2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



PAGE 



The sternal plates of the leg-bearing segments 30 



Sterna in Arachnida 30 



Sterna in Pauropoda 31 



Sterna in Symphyla 31 



Thoracic sterna in Protura 32 



Thoracic sterna in Thysannra and Collembola 32 



Pleurites in leg-bearing segments 34 



The origin of insect pleurites 35 



General conclusions 36 



References to literature Z7 



Abbreviations used on plates 39 



Explanation of plates 40 



INTRODUCTION 



In attempting to work out the homologies of leg segments in a num- 

 ber of the different groups of arthropods, an effort has been made 

 to find the generalized type for such classes as the Crustacea, the 

 Arachnida, and the Insecta, and by comparing intergrades with these 

 established types to interpret the homologies as far as possible for 

 each group. In such a comparison the position of the segment in the 

 series is determined, but the procedure of prime importance is the 

 definite establishment of a known segment in the series which may 

 be used as a point of reference. Different methods have been em- 

 ployed for the establishment of a " land mark " segment or, what 

 amounts to the same, a "land mark" articulation. Borner (1921) 

 lays great stress on the bend of the leg at the knee, usually regarding 

 such a bend as taking place at the distal articulation of the femur. 

 Snodgrass (1927) uses chiefly the trochantero- femoral articulation in 

 insects as a starting point. 



In the Arachnida, where legs appear to show the maximum number 

 of true segments, eight in addition to the claws, it is assumed that all 

 segments are present. Such being the case one may begin at the base 

 of the leg in legs of this sort by calling the first segment the subcoxa. 

 Where there is a reduction of the number of leg segments, it may 

 yet be possible to establish the homology of the remaining segments 

 if the method of reduction can be established. In insect legs it appears, 

 according to the results of recent investigation, that the tarsus never 

 gives rise to muscle fibers, the extensor claw muscle having been lost, 

 and the flexor taking its origin from the tibia or" from the tibia and 

 other segments proximal to it. Using this as an established principle 

 the writer has in many instances regarded the last segment giving 

 rise to muscle fibers as the tibia in the case of insects and also in 



