600 THE MICROSCOPE. 



Gnat's wing is magnified 500 diameters, fig. 273, No. 7. 

 The proboscis is protected on either side by antennae, or 

 feelers. 



The metamorphosis of the larva of Corethra plumi- 

 cornis, one of the Gnat tribe, has been carefully in< 

 vestigated by Professor Bymer Jones, F.RS. 1 This 

 competent observer brings out many points of interest ; 

 one in particular deserves notice, namely, the use of 

 the four remarkable-looking jet-black bodies situated 

 in the body of the larva, two of which are placed in 

 the thoracic region, and two near the centre of the pos- 

 terior half of the body. These have hitherto puzzled all 

 observers, but at length receive elucidation at the hands of 

 Professor Jones, who had the good fortune to witness the 

 metamorphosis. In form these bodies are more or less 

 kidney-shaped, and to all appearance completely isolated 

 in the body. Upon crushing the insect beneath the com- 

 pressorum, they are found to b filled with air, and by their 

 means the creature is enabled to rise and sink at pleasure. 

 They are composed of a series of small vesicles, each of 

 which has several coats : of these the outermost, when feebly 

 magnified, seems of a uniform hue of black, but under a 

 higher power is seen to be made up of many pigment cells, 

 so as to give the organ a reticulated appearance ; and it is 

 only when this black pigment has been removed, together 

 with a dull opaque membrane whereon the black patches 

 rest, that the real air-sac is displayed. When thus denuded, 

 the true walls of the air-sac appear to be composed of 

 a dense membrane, possessing great refractive power, the 

 effect of which upon transmitted light is extraordinary. 

 When highly magnified, it is found to be entirely com- 

 posed of numerous coils of a delicate fibre,., similar to that 

 which maintains the permeability of the trachea of ordi- 

 nary insects, arranged in several superimposed layers, and 

 having the appearance of being closed in on all sides. It is 

 not until the larva thus constituted has arrived at its full 

 size that the appearances described become complicated by 

 intermixture with organs belonging to the pupa condition 

 of the insect. At this period, however, the rudiments of 



(1) See Trans. Micros. Soc. Oct. 1867, "On the Structure and Metamorphosis 

 of the larva of Corethra plumieornis." 



