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 1s impossible to dis- 
tinguish these fine unbranching trachee 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 cesophagus, 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 cesophagus towards the 
pharynx. A posterior nerve (fig. 24, v.n.) runs from the 
ganglion along the dorsal side of the cesophagus 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 
