142 A. HAMMOND ON A COMPARISON OF THE METAMORrHOSIS 



each of them, being, I believe, a continuation inwards of the 

 exterior integument, and both processes and tracheas are cast off 

 with the pupa skin on the emergence of the imago. Although, 

 therefore, an examination of the perfect insect would fail to supply 

 a raison d'etre for the existence of superior prothoracic discs, the 

 hiatus is well filled by a consideration of the intermediate pupal 

 condition ; and, in fact, I have both found these discs, and traced 

 their development into the appendages in question. A statement, 

 therefore, of the limbs of the imago, into the formation of which 

 the several thoracic discs enter, will, I should say, run thus : — Of 

 the prothorax, the upper pair of discs have their development 

 arrested — proceeding no further than the pupa stage, the lower 

 pair forming the anterior legs. Of the mesothorax, the upper pair 

 form the wings, and the lower the intermediate legs. Of the me- 

 tathorax, the upper pair form the halteres, and the lower the pos- 

 terior legs. Of these discs, the antennal and optic, and the three 

 inferior thoracic pairs are certainly connected by prominent 

 nervous cords with the brain and thoracic ganglia, the others — 

 viz., the superior thoracic — are, I think, probably so, though I have 

 not as yet traced the connection. As the nervous system partici- 

 pates but little in the otherwise general law of change, I believe 

 that these connecting nerves are identical with those which subse- 

 quently form the proper nerves of the limbs of the imago, and, if 

 so, their unions with the discs is much more intimate than that 

 which, if I rightly apprehend Mr. Loivne's meaning, occurs in the 

 Blow Fly, for he speaks repeatedly of the discs as being firmly 

 attached to the nerves and trachea? of the larva, as if the connec- 

 tion were merely one of attachment or support. In addition to 

 their connection with the nerves, the discs also appear to have 

 branches of the trachea? distributed to them, but here again I 

 regard the connection as a functional one, involving the necessary 

 supply of air to these parts. As far as my means of observation 

 will carry me, I cannot see any difference between the distribution 

 of some of the tracheal branches to these structures and that of 

 others to the various tissues over which they spread. 



The optic and antennal discs are placed on either side of the 

 oesophagus, between it and the large muscles of the mandibles ; 

 while the thoracic discs are placed immediately beneath the 

 integument, between it and the subcutaneous muscular coat, the 

 inferior ones closely adjoining the ventral cord, and the superior 

 further removed towards the dorsal surface. 



