March, 1905] Powell: Wings of Certain Beetles. 13 



above the wings, and that they generally lie in the cephalic half of 

 their segments, while the wing fundament is centrally situated on its 

 segment. (4) If it can be proven that in any winged insect the 

 mesothoracic and metathoracic spiracles are present and functional, it 

 seems certain that the wings have originated independently of the 

 spiracles and not from any spiracle disc, since it seems pretty hard to 

 believe, as has been stated by Tower, to be the case with the meso- 

 thoracic spiracle, that the spiracles migrate forward while the discs 

 remain behind ; that is to say, the chitinized opening alone moves 

 forward and new hypodermal cells become specialized and take on the 

 function of secreting the chitin of the spiracle, while the old cells 

 (disc) remain behind and later develop into a wing. 



1. The Spiracles. 



In the primitive insects, the ancestors of our present winged forms, 

 a pair of spiracles was present in each of the three thoracic segments, 

 as well as in each of the first nine abdominal segments. The presence 

 or absence of certain of these spiracles and their position on the seg- 

 ments has an important bearing on the theory of the origin of the 

 wings from spiracle discs. 



While the prothoracic spiracle is present in the embryo, as has 

 been shown by Wheeler, it entirely degenerates and is not known to 

 be present in the larva or adult of any .winged insect. In the Lepi- 

 doptera however it is said to have been transformed into the spinneret. 



The mesothoracic spiracle is always present and open except in 

 some aquatic larvae. This spiracle has often been called the pro- 

 thoracic because it has often migrated into that segment, but that it 

 is really the mesothoracic has been proved by Wheeler (1899), whose 

 observations have been confirmed by Tower (1903). This is usually 

 the largest of all the spiracles, but in the aquatic forms, many of the 

 Odonata, Ephemerida and Diptera, this, as well as all the other 

 spiracles of the body are closed or nonfunctional, reappearing as 

 functional spiracles in the adult insect. 



In the larvae of many insects the metathoracic spiracle has en- 

 tirely degenerated, but this is by no means the rule. Among the 

 Coleoptera it has been proved by Graber (1888) for Melolontha and 

 Lina and for Doryphora by Wheeler (1899) that it is present in the 

 embryo. They found however that this spiracle degenerated before 

 the larva emerged from the egg. Complete degeneration however 



