NO. II LEGS OF PRIMITIVE ARTIIROPODS^ — EWING 21 



The great thrusting power of the furcula has l)een matched to a 

 certain degree by the superior deveU)pnient of the posterior legs and 

 their backwardly appressed position. This pecuharity of the legs is 

 probably an adaptation to receive the shock of alighting after each 

 spring into the air. 



According to the external sutures collembolan legs are six-, or 

 seven-segmented. The seven-segmented legs are particularly evident 

 in the family Entomobryidae and such legs have been figured by 

 Carpenter (1916) and by Folsom (1924). The more common type, 

 here regarded as six-segmented, has been considered as five-segmented. 

 Such a leg according to Guthrie (1903) is composed of a coxa, tro- 

 chanter, femur, tibia and tarsus. Borner (1921) has figured this 

 type of a leg in Orche sella. 



The seven-segmented leg will be taken here as a generalized type 

 for the order, and the third leg of Tomocerus arcticus will Idc de- 

 scribed. According to external sutures there are five short basal 

 segments all of about the same length. The most proximal is the 

 stoutest, and the most distal the longest. The sixth segment is long 

 and straight, the seventh is of similar length but tapers distally to the 

 base of the two unequal claws which it bears directly on the end of 

 the segment. An examination of the muscles (pi. 6, fig. 16) shows 

 that this seventh segment bears powerful strands going to the base 

 of the claws. It, therefore, should be regarded as the tibia. The 

 sixth segment becomes the femur, and has the femur type of muscu- 

 lature. Segments four and five, each very short and each with a 

 dicondylic hinge at the base and apex, should be regarded as the two 

 trochanters. They both have the articulations and musculature of a 

 primitive type. There remain the proximal three segments. Muscle 

 attachments show that the first external suture produces only a false 

 segmentation, hence one and two together form the basal segment 

 which should be regarded, mostly on account of its position, as the 

 subcoxa. There remains the third ring, or segment. It is very short, 

 has a dicondylic articulation with the first trochanter and an oblique, 

 incomplete apodeme for the attachment of powerful muscle fibers 

 of the depressor of the second coxa. Although this segment is far 

 from being typical of an insect coxa, it should be regarded as such 

 because of its dicondylic articulation and its position in the segmental 

 series. 



The peculiarities of such a leg are evident ; a falsely divided subcoxa, 

 a reduced and modified coxa and a missing tarsus. 



In most collembolan legs, particularly in the six-segmented legs 

 (pi. 6. fig. 17), there is a small but complete ring inserted between 



