212 THE CAUSES WHICH PROPAGATE 



are invariably the same as that seen in the Drone Flies. But in 

 the Blue-bottle, agreeably to Chabrier's experiment, we find them 

 placed on the spiracular lips, nine on one and sixteen on the other 

 (Plate v.. Fig-. 3, I), and in the large bee-like Tach'ma grossa, 

 that sits on flower clusters at autumn, they are placed on the 

 single lip that shuts the spiracle. Behind these spiracles, 

 situated posteriorly and symmetrically in the metathorax, lie two 

 capacious air-vesicles bounded by the alar muscles, and thus 

 calculated to increase their power of respiration when these 

 muscles are vibrated. 



Regarding- bees, John Hunter had previously recorded in the 

 Philosophical Transactions for 1792 a similar conclusion he 

 had arrived at in respect of the hum of the Hive Bees. " They 

 produce,^'' he says, " a noise independent of their wings ; for if a 

 bee is smeared all over with honey so as to make the wings stick 

 together, it will be found to make a noise which is shrill and 

 peevish. To ascertain this further, I held a bee by the legs 

 with a pair of pincers, and observed that it then made a 

 peevish noise, although the wings were perfectly still. I then 

 cut the wings off, and found it made the same noise. I 

 examined it in water, but then it did not produce the noise till 

 it was very much teased, and then it made the same kind of 

 noise ; and I could observe the water, or rather the surface of 

 contact of the water with the air at the mouth of an air-hole 

 (metathoracic spiracle ?) at the root of the wing, vibrating.'" 

 Dr. Landois considers the spiracular notes of bees due to the 

 large metathoracic spiracles (Plate V., Fig. 4, b), and that the ab- 

 dominal ones may assist in their production. In the Humble Bee, 

 according" to this author, the spiracles (s) appear to want the 

 laminae seen in flies, but their inner lips are chitineous, sharp- 

 edged, and immovable, suited to vibration ; and the lower one (i^) 

 forms internally a cup-shaped cavity, covered by the valve that 

 opens and closes the spiracle [a] . 



Whether or not, then, these Dipterous spiracular laminae are 

 influential in producing their buzz — and there may be those who 

 are inclined to compare them with antennal extensions and 

 consider them an osmeterium — it cannot but be evident it 

 is from resj^iration rather than wing-beat the colour of the 

 aerial notes of insects arises — a physiological view which, if 



