THE NATURE OF METAMORPHOSIS. 63 



become detached, and are gradually developed backwards, and 

 encroach upon the anterior portion of the second segment. This, 

 in accordance with the laws of development as established by 

 Geoffroy St. Hilaire, that in proportion as one part of an organised 

 body is increased beyond its ordinary size, the part or parts in its 

 immediate vicinity are in a corresponding degree arrested in their 

 development, becomes so much reduced, that in the nymph the 

 second segment, which in the larva is of the same size as the third 

 and succeeding ones, has not half its original extent, and being still 

 further reduced in that state, constitutes at length the atrophied 

 and almost obliterated prothorax of the perfect insect. But while 

 the second segment is thus encroached upon by the first, it is in 

 like manner encroached upon from behind by the third — the 

 immense mesothorax, which supports the chief organs of flight in 

 the perfect insect. The fourth segment, from the same cause, is 

 developed backwards ; and the fifth, diminished to a very small 

 size, exists only, as in the Sphinx, as the petiole which connects the 

 thorax with the abdomen, thus leaving the nine posterior segments 

 of the larva to the latter region, as stated when alluding more 

 particularly to the number of segments in hymenopterous larvze. 

 The necessity for this additional segment in the abdomen of these 

 larvae is a matter of much interest, and appears to be connected 

 Avith the development of an apparently additional organ in the 

 females of this class. 



" We have seen that after leaving the larva, or feeding condi- 

 tion, the insect assumes one of a very different form, which is called 

 the pupa, nymph, aurelia, or chrysalis state. The two latter terms 

 were applied by the older entomologists to this stage of transfor- 

 mation in butterflies and moths. The term * aurelia ' was used as 

 expressive of the beautiful golden colours or spots with which many 

 species are adorned, as Vanessa nrticcE, Vanessa Atalanta, and 

 others. The term 'chrysalis' had a similar signification. Linnaeus, 

 desirous of employing a term that would be applicable to this stage 

 of transformation in all insects, adopted that of ' pupa,' because in 

 a large majority of the class the insect is as it were swathed or 

 bound up, just as children were formerly swathed. The kind of 

 pupa, in which the future limbs are seen on the outside of the 

 case, is called ' obtectcd.' Such chrysalides are represented in 



