Fig. 30. Chloeon (Chloeopsis) dipterum. Larva in eighth stage, with wings and respiratory leaflets. × 14. Copied from Vayssière (loc. cit.).
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.
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.
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.