LESSON 74.] THE NERVOUS SYSTEM IN INSECTS. 245 



and in the locomotive organs appended thereto. Most Insects, how- 

 ever, commence their career as worms ; the high form which they 

 are ultimately destined to attain in the articulate series, is at first 

 marked by the guise of a red-blooded worm. 



1111. The larvae, which present larger and more perfect seg- 

 ments, most of which are provided with legs or prolegs, have a gan- 

 glionic centre for each segment, and intermediate chords. 



1112. This worm-like type of the nervous system can be seen in 

 any caterpillar, and an illustration is given (Fig. 370) of the nervous 

 system of the larva of the Goat Moth, Oossus ligniperda immor- 

 talized by the wonderful dissections of Lyonnet, and the engravings, 

 by his own hand, which gave effect to his labors as an Entomotist 

 (one who dissects insects). 



The abdominal nervous columns of insects have been regarded 

 by Lyonnet, Straus, Dufour, and Chiaje, no less than by modern au- 

 thorities, as analogous to the brain and spinal chord of vertebrated 

 animals. There are at first thirteen pairs of approximated ganglia, 

 corresponding with the original segments, and extending along the 

 middle of the ventral surface of the body, and the oesophagus passes 

 downwards, perforating the connecting nervous columns between the 

 first and second pairs of ganglia ; in other words, the oesophagus in 

 insects passes through the centre of the divided brain ; the first 

 pair of ganglia, therefore, are swpra-cesophageal (above the oesopha- 

 gus), or cephalic, and all the succeeding ganglia of the columns are 

 beneath the alimentary canal. 



1113. The ganglia, at first like those of the worm and centipede, 

 are nearly at equal distances, and of equal size, like the segments 

 themselves of the young caterpillar. The columns and ganglia, orig- 

 inally separate (see Fig. 366, of the nervous system of the Sand- 

 hopper, which, although a lowly crustacean, illustrates this point,) on 

 the two sides, early approximate transversely, and unite, and a slow 

 movement of the ganglionic matter is at length observed in a longi- 

 tudinal direction. These transverse and longitudinal movements 

 of the nervous matter proceed to a very variable extent, according 

 to the degree of metamorphosis from the larva condition, to which 

 the individual is subjected in the various adult forms of this class. 

 In insects, as in the Myriopods, the first and second pairs of ganglia 

 are contained within the head, and constitute, together, the brain ; 

 the succeeding pairs are generally placed near the anterior limits of 

 the segments to which they belong. The third pair of ganglia 

 placed in the prothorax (first division of the chest) appear to be 



