274 BENJAMIN P. YouNG 
The spiracles 
The number of abdominal spiracles in Diptera varies from five to eight, 
with seven as the number oftenest met with. Data on this point are 
given in the list of species on pages 258 to 261. A glance at these data will 
show these openings to the tracheal system appearing either in membrane 
or in chitin, but oftener in membrane, as would be expected when it is 
considered that the normal position of the row of abdominal spiracles 
may be assumed to be midway between dorsum and venter in the latus. 
In a large majority of families this region remains membranous, and only 
in relatively few have the so-called tergites usurped the pleural region 
enough to include the spiracles. It amounts to this: when the hypo- 
dermal cells in the pleural region have become stimulated enough to 
deposit chitin, then, in the order Diptera, this region forfeits its right to 
be known as a part of the pleuron and must be called a part of the tergum. 
This arrangement of spiracles in the order certainly goes to show that if 
for any reason there is need for rigidity in a certain region, it is no more 
difficult to lay down chitin around spiracles than at any other place. 
As a rule it may be said that the abdominal spiracles of species of the 
Nemocera and Brachycera groups in the suborder Orthorrhapha are to 
be found in membrane. But immediately the outstanding exceptions, 
in so far as the writer’s study has gone, would have to be mentioned in 
Oncodes incultus (Plate XVI, 23) and in Lonchoptera sp. (Plate XIX, 
31). In the suborder Cyclorrhapha, the representative families of the 
Athericera and the Acalyptratae tend to permit their abdominal spiracles 
to remain in membrane. A number of exceptions would have to be 
raised here, however, such as Platypeza velutina (Plate XIX, 33), Parydra 
limpidipennis (Plate X XIX, 68), and others. Among the families of the 
Calyptratae of this suborder, the tendency is toward the deposition of 
chitin about these spiracular openings because of the excessive downward 
extension of the tergites. It is needless to say that this is not the case 
in all species of every family in the group. A glance at the record of 
the species of anthomyids studied will expel any doubts concerning this 
last statement. 
In most of the Diptera, and especially in the more generalized species, 
the first abdominal spiracle is to be found in the anterior part of the segment 
and very often in the membrane between the metathorax and the first 
abdominal segment. The usual position of the remaining spiracles is 
