THE METAMORPHOSES OF THE RESPIRATORY SYSTEM. 49 
apparatus and some short horny tubes, which are often sur- 
rounded by strong hairs. The tubes are always open at their 
free ends, and generally project beyond the body of the insect. 
The common water scorpions are furnished with these structures, 
and they inspire as well as expire air by means of this curious 
THE LARVA AND NYMPH OF THE GNAT. 
(The breathing-tube of the larva points upwards, and those of the nymph downwards. ) 
respiratory apparatus, without being drowned in their swimming to 
and fro. The larvae of the Dytiscus , the water beetle, possess these 
elongated air-breathing tubes, and after metamorphosis the perfect 
insect, which leads an amphibious sort of life, has the ordinary 
spiracles for its respiratory purposes, but they are situated on the 
