Smithsonian miscellaneous collections . imago, and if this is true in other Hemipterathe coccid pupa is not a part of the imaginal stage. Holometabolismcan be defined only as a type of metamorphosis; the fact that it occursamong the Hemiptera in the male coccid, and also in the Thysanop-tera does not taxonomically relate these insects to each other or tosuch holometabolous insects as Coleoptera, Lepidoptera, Diptera, andHymenoptera. VIII. THYSANOPTERA The Thysanoptera seem to contradict the principle that postembry-onic metamorphosis is due to some structural aberration on the partof the young
Smithsonian miscellaneous collections . imago, and if this is true in other Hemipterathe coccid pupa is not a part of the imaginal stage. Holometabolismcan be defined only as a type of metamorphosis; the fact that it occursamong the Hemiptera in the male coccid, and also in the Thysanop-tera does not taxonomically relate these insects to each other or tosuch holometabolous insects as Coleoptera, Lepidoptera, Diptera, andHymenoptera. VIII. THYSANOPTERA The Thysanoptera seem to contradict the principle that postembry-onic metamorphosis is due to some structural aberration on the partof the young insect that fits it to a special environment or way of liv-ing. The active young thrips in appearance differs from the imagolittle more than a young aphis differs from a winged adult aphis, and no. 9 INSECT METAMORPHOSIS—SNODGRASS 45 it would seem that in like manner it could grow into an adult thripswithout any radical process of transformation. However, after twoactive, feeding nymplike stages (fig. 7 A,B) the young thrips becomes. Fig. 7.—Life-history stages of a thysanopteron, Scirtothrips citri Moulton (out-lines from Horton, 1918). A, first instar. B, second instar. C, propupa. D, pupa. E, adult female. inactive, ceases to feed, moults, and enters a quiescent stage knownas a propupa (C). The propupa in turn is followed by a second rest-ing stage termed the pupa (D), from which after a final moult theadult emerges (E). In the Terebrantia the wings appear in thepropupal stage as straplike outgrowths, which become still more ex-tended in the pupa. In the Tubulifera the propupa differs little in 46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 122 external appearance from the second nymph, since in this suborderthe wings do not appear until the pupal stage. In most of the Tubu-lifera, however, there is a second pupal stage separated from the firstby a moult, making five immature instars in all, but according toPriesner (1926) a propupal stage is absent in some species and i
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