LIFE-HISTORY OF SIMULIUM 259 



are employed exactly like those of a leech, and in a 

 few moments the larva creeps back to its first hold- 

 fast, even if it has to work against the stream. How 

 this is done can be understood by placing a white 

 disc under the leaves. Against this background a 

 web of dirty threads can be seen, and, as the larvae 

 drop, they spin a fresh line ; along this they climb 

 with their suckers. It is on these lines that their safety 

 largely depends, and with their huge spit -glands they 

 are constantly extending them as they move to fresh 

 quarters. 



These glands are also used for another purpose. 

 Before pupation the black-fly larva makes a conical 

 nest against a weed-stalk, and in this nest the pupal 

 stage is passed. The larva, head downstream, con- 

 structs its chrysalis case of the hardened secretion of 

 its glands, and then proceeds to undergo its trans- 

 formation. At first the nest is closed, and only 

 after the last larval skin is cast does the broad pro- 

 jecting end open, and then out there come the pupal 

 head and the thorax, with two sets of breathing fila- 

 ments, like those of the pupa of the harlequin fly. 

 The pupa, fixed within the nest by hooks along its 

 abdomen, is swayed with every movement of the weed. 

 In moorland streams the vegetation is often thickly 

 covered with these strange inanimate-looking tufted 

 excrescences, within which all the organs for aerial 

 life are developing or completed. 



How, from this submerged fixed pupa, the fly 

 could safely emerge was long a problem. Unlike 



