142 Objects for the Microscope. 



which may be applied with a camel-hair pencil, and after- 

 wards mounted in balsam. In this last and most difficult 

 part let the balsam be very fluid, and the warmth gentle, 

 that the air may be quietly dispersed, and the bubbles 

 removed before the final covering with thin glass. 



I recommend the Borborus and Empis stercorea as the 

 easiest specimens to begin with. When caught, if immersed 

 in hollands or spirits of wine, they will keep any length of 

 time until wanted for mounting. 



THE HALTERES, OR POISERS, OF DIPTERA. 



These small organs, which are very apparent as little 

 knobs on a stalk, like drumsticks, just behind the wings of 

 Blow-flies and the Tipulse (especially the Tipula olacea, 

 which flutter against our window-panes), are rudimentary 

 wings. We only find them in the Diptera, which all 

 possess these much-disputed organs. 



They are called poisers, or balances, because it was 

 formerly supposed that the insect used them as the rope- 

 dancer does his pole, to steady itself in the air. Some 

 naturalists fancied that they produced the humming noise 

 in flight by beating against two little scales at the base of 

 the wing, called alulae ; but that can hardly be the case, 

 seeing how many insects buzz who have no halteres, such 

 as Bees, Cockchafers, Dung-beetles, &c., and that so many 

 flies who do possess them fly silently. They certainly do 

 move very rapidly, quivering as the insect flies, and even 

 when at rest I have seen the vibration. They are placed 

 immediately on the margin of the great thoracic spiracle, 

 and the late discoveries of certain organs inside these 

 halteres lead us to suppose they are organs of smell, as the 

 antennae may be of hearing. 



HALTERES OF BLOW-FLY. 



These, if mounted carefully in balsam, have become 

 transparent ; we see that they consist of three parts, the 

 base, shaft, and head. The organs in question are at the 

 base, two distinct groups of vesicles, which look like dots 



