342 NATURAL HISTORY OF AQUATIC INSECTS CH. 



upwards. The two hooks with which each leg is 

 provided secure the body in its place. 



" When the larva has thus fixed itself, internal 

 changes take place. The first visible effect is that 

 the skin cracks along the back of the thorax, and the 

 thorax of the fly becomes visible through the cleft. 

 The part which first appears swells, and, like a wedge, 

 helps to enlarge the cleft, which extends to the fore- 

 edge of the thorax, then along the neck, and finally 

 to the level of the eyes, whence lateral clefts proceed 

 on either side. At this time the fly is able to dilate 

 its head like a Blue-bottle fly under like circum- 

 stances. In proportion as the cleft becomes pro- 

 longed, more and more of the body of the fly is 

 exposed. At length the head is completely with- 

 drawn, and appears so large, that one has difficulty 

 in believing that it could immediately before have 

 been enclosed within the larval head. "When the 

 head and thorax are free, the legs are drawn out of 

 their sheaths, and to disengage these more readily, 

 the fly arches itself backwards. At this time there 

 can be seen on each side two white strands, which 

 remain attached to the thoracic part of the larval 

 skin. These are [the linings of] the four great 

 tracheal trunks, and are drawn out from the four 

 thoracic spiracles. After this the fly throws her head 

 further and further back, until at length the body is 

 supported only by the hindermost segments, which 

 still remain enclosed in the larval skin. 



" At this time the legs of the fly are completely 

 free and distant from their sheaths. For two or 

 *;hree minutes they are moved about, as if to try their 



