14 journal New York Entomological Society. [Vol xin. 



does not always take place. A careful external examination will show, 

 in the larva? of many beetles, that the metathoracic spiracle is present, 

 though usually much smaller than the other spiracles and generally 

 much less heavily chitinized, so that it is quite inconspicuous and easily 

 overlooked and it is in many cases evidently closed or non-functional, 

 at least during the larval state. I have found the metathoracic spiracle 

 present in the larva? of a Trogositid, Thymalus marginicollis (fulgidus) 

 (Fig. 25), of a Pyrochroid sp., of an Erotylid sp. (Fig. 26), in the 

 larvae of several unknown Tenebrionidae, Carabidae and Cerambycidae. 

 In the larva; of the Scolytids T. plastographus and D. valens (Figs. 

 32, 33) I have been unable to find the spiracle by external exami- 

 nation, but cross-sections of the metathorax at the place where the 

 spiracle should be show the spiracular branch extending out from the 

 lateral tracheal trunk entirely through the hypodermis (Fig. 12) and 

 they are probably functional. In the larva of Thymalus marginicollis 

 the metathoracic spiracle can be plainly seen as a small oval disc with 

 a slit in it and it is probably functional (Fig. 25). Breed (1903) has 

 figured this spiracle in both the larva and the pupa of this beetle. In 

 the larva of the Erotylid sp. (Fig. 26) this spiracle is even larger and 

 though considerably smaller than the other spiracles is undoubtedly 

 functional. 



In larva; that are light colored and not heavily chitinized, such as 

 many of the wood borers, the tracheal system can usually be well 

 brought out by clearing in cedar oil. Fig. 24 shows the lateral 

 tracheal system of a larva thus prepared, of a Ptinid, Ozognathus cor- 

 nutus. It will be seen that under the metathoracic and each of the 

 abdominal spiracles there is a tracheal center, that is, the group of 

 tracheal branches which, in the embryo, arose from the end of each 

 spiracular invagination and ramified in all directions. Although, on 

 account of the small size of the larva, I could not make out with 

 certainty a spiracular opening in the metathorax, there is a tracheal 

 center under the spot where this spiracle should be, which is larger 

 than any in the abdomen except the first and it is not probable that 

 this would be the case if there were no functional opening. It will 

 be seen that, while the mesothoracic spiracle has migrated forward 

 into the suture, the tracheal center migrated with it, while the meta- 

 thoracic tracheal center is distinctly in the metathorax and occupies 

 nearly the same position in its segment as is occupied by the abdomi- 

 nal centers in their segments. 



