lOO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



maxillae, shortly behind the boundary between the first and second 

 maxillary segments. The second maxillary spiracles have thus the 

 same relative position on their segment as have all the body spiracles 

 in the embryo, or the abdominal spiracles in adult insects. The em- 

 bryonic invaginations of the labial segment, according to Nelson, 

 give rise to a part of the tracheal system of the head, but are later 

 closed and leave no external trace of their existence in the adult insect. 



Prothoracic spiracles are known to exist as functional organs of 

 the adult only in some of the Sminthuridae (Collembola). They are 

 situated laterally in the neck membrane close to the posterior margin 

 of the head, but Davies (1927) claims that the region bearing the 

 spiracles belongs to the prothorax. These cervical or prothoracic 

 spiracles are the only spiracles present in the Sminthuridae, and no 

 other collembolan is known to possess either spiracles or tracheae 

 in any part of the body. Temporary prothoracic spiracles, followed 

 by the usual series of spiracular invaginations, have been described 

 in the embryo of Blattella by Cholodkowsky (1891), and in the em- 

 bryo of Leptinotarsa by Wheeler (1889). 



The usual first pair of thoracic spiracles of adult, nymphal, and larval 

 insects is always situated either in the posterior part of the prothorax 

 or in the intersegmental membrane between the prothorax and the 

 mesothorax. In the embryos of most insects, however, these spiracles 

 are said to lie anteriorly in the mesothorax ; they would appear, 

 therefore, to be the true mesothoracic spiracles which have become 

 prothoracic in position by a secondary forward migration. The usual 

 second pair of adult thoracic spiracles are the embryonic meta- 

 thoracic spiracles, and they sometimes occur on the anterior part 

 of the metathorax in the adult, though more commonly they lie in 

 the membrane between the mesothorax and the metathorax, or in 

 the posterior part of the mesothorax. The segmental relations of the 

 thoracic spiracles is somewhat complicated by the fact that the muscles 

 of their closing apparatus have their origins in the segments on which 

 the spiracles are situated in the adult. Since, however, the musculature 

 of the thoracic spiracles is not alike in different groups of insects and 

 is often different in the two spiracles of the same insect, it is probably 

 of secondary development in all cases. 



Contrary to the embryological evidence of the segmental relations 

 of the spiracles, there are many points in the anatomy of the tracheal 

 system, and in the innervation of the spiracular muscles, that suggest, 

 as now claimed by several writers, that the spiracles are primarily 

 intersegmental invaginations, and that their definitive positions are 



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