452 THE STUDY OF INSECTS. 



The females of -many species suck blood and are well- 

 known pests. Unlike mosquitoes and midges, the black-flies 

 like heat and strong light. They are often seen in large 

 numbers disporting themselves in the brightest sunshine. 



The larvae are aquatic ; and usually live in swiftly-flowing 

 streams, clinging to the surface of rocks in rapids or on the 

 brinks of falls. They sometimes occur in such large num- 

 bers as to form a moss-like coating over the rocks. There 

 is a disk-like sucker fringed with little hooks at the caudal 

 end of the body by means of which the larva clings to the 

 rocks; and just back of the head there is a fleshy proleg 

 which ends in a similar sucker fringed with hooks (Fig. 535). 

 By means of these two organs the larva is able 

 to walk with a looping gait similar to that of a 

 measuring-worm. It also has the power of 

 spinning silk from its mouth, which it uses in 

 locomotion. The hooks on the caudal sucker 

 and at the end of the proleg are well adapted 

 FIG . - Head to c ^ n gi n g to a thread or to a film of silk 

 of "larva. S p un upon the rock to which the larva is 

 clinging. Respiration is accomplished by means of three 

 much-branched tracheal gills which are pushed out from be- 

 tween the last two abdominal segments. The head bears 

 two large fan-shaped organs, which aid in procuring food. 

 The food consists of microscopic plants and bits of tissue of 

 larger plants. 



When full-grown the larva spins a boot-shaped cocoon 

 within which the pupa state is passed 

 (Fig- 53 6 )- This cocoon is firmly fast- 

 ened to the rock upon which the larva 

 has lived or to other cocoons, for they 

 occur in dense masses, forming a carpet- 

 like covering on the rocks. The pupa, 

 like the larva, breathes by tracheal gills ; 

 but in tin's stage the tracheal gills are FlG . S3 6.-Lar 

 borne by the prothorax. 



