293 



this change of posture, and consequent removal of the apparatus 

 for taking in air from one part of the body to another, will be at 

 once obvious when we consider the circumstances under which 

 the perfect insect, having completed its developement, emerges 

 from its pupa investments and enters upon an aerial existence. 

 The problem to be solved is, how shall the mature gnat escape 

 from the water without being wetted ? and, when we consider 

 that neither the larva nor the pupa possesses instruments of lo- 

 comotion capable of enabling it to leave its native element by 

 crawling on shore, the difficulties attending the change appear 

 almost insurmountable. It is evident that, while swimming in 

 the position in which the larva floats (.fig- 133, c), the last 



Fig. 133. 



change could not by possibility be accomplished, as the bursting 

 of the integument would at once admit the water to the sub- 

 merged gnat, and drown it at the moment of its birth ; but by 

 the new arrangement the metamorphosis is easily effected, and 

 that in a manner so beautiful, that it is hard to say which is 

 most admirable, the simplicity of the contrivance, or the perfection 

 with which the object is accomplished. No sooner has the en- 

 cased imago become fitted for its escape, than the pupa, rendered 

 more buoyant, raises its back above the surface : the protruded 

 portion of the pupa-case soon dries, and gradually begins to split 

 in a longitudinal direction, so as to form by its expansion a boat 



