SOME ANNOYING PESTS OF MAN 



343 



branched tracheal gills pushed out between the last two 

 segments of the abdomen. The length of the larval stage 

 is about four weeks during the summer months. This 

 stage is much lengthened by cool weather. In fact, the 

 insect passes the winter in the larval stage. 



In order to live and flourish, the larvae of black-flies 

 must have fresh, flowing water full of tiny plant and animal 

 life. As soon as the larvae 

 are taken out of the water 

 or even placed in quiet 

 standing water, they die. 



When the larva completes 

 its growth, it spins a boot- 

 shaped cocoon which is 

 securely attached to the 

 rocks by the sole but is open 

 at the top. Within the 

 cocoon the larva changes to 

 a pupa which must also live 

 in fresh, swiftly moving 

 water. The pupa breathes 

 by means of two tufts of 

 respiratory filaments borne 

 on the thorax. Each tuft 

 consists of nine filaments, 

 one of which is slightly 

 shorter than any of the other eight (Fig. 118). The pupal 

 stage lasts about three weeks, at the end of which period 

 the adult fly emerges and quickly rises to the surface 

 inclosed in a bubble of air. There seem to be two or 

 three generations during a season at Ithaca, New York. 



The life histories of other black-flies, at least what we 



FIG. 118. Pupa of a black-fly 

 (5. pictipea). (X 16.) 



