Midge Larvae 395 



These tubes, like the ones on the stems, open to the 

 surface at both ends. The larva, within, holding on to 

 the silken lining of the walls with its claws, swings its 

 body in vigorous undulations, driving a current of 

 water thro the tube. This serves for respiration. It 

 also serves to drive diatoms and other food organisms 

 into net-like barriers spun across the exit; these bar- 

 riers are repaired or renewed after every catch. Food 

 is thus carried into the shelter of the case. But food 

 is also gathered from exposed surfaces whenever it can 

 be reached from open ends of the tube. It is gathered 

 by scraping the sessile diatoms and algae from stems. 

 For such work the mouth of the larva is equipped with 

 elaborate rakes and scrapers. 



The larva of Chironomus is relatively simple. It 

 appears much less complex in organization than are 

 many of its insect competitors. It has a cylindric 

 worm-like pale and naked body with a bifid proleg 

 underneath at the front and a pair of prolegs behind, 

 caudal tufts of bristles, and a few simple gills. The 

 prolegs are armed with hooks and on them it creeps 

 somewhat like a looping caterpillar. From its mouth 

 it spins the fluid silk, and spreads it ere it hardens with 

 the front proleg. All in all, it is a shy and defenseless 

 and secretive creature, without any special gift of 

 locomotion. 



This apparent weakling has been able to possess 

 itself of the entire littoral region of the earth, perhaps 

 by reason of the following characteristics : 



1. Ability to live on foodstuffs that have a very 

 general distribution. 



2. Ability to build its own shelter. 



3. Consequent adaptability to variety of conditions. 



4. Great reproductive capacity. 



5. Brief life cycle. 



