124 



AN INTRODUCTION TO ENTOMOLOGY 



The series of ganglia is really a double one, there being typically a 

 pair of ganglia in each segment of the body ; but each pair of ganglia 

 is more or less closely united on the middle line of the body, and 

 often appear to be a single ganglion. 



In some cases the ganglia of adjacent segments coalesce, thus 

 reducing the nimiber of distinct ganglia in the series. It has been 

 demonstrated that the brain is composed of the coalesced ganglia of 

 three of the head segments, and the suboesophageal ganglion of the 

 coalesced ganglia of the remaining four segments. 



Fig. 140. — Successive stages in the coalescence of thoracic and of abdominal 

 gangUa in Diptera; A, Chironomus; B, Empis; C, Tahanus; D, Sar- 

 cophaga (From Henneguy after Brandt). 



The three parts of the brain, each of which is composed of the pair 

 of ganglia of a head segment, are designated as the protocerebruni, the 

 deutocerebrum, and the tritocerebrum, respectively. The protocere- 

 brum innervates the compound eyes; the deutocerebrum, the 

 antennas; and the tritocerebrum, the labnmi. 



The subcesophageal ganglion is composed of four pairs of primary 

 ganglia ; these are the ganglia of the segments of which the mandibles, 

 the maxillulae, the maxilla, and the labium, respectively, are the 

 appendages. 



The three pairs of thoracic ganglia often coalesce so as to form a 

 single ganghonic mass; and usually in adult insects the number of 

 abdominal ganglia is reduced in a similar way. 



