CLASSIFICAT:ON 23 
Platyptera, as a whole, are most nearly related to Orthop- 
tera on the one hand and to Plecoptera on the other. ‘Termit- 
ide have strong orthopteran affinities and Embiidee have even 
been placed in the order Orthoptera, though the latter family 
is most nearly allied to Termitide and Psocidz. 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 afhnities. 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 1s 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,” 
