1 8 ENTOMOLOGY 



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 thysanuran stem. Collembola, however, are 

 not nearly so primitive as Thysanura, for the former have fewer ab- 

 dominal segments than the latter, exhibit much greater concentration of 

 the nervous system, and are uniquely specialized in several respects, 

 notably as regards the ventral tube and the furcula, or springing organ. 



Returning to Thysanura the genera Machilis and Lepisma show de- 

 cided orthopteran affinities; thus their eyes are compound 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 classification." 



Orthoptera probably arose directly from the original thysanuriform 

 stem. 



Platyptera, as a whole, are most nearly related to Orthoptera on the 

 one hand and to Plecoptera on the other. Termitidae have strong orthop- 

 teran affinities and Embiidae have even been placed in the order Orthop- 

 tera, though the latter family is most nearly allied to Termitidae and 

 Psocidae. These two are approached rather closely by Mallophaga and 

 exhibit, by the way, some collembolan characters, as Enderlein has 

 pointed out. 



Plecoptera, which Packard placed in his group Platyptera, are better 

 regarded as a distinct order with some orthopteran and many ephemerid 

 and odonate affinities. The strong resemblance between nymphs of 

 Plecoptera, Ephemerida and Odonata indicates community of origin. 



Ephemerida and Odonata are well circumscribed orders, most nearly 

 related to each other, but sharply separated, nevertheless, by differences 

 in the wings, mouth parts and other organs. Ephemerida are almost 

 unique among insects in having a pair of genital openings a primitive 

 condition. 



Thysanoptera form a distinct order, which is usually placed next to 

 Hemiptera, chiefly on account of the suctorial mouth parts, though even 

 in this respect there is no close agreement between the two orders. 



Hemiptera stand alone and give few hints of their ancestry. They 

 are least unlike Orthoptera and possibly originated with Thysanoptera 



