M. E. MacGregor 
177 
the subject has hardly been given the attention it deserves. I have 
endeavoured therefore to secure actual photomicrographs of these 
organs, under the moderately high powers of the microscope, and while 
these are not in every case quite as clear as some of the other illustra¬ 
tions already mentioned, they are both more accurate and show the 
remarkable differences more clearly. 
Technique. 
The aim has been to obtain a method that will envolve the least 
difficulty in the determination of any particular larva, and the method 
of preparing the specimens for identification will therefore first be 
briefly dealt with. This rests on the every-day principle of removing 
the whole of the softer tissues by boiling in potash, so as to leave only 
Text-fig. 1. Diagram showing method of splitting the edge of the cuticle in specimens 
before mounting. 
the chitinous structures intact. The larvae are hardened in absolute 
alcohol for several days, and are then sectioned by hand, without 
imbedding, with a keen microtome knife or, what does almost better, a 
carefully sharpened Gillete razor blade. With the larva held between 
the finger and thumb, posterior end uppermost, about -%■%" is cut off, 
care being taken that the blade passes below the stigmal plates which 
are seen usually as two little yellowish-brown dots lying on each side 
of the middle line at the posterior end of the animal. The section is 
then examined under the low power of the microscope, to ascertain 
that the stigmata have suffered no injury, and if possible when the 
section is large enough the periphery of the cut edge of the cuticle is 
carefully split by slight incisions with the razor at four points at right 
angles to each other, as shown in Text-fig. 1. 
