144 A. HAMMOND ON A COMPARISON OF TIIE METAMORPHOSIS 



a series, probably of twelve pairs, of ganglia beneath the oeso- 

 phagus, sub-cesophageal." A system of muscular bands clothes 

 the inner surface of the integument as in the Crane Fly. Mr. 

 Lowne has described seven pairs of imaginal discs as follows, 

 viz., four pairs, the antennal and optic and the inferior pro- 

 thoracic and mesothoracic, all of which are attached through 

 the medium of short-nerve trunks to the nerve centres of the 

 larva ; and tbree otbers, viz., the superior mesothoracic and 

 the superior and inferior metathoracic, which are attached to the 

 lateral tracbeal vessels. It will be observed that the superior 

 prothoracic discs are not mentioned in this enumeration. It 

 is true that Mr. Lowne adverts to a horseshoe-shaped arch of 

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

 cesophageal ganglia and above the antennal discs, which he be- 

 lieves may be the disc in question. I have unfortunately not 

 been able as yet to recognise this structure with certainty ; but 

 from the foregoing description and the accompanying plate of Mr. 

 Lowne, I venture to think that this is not exactly the place where 

 I should have expected to find the disc. It appears to me that the 

 right rule which should guide our search for it should be to con- 

 sider what is the proper limb or appendage into the formation of 

 which it enters. And here, again, we are confronted by those pro- 

 cesses on the prothorax of the pupa to which I have already called 

 attention in the Crane Fly. Two button-shaped bosses appear in 

 this situation in the Blow Fly pupa, upon which the scars 

 of the anterior thoracic spiracles are seated, and in the larva I 

 have found attached to the terminations of the main tracheae 

 minute cellular structures, which I believe to be the objects for 

 which we are in search. One curious and apparently anomalous 

 circumstance I must not omit to mention here, viz., that while in 

 the Crane Fly the three pairs of inferior thoracic or leg discs 

 appear ranged on either side of the nerve centres, to which 

 they are attached by thick nervous cords ; in the Blow Fly, ac- 

 cording to Mr. Lowne's statement, the inferior metathoracic 

 pair, representing the posterior legs, is detached from this con- 

 nection, and ranged together with the superior discs, viz., those 

 of the wing, the haltere, and what I believe to be the superior 

 prothoracic pupal appendage, along the course of the main trachea?, 

 to which they are also seemingly affixed. I have not been able 

 entirely to verify this statement, but I believe it to be true, inas- 



