ITS OUTER SKELETON. 67 



Palmen* has subjected Gregenbaur'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 morpho- 

 logically equivalent. In Ephemeridfe, 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.-f* In Perlids-e the 

 tracheal gills may have a tergal, pleura), sternal, or anal 

 insertion. In some Libellulidae also, anal leaflets occur.} 



3. Tracheal gills never perfect^ 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 Perlidae 

 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 



* Zur Morphologic des Tracheensystems (1877). 



f We take these instances from Eaton, Monograph of Ephemeridse, Linn. Trans., 

 1883, p. 15. 



Charles Brougniart has lately described a fossil Insect from the Coal Measures of 

 Commentry, which he names Corydaloides Scudderi, and refers to the Pseudo- 

 Neuroptera. In this Insect every ring of the abdomen carries laminae, upon which 

 the ramified tracheae can still be made out by the naked eye. Stigmata co-existed 

 with these tracheal gills. (Bull. Soc. Sci. Nat. de Rouen, 1885.) 



Some Crustacea are furnished with respiratory leaflets, curiously like those of 

 Tracheates, with which, however, they have no genetic connection. In Isopod 

 Crustacea the exopodites of the anterior abdominal segments often form opercula, 

 which protect the succeeding limbs. In the terrestrial Isopods, Porcellio and 

 Armadillo, these opercula contain ramified air-tubes, which open externally, and 

 much resemble tracheae. The anterior abdominal appendages of Tylus are provided 

 with air-chambers, each lodging brush-like bundles of air-tubes, which open to the 

 outer air. Lamellae, projecting inwards from the sides of the abdominal segments, 

 incompletely cover in the hinder part of the ventral surface of the abdomen, and 

 protect the modified appendages. (Milne Edwards, Hist. Nat. des Crustacea, "Vol. III.) 



