The zoological rank of the Insects (Ephemeridæ, Perlidæ, and Libellulidæ), in which tracheal gills are most usual, is not unfavourable to such an explanation. Lubbock has given reasons for regarding _Campodea_ and the Collembola (of the order Thysanura) as surviving and not very much altered representatives of the most primitive Insects, and he has shown that no great amount of modification would be required to convert the terrestrial _Campodea_ into the aquatic _Chloeon_-nymph.[74] We must not forget, however, that tracheal gills are by no means restricted to these families of low grade. Trichoptera, a few Diptera, two Lepidoptera (_Nymphula_ and _Acentropus_), and two Coleoptera (_Gyrinus_ and _Elmis_),[75] have tracheal gills, and a closed tracheal system in the larval condition. We cannot suppose that these larvæ of higher orders represent an unbroken succession of aquatic forms, but if we refuse to adopt this alternative, we must admit that the closed tracheal system with tracheal gills may be an adaptive modification of the open system with stigmata. [74] Palmén cites one striking proof of the low position of Ephemeridæ among Insects. Their reproductive outlets are paired and separate, as in Worms and Crustacea. [75] These examples are cited by Palmén. It is well known[76] that in certain Ephemeridæ (_e.g._, _Tricorythus_ and _Cœnis_) a pair of anterior tracheal gills may become transformed into large plates, which partly protect the gills behind (fig. 31). A similar modification of the second and third thoracic gills in _Prosopistoma_ and _Bætisca_ brings all the functional respiratory organs under cover, and these enlarged plates resemble stiff and simple wings very closely. [76] Eaton, Trans. Ent. Soc., 1868, p. 281; Vayssière, Ann. Sci. Nat., Zool., 1882, p. 91. [Illustration: Fig. 31.--Tricorythus. Adult larva, with three functional leaflets. The next leaflet in front is converted into a protective plate. × 7. A, protective plate of Tricorythus larva, seen from the outside. × 26. B, the same from within, showing the attached respiratory appendage. C, protective plate of Cœnis larva, without respiratory appendage. All the figures are copied from Vayssière.] Palmén[77] has subjected Gegenbaur’s hypothesis to a very searching examination. He observes that:-- [77] Zur Morphologie des Tracheensystems (1877). 1. In _Campodea_, and presumably in other primitive Insects, the tracheal system is not closed and adapted for aquatic respiration, but open. Tracheal gills are not by any means confined to the lowest Insects. (See above, p. 65.)