34 THE CLASS OF INSECTS. 



close approximation to them." Newport also believes that the 

 ganglionless upper, or internal, column of fibres is analogous 

 to the motor column of Vertebrata, while the external, or under 

 one, corresponds to the sensitive column, thus representing the 

 cerebro-spinal system of the Vertebrata. 



From each pair of ganglia are distributed special nerves to 

 the various organs. In the larva of Sphinx the normal num- 

 ber of double ganglia is thirteen, and the nervous cord of the 

 Neuroptera and other lowly organized and attenuated forms of 

 insects corresponds in the main to this number. In the adult 

 insect, especially in the Coleoptera, Diptera, Lepidoptera, and 

 Hymenoptera, the three thoracic ganglia are fused together, 

 following the fusion and general headwise development of the 

 segments of the tegument. Besides the central nervous cord, 

 corresponding to the spinal cord of the Vertebrates, there is a 

 vagus, or visceral nerve, representing the sympathetic nerve of 

 higher animals. This nerve "arises, in the larva, from the 

 anterior part of the cerebrum, and, forming a ganglion on the 

 upper sm'face of the pharynx, always passes backward beneath 

 the brain, along the middle line of the oesophagus." In its 

 microscopic structure the nervous cord, like that of Vertebrata, 

 consists of a central "gray" substance, and an outer or periph- 

 eral part, the "white" substance. 



In the embryo the ganglia are very large and close together, 

 the commissures, or connecting filaments being very short, and 

 small in proportion. 



Organs of Nutrition. These consist of the alimentary canal 

 and its appendages, or accessory glands (Fig. 44). We have 

 already treated of the external appendages (mouth-parts) 

 which prepare the food for digestion. The simplest form of 

 the alimentary canal is that of a straight tube. In the larva 

 of Stylops and the sedentary young of Bees, it ends in a blind 

 sac, as they live on liquid food and expel no solid excretions. 

 When well developed, as in the adult insect, it becomes a long 

 convoluted thick muscular tube, subdivided into different parts 

 which perform different functions and have distinct names, 

 taken from analogous organs in the vertebrate animals. This 

 digestive tube is composed of three coats, the outer, or j;er/- 



