EDWARD M. XELSOX ON VARIOUS INSECT STRUCTURES. 595 



artery (fig. 5) . A 1/4 th inch will be necessary to demonstrate these 

 structures. The saw on the wing of Agrion is a comparatively 

 bold structure, but if we examine the mandibles of a gad-fly 

 (Tabanus bovinus) we shall find upon one edge the most wonderful 

 saw in the world, having ten to sixteen thousand teeth per inch 

 on it, while the other edge is the keenest blade in existence 

 (fig. 7). As a point of " microscopy " these teeth on the saw on the 

 the lancets or mandibles of this insect form the most delicate 

 optical test I know. This is a matter of some importance, as 

 Podura test-scales are now not to be had for, sad to say, one 

 may pay 20s. for a slide of Podura scales and not find a single 

 test-scale upon it ! If any member of the Club has an objective 

 that will show these saw-like teeth with a large or full cone he 

 should take great care of it, as it may be some time before he 

 finds another that will do so. A 1 J inch * that will demon- 

 strate these teeth at the point of the mandible with axial 

 illumination must be a good lens. This test, however, is not 

 confined to low powers, for high powers such as a l/4th or 

 a l/6th that will show the teeth with a large working aperture 

 cannot have much wrong with them. *As the aperture of the 

 substage condenser is opened a point will be found when, owing 

 to spherical aberration in the objective, the image of the teeth 

 will vanish suddenly. This test rivals in sensitiveness all others 

 with which I am acquainted, and it is scarcely necessary to add 

 that a precise adjustment of tube length is necessary ; but it is 

 important to bear in mind that with a small or moderate sized 

 cone it is no test at all. 



The teeth are coarser at the point, where they count 10,000 

 per inch, and finer at the base of the mandible, where they 

 count 16,000 per inch. Those on the mandible of Haematopoda 

 fluvialis are still finer and count from 15,600 to 19,200 per inch. 

 The stout hairs on the palpi of this insect issue from a delicate 

 cup. The hairs on the wing of Tricho'pteryx atomaria have 

 secondary hairs on them ; a secondary hair measured in length 

 1*1 /./, thickness 0*18 /a = yiiVoo" mc h. This beautiful micro- 

 scopical object cannot be seen with an objective of less than 

 0'58 N.A. These few instances are mentioned to show that 

 a critical examination of the hairs of insects is not only a 



* Some H inches are engraved 2 inches; such lenses should also 

 show them. 



