11 
PEEFACE. 
more difficult, on account of the unequal foreshortening 
of the limbs, &c. 
This disadvantage has been partly obviated by the 
addition to the text of micrometrical measurements, 
which are given in thousandths of an inch, and for the 
use of those who employ the metric scale reductions 
into parts of a millemetre have been made. A refer- 
ence to these measurements will prevent a wrong 
impression as to the relative size of certain specimens, 
for slightly different magnifying powers have neces- 
sarily been employed in the figures. Had it not been 
so, some forms in the winged state would have occu- 
pied the greater part of a page. 
For rendering the insects sufficiently stationary for 
drawing under the camera, they were generally first 
stupefied with vapour of chloroform, or, what was 
found to be equally efficacious, with the vapour of 
light petroleum oil. At other times good positions of 
the limbs were obtained by attaching the live insect to 
the middlemost of three minute spots of Canada 
balsam dotted on a slip of glass. By means of a fine 
bristle, the wings were laid out and secured in their 
places by attachments to the remaining outer spots of 
balsam. This method permitted the insects to be 
permanently mounted for the microscope after the 
drawings were finished. 
The Author wishes here freely to acknowledge the 
help he has obtained from a goodly list of both 
present and by-gone workers. In most places refer- 
ences to the various authorities have been given in 
the footnotes. The author has gained much infor- 
mation as to the life-history of the group from the 
treatises of Bonnet, He Geer, Kaltenbach, Koch, and 
Passerini. 
The writings of Morren, Huxley, Balbiani, and 
others, have been very suggestive, and have furnished 
valuable materials for treating on the subjects of mor- 
phology and reproduction. It is purposed briefly to 
discuss these subjects in a subsequent volume. 
