CLASSIFICATION 23 



Platyptera, as a whole, are most nearly related to Orthop- 

 tera on the one hand and to Plecoptera on the other. Termit- 

 idae have strong orthopteran affinities and Embiidse have even 

 been placed in the order Orthoptera, 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 lately 

 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 re- 

 semblance 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, nev- 

 ertheless, by differences in the wings, mouth parts and other 

 organs. Ephemerida are almost unique among insects in hav- 

 ing 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 from some mandibulate and winged form. The 

 conversion of mandibulate into suctorial organs may be seen 

 within the order Collembola, but it is highly improbable that 

 Hemiptera arose from forms like Collembola. Hemiptera are 

 exceptional among insects with a direct metamorphosis in 

 their highly developed type of suctorial mouth parts. 



Metamorphosis offers, upon the whole, the broadest criteria 

 for the separation of insects into primary groups. All the 

 orders considered thus far are characterized either by no meta- 

 morphosis or by a slight, or so-called " direct," or " incom- 

 plete," transformation. The following orders, on the con- 

 trary, are distinguished by an " indirect," or " complete," 



