( I \ 



iral type of arthropod. Tin's pi imordial type, then probabh 

 three simple and equal thoracic sr^im-m lightly 



the ten abdominal Segments; t liree pai; and no pairs 



posed biting mouth parts; a pair of loni^, many jointed 

 and a pair of cerci of the same description; a thin naked inte- 

 simple straight alimentary canal distinctly divided into three pri 

 ttSJ a ijan^lion and a pair of spiral les for each of the three th' 

 and the lirst ei^ht alxloniinal segment .-. if not all the latter; no : 

 morphosis; functional abdominal legs and active terrestrial hah 



Tin- existing form that best nuvts these requirements is Scolopm- 

 drclla, 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 Cramp ton, 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 JapyXj 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 se 

 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. 



