THE FORMATION OP THE PUPA. 117 



cellular expansions, called by Dr. Weismann imaginal discs, 

 and partly from cells formed upon the inner surface of the 

 larval integument. When mature, it is an extremely delicate, 

 apparently structureless transparent membrane, presenting no 

 trace of its cellular origin, and, after the escape of the imago, 

 remains as a fine silvery membrane in the interior of the pupa 

 case. 



The imaginal discs have received the following names from Dr. Weis- 

 mann, which I have retained, and simply translated into English, the 

 antennal and optic discs* which enter into the formation of the head, and 

 the inferior pro-thoracic and meso-thoracic discs f which are chiefly con- 

 cerned in forming the anterior and intermediate limbs. These are all 

 attached firmly through the medium of short nerve trunks to the nerve 

 centres of the larva. (Plate VI., fig. 1 ) 



The superior meso-thoracic disc, J and the superior and inf erior meta- 

 thoracic || discs are firmly attached to the lateral tracheal vessel near the 

 anterior spiracle. The superior discs are chiefly concerned in forming the 

 wings and halteres, and the inferior in the formation of the posterior limbs. 

 (Plate VI., fig. 2) . Beside these a horse-shoe shaped arch of similar 

 tissue is attached to a pair of tracheal vessels, in front of the supra- 

 cesophageal ganglia, and above the antennal discs. It is called the ring 

 by Dr. Weismann, but I believe it is the upper pro-thoracic disc. 



The abdominal portions of the pupa skin are formed from cells developed 

 upon the inner surface of the corresponding larval segments. 



Each imaginal disc is enclosed in a capsule of structureless 

 membrane. The capsules of those attached to the nerve centres 

 appear to consist of the neurilemma of the nerves which sup- 

 port them, but the capsules of those attached to the trachea 

 seem to be derived from the outer coat of the tracheal vessels. 

 The discs are formed of minute cells, having a granular appear- 



* Autennen and Augensheiben. 



t Unter prothoral scheibe,and meso-thoral sheibe. 



J Ober meso-thoral sheibe. 



II Ober and unter meta-thoral eheiben. 



