40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



it is disconcerting to observe that the dorsal muscle from the closing 

 lever of the first spiracle of a caterpillar is attached dorsally to the 

 lateral margin of the protergal shield, and still more so to find that 

 there is a spiracle in the protHorax of Japyx and related genera in 

 addition to one in the mesothorax. 



The usual absence of prothoracic spiracles in adult insects has never 

 been satisfactorily explained. Though Wheeler (1889) says the tra- 

 cheal invaginations of the first thoracic segment in the embryo of the 

 potato beetle {Leptiiwtarsa decnnlincata) are small, and soon close 

 over and disappear, and though Cholodkowsky (1891) mentions the 

 presence of a pair of such invaginations in each segment of the 

 embryo of Blatta, students of insect embryology in general have not 

 been able to find traces of spiracles or tracheal pits in the prothorax 

 at any stage of development (Lehmann, 1925). In Smynthurus, one 

 of the Collembola, the single pair of spiracles present is located in 

 the neck. Perhaps these are the prothoracic spiracles. In Campodea 

 and Japyx, and related genera, there is a jjair of spiracles on each of 

 the thoracic segments ; in Japyx there is an additional pair on the 

 metathorax. In Hctcro japyx the first spiracle (fig. 10 B, Spx) is on 

 the side of the prothorax posterior to the base of the leg, the meso- 

 thoracic spiracle (Sp^) lies between a dorsal (tergopleural) scelerite 

 of the pleuron and the edge of the tergum, the corresponding spiracle 

 of the metathorax has a similar position, while the fourth spiracle 

 {Sp) lies latero-ventrally before the base of the hind leg, between the 

 lateral ends of the first and second apotomal folds of the pleuron. 

 The significance of the number and position of the thoracic spiracles 

 of Campodea and Japyx is by no means clear. The dorsal spiracles 

 that occur on the prothorax and at the posterior end of the abdomen 

 in the larvae of Diptera are probably si)ecial larval organs, since the 

 eight normal lateral spiracles apj^ear on each side of the abdomen in 

 some forms ( Rliagolctis, Braula ) toward the end of the larval period. 



The second thoracic spiracles are very small in some adult insects ; 

 in certain larv?e they are lacking. In caterpillars and in the larvae 

 of beetles, the site of each is marked externally by a minute disc in 

 the cuticula just behind the intersegmental fold between mesothorax 

 and metathorax, and the disc is connected l)y a degenerate tracheal 

 strand with the main lateral trunk of the respiratory system ; in the 

 adults, these spiracles are restored as functional organs. The thoracic 

 spiracles usually dififer from the abdominal spiracles in some manner, 

 either in the relative positions of their parts, in their structure, or ir 

 the type of their closing apparatus, a condition as yet unexplained. 



