48 



AN INTRODUCTION TO ENTOMOLOGY 



In addition to the maxillular ganglia, which have been almost universally 

 overlooked, and the existence of which has been denied bv some writers, the sub- 

 cesophageal ganglion is formed by the union of three pairs of primitive ganglia, 

 pertaining respectively to the mandibular, the maxillary-, and the labial segments 

 of the embryo. 



LIST OF THE SEGMENTS OF THE HEAD 



First, ocular, or protocerebral. 



Second, antennal, or deutocerebral. 



Third, second antennal, or tritocerebral. 



Fourth, mandibular. 



Fifth, maxillular. 



Sixth, maxillary. 



Seventh, labial, or second maxillary. 



III. THE THORAX 



a. THE SEGMENTS OF THE THORAX 



The prothorax, the mesothorax, and the metathorax. The thorax 

 is the second or intermediate region of the body ; it is the region that 

 in nymphs, naiads, an i adults" bears the organs of locomotion, the legs, 

 and the wings when they are present. This region is composed of 

 three of the body-segments more or less firmly joined together; the 

 segments are most readily distinguished by the 

 fact" that each bears a pair of legs. In winged 

 insects, the wings are borne by the second and 

 third segments. The first segment of the thorax, 

 the one next the head, is named the prothorax; 

 the second thoracic segment is the mesothorax', 

 and the third, the metathorax. 



f-~ The simplest form of the thorax in adult 



JK IL insects occurs in the Apterygota (the Thysanura 



1 7\ Fv and the Collembola) where although the seg- 

 ments differ in size and proportions, they are 

 distinct and quite similar (Fig. 60). 



In the Pterygota, or wirg3d insects, the 

 prothorax is either free or closely united to the 

 mesothorax ; in many cases it is greatly reduced in 

 size; it bears the first pair of legs. The meso- 

 thorax and the metathorax are more or less closely 

 united, forming a box, which bears the wings and 

 the second and third pairs of legs. This union of 

 these two segments is often so close that it is very difficult to distin- 

 guish their limits. Sometimes the matter is farther complicated by 

 a union with the thorax of a part or of the whole of the first 



Fig. 60. Lepisma 

 saccharina (After 

 Lubbock). 



