17 



ventral, while in vertebrate animals the nervous system is 

 dorsal and the circulatory one ventral. 



In insects, the nervous system is not concentrated' into 

 a large brain, but is divided among a series of nerve centres, 

 or ganglia, arranged in pairs often fused together. The 

 ventral line in Fig. 11 shows the position of these ganglia 

 and their connecting nerve cords, and Fig. 14 shows the 

 ventral nervous system, with its small nerve-branches. 



The double mass shown at the extreme end, and the 

 next portion connected by the heavy lines, together form 

 the brain. The former portion is above the oesophagus, or 

 gullet, and the latter below, and the heavy lines referred to 

 are nerve cords which connect the two parts, the gullet 

 running through the opening thus made. The first portion 

 of the nervous system is the supra- oesophageal ganglia, the 

 second the sub-oesophageal ganglia ; these are connected by 

 the oesophageal nerve collar. Next follow three thoracic 

 and seven abdominal ganglia, all connected by a central 

 nerve cord. 



The nervous system varies according to general struc- 

 ture, usuaily by fusion of ganglia ; but, with the exception of 

 the supra-oesophageal ganglia, it is always ventral in 

 position. Nerves extend to all the organs and muscles, and 

 permit the insect to respond to a stimulus of any sort. 



The nerve-endings in certain insect structures, such as 

 antennae, palpi, etc., are very delicate, and enable the 

 insect to be conscious of very slight stimuli. 



DIGESTION. 



i. 



The digestive system in insects, in its simplest form, 

 oonsitss of a cylindrical tube, extending in a direct course 

 from the mouth to the anus. In the more specialized insects, 

 the alimentary canal is very modified, being folded and 

 doubled on itself in such a way as to give it a length con- 

 siderably in excess of the total length of the body. The 

 organs which are concerned with feeding and digestion 

 occupy a very large proportionate part of the entire body- 

 cavity, as may be seen by reference to the drawing at 

 Fig. 11. Food taken in at the mouth passes by means 

 of the oesophagus, or gullet, to the stomach. Sometimes 

 a crop and gizzard are developed the crop for storing 

 a portion of food, the gizzard with strong muscles and 



