156 INSECT LIFE. 



each pair. Consider how well fitted this insect is 

 both for life in the water and on the land. 



A more detailed account of the structure of this 

 larva is given in The Elements of Insect Anatomy, 

 by Comstock and Kellogg. 



The Near Relatives of the Dobson or Cory- 

 DALIS {Field Work). — In searching for the dobson the 

 pupil is apt to find certain other members of the 

 same family which live in similar situations. Some 

 of these resemble Corydalis very closely, except that 

 they are smaller when full grown and do not have 

 tufts of tracheal gills ; these belong to the genus 

 Chauliodes (Chau-li' o-des). A still smaller larva which 

 is similar in form and also lacks tracheal gills is Sialis 

 {Si'a-lis) ; this differs from both of the preceding in 

 having at the hind end of the body a long, tapering 

 appendage instead of a pair of prolegs. Place any 

 specimens that you may get of either of these in your 

 collection under the family Sialidce (see page 155). 



INSECTS THAT LIVE EXPOSED IN TORRENTS. 



There are many insects that find in rapids places 

 best fitted for their existence, but most of these are 

 more or less protected from the rush of the water by 

 the fact that they live beneath stones. Some insects, 

 however, scorn any protection, but live exposed 

 where the water flows the swiftest ; two of these are 

 mentioned below. 



The Black-flies {Field Work). — The larvas of the 

 black-flies should be sought in streams flowing down 

 steep descents. If present, they are easily found, for 

 they occur clustered together in large numbers, form- 

 ing a black coating over the rocks. They prefer the 



