarded as a suborder of Thysanura by 

 those who are familiar with the morphology of the two groups. All the 

 specialists in Thysanura and Collembola agree in regarding them as 

 two distinct orders. 



