THE CANADIAN ENTOMOLOGIST 423 



can subsist without water, provided the condition is not too dry. 

 In the laboratory I have some fully grown larvae which had made 

 their cases with their secretion and plant materials on the wall of 

 glass containers when the water was present. Upon removal of 

 the water they continue to live in their cases for almost a month 

 until entirely dry. 



Pupa, — Upon pupation the larval skin splits along the mid- 

 dorsal line of the thorax, a part of the pupal thorax will emerge 

 first, and the horn-shaped processes on the top of the head, the 

 compound eyes, a part of the antennae, and the tracheal filaments 

 are now outside the skin. As the development advances, the skin 

 splits further along the dorsal surface, a part of the pupa's body 

 emerges, the compound eyes soon appear in a perfect shape, the 

 tracheal filaments longer, and the legs of the imago have their 

 claws, hairs and segments all visible through the transparent 

 pupal sheath (Fig*. 4). 



Finally matured, its tracheal filaments are very bushy and 

 white, and its antennte brownish black. The abdomen is con- 

 tracted within the pupal skin, leaving the last two segments empty. 

 The genitalia of the adult are visible through the transparent 

 skin. The mature pupa measures about 7 mm, in length and now 

 becomes very active, swimming tadpole-like under the surface of 

 water and moving its depressed abdomen very rapidly back and. 

 forth. The thorax closely touches the surface film with the res- 

 piratory filaments slightly indenting it. The anal appendages 

 are much flattened and have matted hairs well developed for 

 locomotion. While floating against the surface film it resembles 

 in habit the mosquito wriggler. Sometimes it lies stationary be- 

 neath the surface. 



Emergence of Adult. — The pupal skin splits longitudinally 

 down the dorsum of the thorax. The head of the adult is pulled 

 backwards and upward, and the mouth-parts, palpi and antennae 

 gradually emerge. The base of the wings and the legs soon ap- 

 pear. When the front and middle pairs of legs are freed, or almost 

 so, the fly, in trying to raise its body, has its wings immediately 

 straightened out. At this time a greater portion of the posterior 

 legs and the entire abdomen are still retained within the pupal 

 covering. The posterior extremity of the abdomen is always 



