352 INSECTA. 



tion in which the animal now swims, the back of the thorax, and not 

 the tail, being nearest to the surface, as represented in the drawing (D). 

 The necessity for 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 development, 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 locomotion 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 swim- 

 ming in the position in which the larva floats (fig. 177, c), the last 

 change could not by possibility be accomplished, as the bursting of the 

 integument would at once admit the water to the submerged 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 encased 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 

 wherein the gnat swims upon the top of its native pond ; and sustained 

 in this frail bark, formed by its late skin, it gradually extricates its legs 

 and wings from their coverings, and is kept perfectly dry until the 

 expansion of its instruments of flight enables it to soar into the air and 

 quit for ever the raft so singularly provided for its use. 



(916.) Having thus become acquainted with the various conditions 

 under which insects arrive at maturity, and the principal forms that 

 they exhibit during the different stages of the metamorphosis, the reader 

 will be prepared to investigate more minutely the changes in progress 

 during the process, and the gradual development of the organs which 

 successively make their appearance. On examining the viscera of a 

 Caterpillar, they are found scarcely at all to resemble those of the 

 Butterfly or Moth, into which a larva of this description is ultimately 

 matured. The jaws (fig. 180, b 6), widely different, both in structure 

 and office, from the proboscis which represents them in the perfect 

 insect (fig. 160), are strong and horny shears adapted to cut the leaves 

 of vegetables and other coarse materials used as food ; the oesophagus 

 (fig. 178, g h) is strong, muscular, and capacious ; and the stomach 

 (h i), in capacity corresponding with the extraordinary voracity exhi- 

 bited by the larva, passes insensibly into a wide intestine (I m), the 

 line of separation being only indicated by the entrance of the biliary 

 vessels (h) that wind in numerous convolutions around the posterior half 



