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tral diaphragm. This is a transparent, apparently structure- 

 less membrane, stretched over the floor of the abdominal 

 cavity in such a way as to protect the central part of the 

 nervous system. Owing to the transparency of this mem- 

 brane, it is very difficult to see it unless it is stained. The 

 ventral diaphragm is attached along each side of the body 

 just laterad of the great ventral muscles ; the points of at- 

 tachment are on the lines separating the segments of the 

 body. Between the points of attachment, the margins of 

 the membrane curve mesad, giving it the appearance of be- 

 ing strongly stretched at the points of attachment. 



This diaphragm has been described as a ventral heart. 

 See Packard, Text Book of Entomology^ p. 403. We believe its 

 function is to protect the central nervous system from the 

 peristaltic movements of the alimentary canal. 



The nervous system. — After removing the alimentary 

 canal from the specimen opened along the back, the central 

 nervous system will be exposed. 



Make a diagram showing the disposition of the ganglia 

 and of the principal nerves of the thorax and abdomen. 

 These can be seen better in a specimen which has been 

 opened along the venter. Take for this purpose either a 

 fresh specimen or one that has been slit on the dorsal side, 

 and open it carefully on the ventral side so as to cut through 

 only the body-wall, leaving the nervous system in place. 



Make careful dissections of the ganglia and nerves found 

 in the head, and make a diagram showing their arrangement. 



The following parts should be observed and figured : — 



The supraoesophageal ganglia. — These are two, large, ovoid 

 ganglia, lying above the oesophagus, and connected by a 

 short, thick commissure. They are sometimes termed the 

 brain. 



The antennal nerves. — These arise from the latero-cephalic 

 angles of the supraoesophageal ganglia. 



