2)6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



coxa in the Protura is entirely with the sternum. It is probable that 

 the ventral trochantinal articulation found in many insects is a sec- 

 ondary development. 



GENERAL CONCLUSIONS 



Some of the more important conclusions resulting from this in- 

 vestigation may be enumerated as follows : 



1. The generalized type of an arachnid leg appears to possess one 

 more segment than the maximum number of eight allowed by Hansen 

 for the Crustacea, but this point needs further investigation. An 

 arachnid leg so constituted should have the following segments, 

 named from the base to tip : Subcoxa, coxa, coxal trochanter, femoral 

 trochanter, femur, patella, tibia, tarsus, and pretarsus. 



2. The generalized pauropod leg is composed of eight rings, which 

 represent, however, only six or possibly seven true segments, the first 

 three of which probably represent the coxa, first trochanter, and 

 second trochanter. The last four true segments of the pauropod leg 

 are the femur, tibia, tarsus, and pretarsus. 



3. The generalized symphylid leg is composed of seven true seg- 

 ments, the first being represented by a condyle-bearing plate. They 

 are: Subcoxa, coxa, a greatly enlarged trochanter, a much reduced 

 femur, a tibia, tarsus, and pretarsus. 



4. The generalized thysanuran leg is completely homologous with 

 the pauropod type, with the exception that it possesses a subcoxa, 

 usually in the form of a platelike structure. 



5. The typical collembolan leg possesses a subcoxal segment and 

 either lacks the tarsus entirely or has it represented by a short rudi- 

 ment at the base of the claws. 



6. The so-called coxal appendages in the Pauropoda, Symphyla, 

 and the Thysanura are not true appendages and have no muscle fibers 

 attaching to them. They are probably not homologous among them- 

 selves. Some may represent structures analogous, or possibly even 

 homologous, with either the epipods or the exopods of Crustacea. 



7. The primitive insectan type of tarsus was three-clawed. Two- 

 clawed or one-clawed types found in the Thysanura or the Collem- 

 bola are clearly not primitive but evidently were derived from the 

 three-clawed type as found today in the Pauropoda, Symphyla, and 

 certain Thysanura. 



8. Much evidence exists indicating the presence of an additional 

 segment to the insect thorax, which should probably be called the 

 cervical. It should be considered as being homologous with the legless 

 postcephalic segment of paviropods and certain symphylids. 



