246 ANIMAL PHYSIOLOGY. [LESSON 74. 



generally smaller than the fourth ; and the fifth, in the mesothorax 

 (middle division of the chest), smaller than the sixth, contained in 

 the metathorax (last division of the chest). 



1114. The ganglia contained in the abdomen, like the segments 

 of that part of the body, are generally the least altered by develop- 

 ment from their primitive condition ; in the Coleopterous insects 

 (Beetles), however, the majority of them become obliterated in the 

 progress of development, and the like occurs in many of the Hemip- 

 tera (Tree bugs). The last pair of ganglia are generally the first 

 to advance forward and unite with the penultimate pair in the larva 

 state. 



1115. In the larva of a Lepidopterous insect (butterfly), the co- 

 lumns are lengthened, and the cephalic ganglia widely separated to 

 allow the oesophagus to pass between them. The first of these ganglia, 

 equivalent to the cerebrum of man, gives off the nerves for the sup- 

 ply of organs of special sense ; these amount in caterpillars to eight 

 pairs, in addition to the nerves which connect the upper with the 

 lower part of the brain ; the latter are called the crura. They like- 

 wise give off filaments to the small lateral ganglia (c) of the head, 

 and to the commencement of the sympathetic series of ganglia 

 (Fig. 370). 



1116. Between all the succeeding pairs of ganglia, a solitary 

 branch is seen coming through the loop formed by the division of the 

 chords in the upper part of the figure ; it divides into two branches, 

 one passing to the orifice of the spiracle (breathing mouth) on the 

 right side, and the other to the like organ, on the left. These 

 branches invariably come off above the ganglia. 



1117. Ninety years ago these structures were described by Ly- 

 onnet, as motor nerves, and the fact that their ramification was dis- 

 tributed to the muscles employee! in respiration, favored this opinion. 

 The opinions of Lyonnet were adopted and maintained by all suc- 

 ceeding authorities, but 



1118. In the Limulus polyphemus, or King Crab, found in con- 

 siderable numbers on the Jersey coast, the heart agrees with the 

 figures given of the heart in Insects, i. e., a straight vessel, from the 

 anterior chamber, terminating in three vessels, two of which are later- 

 al, and descend down to the nervous ring surrounding the oesopha- 

 gus, the middle one being lost in the head : thence a branch passes 

 along the dorsal surface of the ganglionic ventral chord, invariably 

 giving off branches in front of each ganglion, in fact the motor 

 and as some authors have designated it, respiratory tract of Insects. 



