132 NATURAL HISTORY OF AQUATIC INSECTS CH. 



Then six larvae were introduced, the small space 

 above the water being at the same time filled up with 

 carbonic acid. The bung was replaced, and the 

 larvae were watched from clay to day. Four of the 

 larvae survived for forty-eight hours, and one till the 

 fifth day. Two of them changed to pupae. Never- 

 theless, the water was from the first exhausted of 

 oxygen or nearly so. 



The special provisions which enable Chironomus 

 larvae, or at least many of them, to inhabit waters 

 poorly supplied with oxygen, probably explain their 

 occurrence at great depths. They have been fished 

 up, together with larvae of Tanypus, from the bottom 

 of the lake of Geneva, and have been found alive at 

 considerable depths in the sea in Denmark, Maine, 

 and elsewhere. 



I have described the adaptation of Chironomus 

 larvae to life at the bottom of slow streams and pools, 

 but we must notice that some of the species have 

 different habits, keeping near the surface of the water, 

 and supporting themselves on confervae, floating pieces 

 of wood and similar objects. It is interesting to see 

 the effect of this change of situation upon the respira- 

 tory organs. The surface-larvae have no tubules at 

 the hinder end of the body, and the blood rarely 

 exhibits a red tinge. I believe that the pupae which 

 succeed to these surface-larvae are similarly modified 

 to their peculiar conditions. 



As the time of pupation approaches, the thorax of 

 the Chironomus larva becomes swollen, and its seg- 

 ments lose their distinctness. At this time the legs 

 and wings of the future fly may be seen beneath the 



