(82) 



522 C. GORDON HEWITT. 



mistook them for such, even when dissecting with a magnifi- 

 cation of sixty-five diameters, until my serial sections showed 

 their real nature. Without sections it is impossible to dis- 

 tinguish these fine unbranching tracheae from accessory 

 nerves. I have mentioned this fact as showing the necessity 

 of supplementing the one method by the other. 



The visceral or stomatogastric nervous system 

 (PL 31, fig. 20) consists of a small central ganglion (c. g.) lying 

 on the dorsal side of the oesophagus, immediately behind the 

 transverse commissure of the cerebral lobes from the bases of 

 which two fine nerves are given off to join a fine nerve from 

 the ganglion, which runs dorsally towards the anterior end of 

 the dorsal vessel. A fine nerve from the ganglion runs 

 forward on the dorsal side of the oesophagus towards the 

 pharynx. A posterior nerve (fig. 24, v.n.) runs from the 

 ganglion along the dorsal side of the oesophagus to the neck 

 of the proventriculus, where it forms a small posterior 

 ganglion (fig. 20, pv. g.), from which fine nerve-fibres arise 

 and run over the anterior end of the proventriculus. 



Sensory organs. The only sensory organs which the 

 larva possesses are the two pairs of conical tubercles (fig. 9, 

 o. t.), which have been described already on the oral lobes. 

 In section each consists of an external transparent sheath of 

 the outer cuticular layer ; beneath this and surrounded by a 

 chitinous ring are the distal cuticularised extremities of a 

 number of elongate fusiform cells grouped together to form 

 a bulb. These are nerve-end cells and their proximal extre- 

 mities are continuous with nerve- fibres by means of which they 

 are connected to the ganglion. Both sensory organs of each 

 oral lobe are supplied by the same nerve from the second of 

 the two anterior nerves. Judging from their structure these 

 organs appear to be of an optical nature, and this is the usual 

 view which is held with regard to their function. They 

 would appear merely to distinguish light and darkness, which, 

 for such cryptophagous larva, is no doubt all that is necessary. 

 The negative heliotropism of the larva of the blow-fly has 

 been experimentally proved by Loeb (1890), and my own 



