1919] Metcalf — Respiratory Apparalus of Dipterous Larvce 57 



of which it is very easy to gauge the angle of divergence of these 

 parts. The micro-protractor (see figure 2) is a simple glass disk, 

 21 mm. in diameter, ruled in angles so designed and arranged that 

 by using different combinations of adjacent larger and smaller 

 angles as indicated by the concentric lines in figure 2, it is possible 

 to measure very accurately any angle from 5° to 360° by 5° inter- 

 vals. Even single degrees of divergence may be estimated with 

 considerable accuracy by this method. 



In order to use these points it is only necessary to place the 

 quiescent (killed or paralyzed) larva or the puparium under the 

 microscope in such position that the plane of the stigmal plates is 

 perpendicular to the axis of vision. The ocular with the micro- 

 protractor resting on its diaphragm is then revolved until some 

 combination of its measured angles exactly coincides with the angle 

 of divergence of the spiracles to be measured.^ 



The amount of divergence of the dorsal spiracles from each other 

 I have called angle A (See figure 1). In cases where the two 

 stigmal plates are widely separated it may be easier to measure the 

 divergence of either dorsal spiracle from the median line and this 



may be designated The amount of divergence between 



the dorsal and median spiracles on either side is angle B: that 

 between the median and ventral spiracles on either side is angle C; 

 and that of the right and left ventral spiracles from each other is 

 angle D. Here again the divergence of a ventral spiracle from the 



median line may be designated - — '■ In all cases where the 



outer (peripheral) ends of tivo spiracles are closer together than their 

 inner (central) ends, the fact is indicated by designating the corre- 

 sponding angle a minns one. This is often the case with angles A 

 and D in the Syrphidae. 



In addition to its use as described above, I believe the micro- 

 protractor may prove useful to biologists in many lines of work 

 where it is desired to measure the angles of microscopic objects. 



' Whenever it is possible to do so, much better results will be achieved by preparing the 

 stigmal plates with 10 per cent KOH solution, as described by MacGregor and Herms in the 

 papers cited above. 



