NEUEOPTEEA. 433 



tractile tubes. They construct for themselves fixed places of shelter, 

 more or less imperfect, at the bottom of the water, and against large 

 stones, which they leave occasionally for a few moments. Some- 

 times these cases contain many larvae at the same time. Fig. 420 

 represents the various states of a Hy dropsy che ; the larva is seen 

 on the left, the pupa on the right, the winged insect in the middle. 

 Two of the insect's tents or places of shelter are represented below. 

 Fig. 421 shows the different states of Rhyacophilus vulgatus, larva, 

 cocoon, pupa, and imago. The genus Rhyacophilus has this pecu- 

 liarity, that the larva spins itself a cocoon in the interior of its 

 dwelling, before changing into a pupa. 



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