STRUCTUREjDEVELOPMENTjAND BIONOMICS OF HOUSE-FLY. 417 



united by a single median ventral cord running from the 

 suboesopliageal ganglia to the anterior end o£ the thoracic 

 nerve-centre. 



The cephalic ganglion consists of the supracesophageal 

 ganglion and the suboesophageal ganglia so closely united 

 that the commissural character of the circumoesophageal con- 

 nectives is quite lost. Externally, on the dorsal side of the 

 brain three longitudinal fissures can be seen, a median fissure 

 aud two lateral fissures marking* the origin of the optic lobes. 



The supracesophageal ganglia. The characters of the ganglia 

 composing the brain are hidden by the sheath of cortical 

 cells which fills up the spaces between tlie ganglia, the 

 characters of these can be ascertained by the serial sections. 

 The median mass the procerebrum is formed by the fusion of 

 the procerebral lobes. These ai-e united before and behind, 

 and enclose a central ganglionic mass — the central body. 

 Behind the procerebrum two pairs of fungiform bodies arise. 

 On the anterior face of the procerebrum the antennal or olfac- 

 tory lobes which represent the deutocerebrum are situated 

 laterally. Each sends a nerve (figs. I and 11, an.n.) to the an- 

 tenna. Above these and on the dorsal side are a pair of lobes 

 — the frontal lobes contiguous with each other in the median 

 line — these belong morphologically to the tritocerebrum. 

 Posterior to these in the median dorsal line of the cerebrum 

 a single median nerve, the ocellar nerve (figs. 1 and 11, 

 oc.n.), arises ; this runs vertically to the ocelli. A pair of 

 lobes which correspond to Lowne's thalami of the blowfly are 

 situated external to and between the frontal and antennal 

 lobes. The peduncles of the optic lobes have their origins 

 from the sides of the procerebrum. Each optic peduncle (fig. 

 11, O.P.) contains three ganglionic masses which Hickson 

 (1885) has termed from the brain peripherally the opticon, 

 epiopticon, and periopticon (fig. 1, P.O.) respectively. 



The suboesophageal ganglia (fig. 1, 8.0.) . The commissures 

 uniting the supracesophageal ganglia to the oesophageal 

 mass cannot be recognised as such, owing to the extreme 

 state of cephalisation of the cephalic ganglia. They are 



