256 BENJAMIN P. YounG 
number of abdominal segments in different families. Some have counted 
only definitive segments and used such for key characters, while others 
have taken into consideration the true number as indicated by the spiracles. 
One of the reasons for figuring herein the pleurites of the meso- and the 
metathorax and the sclerites of the proximal abdominal segments of some 
of the commoner species of fifty-seven families of this order, is to aid in 
bringing about uniformity in interpretation. 
Having studied but a few species of each family, the writer cannot say 
that the characters of the one figure hold throughout the family, but 
nevertheless a typical species should afford external facies more or less 
characteristic of the family. 
METHODS AND TECHNIQUE 
Practically all of this study was made from dried specimens which had 
been soaked for from three hours to seven days in a 10-per-cent solution of 
potassium hydroxide. Some forms were already clear enough for study 
but were allowed to remain in the clearer for a few hours in order to relax 
them sufficiently for study and handling. The specimens were then 
washed in distilled water to which a few drops of acetic acid had been 
added, and preserved in 70-per-cent alcohol. 
It was soon found impossible to see sutures in some forms, especially 
the smaller species, without dissection, and it was only after each form 
had been halved by a median longitudinal cut with a scalpel that the 
work progressed with dispatch. This operation, which was done under 
70-per-cent alcohol, was followed by the removal of the viscera of each 
half, but during the latter operation close attention was given also to 
the tracheal branches leading to the spiracles in order to learn the position 
~and number of these in the abdomen. The left half was then available 
for external study, while the right was reserved for internal study of 
phragmas, apodemes, and apophyses as an aid to the more definite location 
of sclerites. 
The binocular microscope was used both in making dissections and 
for the study of specimens in 70-per-cent alcohol. Drawings were made 
on coordinate paper with the aid of an ocular micrometer laid off in squares. 
To insure the object’s remaining in the same place, a small piece of plasti- 
cine, used in modeling, was stuck to the bottom of a watch glass and the 
fly was held against the bottom of the glass by means of two bent pins 
