362 COLLECTING, REARING, ETC. [CH. XJX 



studies of the heart in situ, etc. It should also be used in sectioning those parts 

 (e.g. caudal gills) in which the cuticle is very thick compared with the rest of 

 the organ ; for, otherwise, the cuticle may tear away and carry most of the 

 tissues with it. Method A yields excellent results with the nervous-system, 

 sense-organs, alimentary canal, rectal gills, etc., and also offers a less resistant 

 cutting-surface to the razor. It is, also, of course, a much shorter method. 

 Insects are, at the best, very difficult objects to section well, and the student 

 must not expect to obtain first-rate results without long practice and many 

 disappointments. The success of both methods depends chiefly on the softness 

 of the chitin. There are two safe rules to be followed: 



1. Always use a specimen fresh from ecdysis or metamorphosis, if possible. 



2. Avoid xylol, and substitute cedar oil, in all methods used. 



Tracheal Studies. The camera-lucida and a good photo-micrographic 

 apparatus are, of course, invaluable adjuncts for which the methods of use need 

 not be gone into here in detail. The latter may, however, be specially applied 

 to the study of the tracheal system of the gills and wings of Odonate larvae. 

 As the air passes out of the finer branches between one and two hours after 

 death (or even sooner), it is clear that permanent preparations cannot be 

 obtained. The part to be studied should be cut off immediately the larva 

 is dead (a drop or two of chloroform shaken up in water is the best killing- 

 agent) and placed on a slide, in pure water. It may then be quickly examined, 

 and, if necessary, a special portion selected and cut off (in the case of gills). 

 This should be floated on to a clear slide, with water, and a cover-glass lowered 

 gently upon it. It should then be photographed to the desired magnification 

 without loss of time. The tracheae appear black on a pale ground. 



Wing- Venation Studies. For this purpose I employ a "triple-extension" 

 camera with a total bellows-length of 2 feet, and a special magnifier (or micro- 

 summar) to be used with the lens. A strong artificial light and a condenser 

 are also needed. The wings may either be cut off and placed between two 

 glass slides, or the set insect may be used (by sticking the pin into a small 

 lump of putty placed on a piece of glass, or otherwise arranging it so that 

 the wings are fixed in the desired plane). Slow plates must be used. The 

 negative should be under-exposed and over- developed, to give a transparent 

 venation on a black ground. The magnifications obtained by this camera are 

 as follows : 



(a) Without the magnifier : from one-half to a little over natural size. 



(b) With the magnifier : from twice to four times natural size, according 

 to the amount of extension of the bellows and the position of the object. 

 The figures of wing- venation in this book were nearly all made from photo- 

 graphs taken by this method. 



