OF THE CRANE FLY AND THE BLOW FLY. 143 



Mr. Lowne has described the discs of the Blow Fly as being 

 each enclosed in a capsule of structureless membrance, and E was 

 led at first to suppose that those of the Crane Fly were so like- 

 wise — indeed, I have so described them in my papers in " Science 

 Gossip." Subsequent examination, however, bas induced the 

 opinion that the supposed capsules are really nothing more than 

 infoldings of the newly forming pupa skin surrounding each disc ; 

 and that as the discs gradually acquire the form of pouches fitted 

 for tbe elaboration of their respective occupants, the sacs or cap- 

 sules thus surrounding them become continuous with them at the 

 point of insertion of the limb. This, of course, alters the whole 

 aspect in which these structures should be viewed. Instead of 

 being regarded as so many separate centres of growth, which by 

 their union form the bead and thorax of the fly, they must now be 

 looked upon as so many prolongations of the pupa integument 

 in an internal, and subsequently by invagination in an external 

 direction. In support of this the sacs, instead of being structure- 

 less, exhibit clear evidence of cellular origin, the same as the pupa 

 skin ; but further than this, I lately obtained a section of tbe leg 

 discs in situ in a plane perpendicular to the integument, which ex- 

 hibited the form and connection of the investing sacs. 



The antennal and optic discs take the form of flattened cellular 

 expansions, within which sooner or later the commencing areola- 

 tion of the facets of the eye and the articulations of the antennae 

 become discernible. The leg discs present at first spiral folds, and 

 subsequently develop into long wrinkled pouches coiled round 

 within the sacs ; those of the superior prothoracic appendages are 

 small and of an irregular shape, those of the wings and halteres 

 are broad and flat, of similar outline, but differing in size, the 

 latter being considerably the smaller. The similarity of outline 

 is singularly confirmatory of the view that regards tbe latter 

 organs as mere modifications of the former. 



An examination of the larva of the Blow Fly shews us a 

 creature presenting many points of resemblance to that I have just 

 described, and yet others of striking difference. The sixth 

 segment is here furnished with a pair of anterior spiracles, an 

 adjunct which is wholly wanting to the Crane Fly larva. The 

 nervous system is very peculiar, to use Mr. Lowne's words — "All 

 the nerve centres are concentrated in the anterior segments. They 

 consist of a pair of hemispherical ganglia above the oesophagus, 

 supra-oesophageal ; and a large flattened nerve centre, composed of 



