LIFE-HISTORY OF CHIRONOMUS 255 



probability help to increase the respiratory surface 

 of the skin. 



The length of the larval life of Chironomus varies 

 with the seasons. Spring larvae grow rapidly, and in 

 a few weeks pass into the pupal stage, whereas the 

 autumn broods grow more slowly and do not pupate 

 till the winter is over. Before this stage is reached 

 the rudiments of the future wings and legs can be 

 detected as coiled tubules lying in the thoracic seg- 

 ments. When the last larval skin has been cast, 

 these and other organs of the fly can be seen more 

 clearly. They form a large mass occupying the 

 anterior end of the pupa, from the thorax of which 

 a pair of tufted filaments projects freely. This portion 

 of the body sways above the mud, below which the 

 rest is concealed. The tail-tubules have disappeared, 

 and a broad fin has formed which is used for move- 

 ment through the water. In two or three days the 

 thoracic filaments have extracted sufficient oxygen 

 from the surrounding water to render the pupa buoyant. 

 The oxygen is stored in the tracheal system, which 

 rapidly becomes fully developed. The pupa now 

 floats at the surface, and on the last day or so inhales 

 air directly from the atmosphere through the spiracles 

 or breathing holes. At length the pupal skin splits 

 along the thorax, the fly creeps out, dries its wings, 

 and in less than a minute flies away. 



The life-history of Corethra, a close ally of Chiro- 

 nomus, is almost equally easy to follow. In still pools 

 the early stages of this gnat-like insect are developed 



