158 NATURAL HISTORY OF WASPS. 
urea, uric acid, and other such like organic com- 
pounds, occur as the results of insect digestion, 
whenever the conditions are analogous to those 
under which these substances are produced by the 
higher animals. But analogy scarcely justifies us in 
the belief that these. organic compounds take such 
an active part im insect digestion as they do in that 
of the higher animals, either initiating secondary 
changes, or marking any particular stage in the 
process. We scarcely know them in insects, except 
as excretory products. 
The anatomical investigation of this system of 
organs is comparatively easy. Having killed one of 
the large female wasps with benzol or chloroform, we 
_ begin by cutting up the abdominal scales on each 
side, from the sting to the pedicle, with a fine-pointed 
pair of scissors. Next fasten the specimen to a 
loaded cork, under water, by a small pin through 
the last dorsal scale, and a stronger one through the 
thorax; keeping the parts just on the stretch. It 
will be convenient to clip off the legs to prevent 
their getting in the way. On turning aside the 
ventral scales which have been cut through, these 
are seen to be coated internally with a woolly-look- 
ing substance. Under the microscope, which should 
stand at our elbow all the time we are dissecting, 
this substance is shown to be composed of very fine 
membranous bags containing collections of minute 
oil globules. These bags are arranged, in regular 
order, on the terminal branches of trachee, just as 
the acini of the lobular glands of the higher animals 
are arranged on their vessels, or like grapes on their 
