22 



APPLIED ENTOMOLOGY 



Below or behind the oesophagus is a second ganglion, also in the head, called 

 from its position the suboesophageal ganglion. As the oesophagus lies directly 

 between this and the brain, the commissures connecting the two do not lie close 

 together, but separate far enough to permit the oesophagus to pass between them. 

 The suboesophageal ganglion besides being connected with the brain in front, 

 and the first thoracic ganglion behind it, by commissures, sends nerves to the 

 mouth parts and other nearby- regions of the head. 



FIG. 28. Diagram showing various degrees of concentration forward of four species of 

 flies. A, of Chironomus plumosus, little concentrated; B, Empis stercorea; C, Tabanus 

 bovinus; D, Sarcophaga carnaria, most concentrated. (After Brandt, from Lang's Lehrbuch.) 



The thoracic ganglia may be more or less separate or fused and may have fewer 

 or more of the abdominal ganglia added. Commissures, however, connect all 

 separate ganglia, and these also send out nerves to all the parts of the segments to 

 which they belong, no matter what their final location may be. In this way, the 

 wings, legs, muscles and other parts receive their nerve supply. A small "sympa- 

 thetic nervous system" also present, appears to be concerned chiefly with the 

 nerve supply of the alimentary canal and tracheae. 



Sense Organs. All the more evident senses possessed by man appear to be 

 present in insects, but not in all cases in the same individual. Thus some cave- 

 inhabiting insects have no eyes. It is at least probable that insects may have 

 other senses not possessed by man. 



Reproductive Organs. Insects are of distinct sexes, male and female. 

 In many cases, however, individuals occur, incapable of reproduction, 

 their sexual organs not having become fully developed, and such insects 

 may be termed neuters. Most of these appear to be really undeveloped 

 females, though undeveloped males are also known. They are found in 

 colonial insects where division of labor occurs, as in the honey bee, ants, 

 termites, etc., and are known according to their duties, as workers, soldiers, 



