Palmén[77] has subjected Gegenbaur’s hypothesis to a very searching examination. He observes that:—
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].)
2. Tracheal gills are not always homodynamous or morphologically equivalent. In Ephemeridæ, some are dorsal in position, some ventral (first abdominal pair in Oligoneuria and Rhithrogena); they may be cephalic, springing from the base of the maxilla, as in Oligoneuria and Jolia; Jolia has a branchial tuft at the insertion of each of the fore legs.[78] In Perlidæ the tracheal gills may have a tergal, pleural, sternal, or anal insertion. In some Libellulidæ also, anal leaflets occur.[79]
3. Tracheal gills never perfectly agree in position and number with the stigmata throughout the body. Sometimes they occur on different rings, sometimes on different parts of the same ring. Gegenbaur’s statements on this point are incorrect.
4. Tracheal gills may co-exist with stigmata. In Perlidæ the tracheal gills persist in the imago, and may be found, dry and functionless, beneath the stigmata. In Trichoptera they gradually abort at successive moults, and in some cases remain after the stigmata have opened.
5. Stigmata do not form by the breaking off of tracheal appendages, but by the enlargement of rudimentary tracheal branches, which open into the main longitudinal trunks. In larvæ with aquatic respiration these branches exist, though they are not functional.
Palmén’s objections must be satisfactorily disposed of before Gegenbaur’s explanation, interesting as it is, can be fully accepted. Palmén has proved, what is on other grounds clear enough, that stigmata are more ancient than tracheal gills, aerial tracheate respiration than aquatic. But there is nothing as yet to contradict the view that the first Insect-wings were adapted for propulsion in water, and that they were respiratory organs before they became motor. It is Gegenbaur’s explanation of the origin of stigmata, and not his explanation of the origin of wings, which is refuted by Palmén.