128 



THE SPIRACLES OF HYMENOPTERA. 



The investigations of M. G. Carlet have recently shown that the spira- 

 cles of Hy u^uoptera are provided with a peculiar organ for closing and 

 opening them which has been hitherto overlooked. 



The spiracles of insects are, for the most part, capable of being closed 

 in various ways. This is effected by the sides being caused to approach 

 each other or by the action of a single or double lid or lips. The 

 spiracles of these classes are provided with an internal muscular ap- 

 paratus, by means of which the insects can exclude or admit the air at 

 will. In other cases, however, the opening in the spiracle is rigid, and 

 is commonly protected from the entrance of extraneous matter by the 

 presence of simple or plumose hairs on the edge of the opening. The 

 spiracles of Hymenoptera previous to Garlet's studies have been con- 

 sidered to belong to the latter class, and this is true of the external 

 opening. It was found, however, in the first instance in the case of the 

 anal spiracle of the bee,* and afterwards t to be true of all the spiracles 

 of Hymenoptera, that the closing was effected not at the mouth of the 

 spiracle, but a short distance below it on the trunk of the trachea, thus 

 forming a small cloaca-like cavity. The closing is effected by means of 

 a chitinous flap or operculum, which in its normal position forms a por- 

 tion of the wall of the enlarged cavity immediately below the spiracle. 

 By the action of a special muscle this flap may be drawn backward so 

 that it crushes in the wall of the trachea and closes it much as a rubber 

 tube may be closed by bending it at an angle. This form of shutting 

 out the entrance of air into the trachea is given the name of the oper- 

 culate closure (fermeture operculaire). The dissection of this tracheal 

 muscle is a matter of extreme difficulty, as it is smaller than the finest 

 thread of silk and is confused among the bundles of the other muscles 

 which surround the stigmata. 



MOUTH PARTS OF THYSANOPTERA. 



A peculiar asymmetry of the head and mouth parts of Thysanoptera, 

 which seems to have been hitherto overlooked, is described and figured 

 by Prof. H. Garman in Bulletin of the Esse:^ Institute, Vol. XXII, IN'os. 

 1-3, 1890. A well-developed organ supposed to be a mandible is found 

 to occur on one side of the head and to be represented by a mere rudi- 

 ment on the opposite side, and that is accompanied with a lack of sym- 

 metry in the clypeus labrum, and also in the chitinous endocranium of 

 the head. The long styliferous organs heretofore taken for the mandi- 

 bles are supposed to be rather the laciniae of the maxillae. The explana- 

 tion of the organs is given with some hesitation by the author, his stud- 

 ies having been limited to species in two genera. 



* Comptes Rendus, November 5, 1888. t Comptes Eendus, April 23, 1889. 



