460 TEXT-BOOK OF ENTOMOLOGY 



only exist, are aborted, becoming simple solid cords not filled with 

 air (Fig. 436, vf, and 447, /, f uniculus or stigmatic cord). In the 

 imago, however, they resume their function, connecting with the open 

 functional stigmata. In Corethra, in its earliest stages, the entire 

 tracheal system is, like the stigmatic branch, a system of solid cords 

 and empty of air. (Palmen.) 



Embryology shows that these stigmatal branches are well developed, and are 

 formed at the same time as the stigmata. It was also shown by Dewitz, in a 

 posthumous paper (1890), that in the young larval stage of the Odonata and 

 Ephemeridae the tracheal system is at first an open one, and in some of the 

 families (Libellulidae, Agrionidae, and Ephemeridae) thoracic stigmata are seen 

 at a very early stage. From numerous experiments Dewitz concludes that in 

 the young stages of Odonata and Ephemeridae there is an open tracheal system ; 

 certainly in very young nymphs the thoracic spiracles allow the air to pass out. 

 Fully grown nymphs of .JDschnidae, Libellulidae, and Agrionidae are capable not 

 only of forcing the air out, but also, like the perfect insect, of inhaling it. 

 Moreover, he proved that the gills of Ephemeridae and Agrionidae are not indis- 

 pensable for the maintenance of life, as the insects can live without them, 

 breathing either through the skin or by the rectum, or in both ways. It would 

 seem that while in freshly hatched or very young larvae of aquatic insects of 

 different orders the skin is so delicate as to allow of dermal respiration, in after 

 life, when the skin becomes thicker and denser, these expansions (gills), pro- 

 vided with a very thin and delicate skin, of a necessity grow out from the walls 

 of the body. 



It thus appears that the closure and total or partial abolition of the stigmata 

 are in adaptation to aquatic life, and that such insects have descended from 

 terrestrial air-breathing winged forms. This is an important argument against 

 the view that the wings are modified tracheal gills. 



In this connection may be noticed the closure of the 2d and 3d thoracic stig- 

 mata in holopneustic insects. We have found on laying open the body of a 

 Sphinx larva that a large number of tracheal branches are seen to arise from the 

 prothoracic and from the first pair of abdominal stigmata. Now between these 

 points there are no spiracles or any external signs of them, there being in 

 Lepidoptera no mesothoracic or metathoracic spiracles. Yet the main lateral 

 trachea between the prothoracic and first abdominal segments deviates from its 

 course and bends down to send off a small shrivelled stigmatal branch or cord 

 to a place where, did a spiracle exist, we should look for it. In the larva of 

 Plati/samia cccrnpia, a similar vestigial stigmatal branch is present. 



In the larva of Corydalus, also, a trachea as large as the main longitudinal 

 one takes its origin and passes directly under the main trachea. Now both 

 tracheae send a stigmatal branch opposite to where the mesothoracic stigma 

 should be, if present, i.e. on the hind edge of the segment. 



Verson, moreover, has found in the freshly hatched silkworm vestiges of 

 meso- and metathoracic stigmata, each consisting of a circle of high hypodermal 

 cells radially arranged around a common centre. The stigmatal branch is long, 

 but shrivelled ; its peritoneum is widened out into several berry-like saccules 

 filled with cell-elements. In profile these rudimentary stigmata appear as a 

 series of high hypodermal cells, which form the basis of a short blind tube. 



After the second moult there begins a peculiar transformation of the rudi- 

 mentary stigmata. The stigmatal branch connected with them sends off at 

 various points thick tufts of capillary tracheae which press against the base of 



