112 LETTERS ON ENTOMOLOGY. 
There are fourteen thousand hemispheres or 
eyes distinguishable in the large eyes of the 
drone-fly, and each of these is a perfect eye, 
being furnished with a cornea, a transparent 
humour, and a retina. The most remarkable of 
insects for its eyes, is the libella, or dragon-fly : 
Leeuwenhock reckons in both twenty-five thou- 
sand and eighty-eight lenses, placed in an hex- 
angular position. He also numbered six thou- 
sand two hundred and thirty-six in those of the 
silk-worm in the image state, and eight thousand 
in the common fly. These large eyes are all 
immoveable, and so placed that the insect can 
see on all sides without turning. Reaumur 
asserts, that many kinds have besides three small 
eyes on the back of the head. 
Flies, in common with other insects, breathe 
by means of spiracles or breathing holes. All, 
whether two or four- winged, which have an in- 
dividual corslet or thorax to which the six legs 
are fastened, have four of these breathing-holes, 
two on each side the corslet. They have also 
others on the wings or segments of the body, 
but less considerable. They are placed length- 
ways on the body, being oblong, with raised 
edges, and generally of a different colour from 
the body ; they are most easily discovered in the 
