

CLASSIFICATION - 23 



tera, Ephemerida and Odonata. Of these, Plecoptera is probably the 

 most generalized order; though Ephemerida has retained some very 

 primitive structures, notably the paired genital openings and ducts. 



It is often stated that Plecoptera are the most primitive of winged 

 insects. According to this view, then, Orthoptera have arisen from 

 the plecopteran stem. They show, however, no evidence of an aquatic 

 ancestry; and everything indicates that terrestrial insects preceded 

 aquatic. Doubtless plecopteroid and orthopteroid insects both arose 

 from a type that was winged, with many wing veins, mandibulate, and 

 terrestrial a form like a thysanuran but with wings. 



On the basis of metamorphosis, Plecoptera, Ephemerida and Odo- 

 nata form a natural group, Hemimetabola, in which the changes in 

 form during development are greater than in other Heterometabola, 

 the aquatic nymphs of these three orders being termed naiads by 

 Comstock. 



Odonata are naturally placed next to Ephemerida but are strongly 

 aberrant forms with a unique kind of specialization. 



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. 

 They are aberrant and hard to place. Borner and Crampton find 

 resemblances between Thysanoptera and Corrodentia. 



Hemiptera form a homogeneous and monophyletic order, charac- 

 terized by the unique shape and arrangement of the mouth parts, 

 which are always of the same type (Muir). Hemiptera are somewhat 

 like 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, though it is 

 improbable that Hemiptera arose from forms like Collembola. Hemip- 

 tera are exceptional among insects with a direct metamorphosis in 

 their highly developed type of suctorial mouth parts. Homoptera 

 are on the whole more primitive than Heteroptera. 



Parasita, long a suborder of Hemiptera, should rank as an order, 

 apparently; though opinions differ in regard to this. 



In the early days of the classification, the sucking lice and the biting 

 lice were always grouped together, on account of their resemblances. 

 Then it was found that these similarities, correlated with parasitic 

 existence, were only superficial; and the two groups were separated. 

 Some recent authors have, however, followed one another in the opinion 

 that the two kinds of lice are closely related to each other an opinion 



