518 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxvi. 



and I see no reason why it should not apply to other regions of the body. 

 Sclerites originate from mechanical causes and do not necessarily have any 

 relation to the primary segmentation. 



LoAvne (1892) in discussing the prevalent notion of the dual struc- 

 ture of the thoracic segments states that he does not admit it proved, 

 and does not see that it helps in the understanding of the morphology 

 of the insect segment. 



The writer, then, Avishes to say emphatically that he discards every- 

 thing but plain statements of the facts in the description of the 

 thorax. Since, however, modifications of the same plan of thoracic 

 structure recur throughout the insect orders, this fact can be taken 

 as evidence that all the sclerites, especially those of the pleurum, 

 have not been produced independently in the different orders. 



Many writers have supposed that each thoracic segment consists 

 of two united segments. The arrangement of the plates on any 

 typical segment w^ould suggest this — the dividing line on the side 

 passing between the episternum and the epimerum, on the back be- 

 tween the scutum and scutellum, and on the venter between the 

 sternum and sternellum. Some authors have adduced further evi- 

 dence of the dual nature of the segment from the apparent division 

 of the coxa in some orders into an anterior and a posterior part. 



Banks (181);^), arguing from the coalescence of segments in the 

 Chilopoda, concluded that the thorax of insects is formed of five 

 segmentiH, the first, third, and fifth retaining the legs, the second and 

 fourth bearing the wings. He regards the coxie as double and cites 

 the meso- and metacoxal appendages of MdchUix as examples of 

 remnants of the ventral appendages of segments two and four. He 

 points out that in Scutujera (the highest chilopod) the small terga, 

 after the first segment, are united with the larger ones so that the 

 first segment bears only one pair of legs while the following bear two 

 pairs each. It is only a step from this to suppose that in MachUis 

 the second leg of each pair has become rudimentary, forming the 

 coxal appendages, while the first of each pair has persisted as the 

 functional walking a])pendage. Banks does not seem to regard the 

 cervical sclerites of insects as representing a segment in the thoracic 

 series. 



Patten (1890) gave other reasons for regarding each segment as 

 double, adduced from a study of the mouth parts and the nerves. 



Walton (1900) still further supports this theory by a study of the 

 coxae. He concludes that in both the Chilopoda and the Hexapoda 

 the coxa is composed of an anterior part, " coxa genuina," and a 

 posterior part, " coxa meron." These two coxal segments falling in 

 line with the episternum and epimerum, and the arrangement of the 

 thoracic muscles, form his basis for believing the entire segment a 

 compound of two primitive segments. 



