NO. I INSECT THORAX — SNODGRASS 8l 



it consists of a single leg segment (figs. 16 A, 32 A, 41 B, Tar), as 

 it does in adult Protura (fig. 41 A), which segment is probably the 

 propodite of the generalized arthropod limb (fig. 42). The subseg- 

 ments, or articles, of the adult tarsus, conveniently called tarsal 

 " segments," therefore a])pear to be subdivisions of a single primitive 

 shaft ; they have no articular hinges with each other, though they 

 are usually freely movable by inflected connecting membranes (fig. 

 39), and they never have individual muscles. The tarsus is moved 

 as a whole by muscles inserted upon its base, or by tension of the 

 claw muscles on the tendon which traverses it (fig. 39, x). Tarsi 

 having fewer than five segments, therefore, represent either a stage 

 of progress in the division of the primitive segment, or a retrogressive 

 condition in which some of the articles of a five-segmented tarsus 

 have been lost or have coalesced. The tarsi of the Apterygota may 

 be supposed to be of the first class, as may likewise those of the 

 Odonata with three segments, but in the rest of Pterygota, the adult 

 tarsus appears to have been standardized with five segments, and 

 all reductions from this number are most likely of a secondary nature. 



The basal segment of the tarsus is often larger than the others, 

 and is distinguished as the basitarsus, metatarsus, or planta, the first 

 term being preferable. On the under surfaces of the segments, ex- 

 cept the last, there are sometimes small pads, the euplantitlcc (Cramp- 

 ton, 1923), or tarsal pulvilli (fig. 31 A, q). 



The prctarsus. — Entomologists generally have not found it neces- 

 sary to refer collectively to the terminal parts of the insect leg (fig. 

 31 A, Ar, Cla), and consequently we have no satisfactory name for 

 the group of organs at the end of the tarsus, which in some cases 

 might appropriately be termed the " foot," but not with those insects 

 that place a part or all of the tarsus on the supporting surface. The 

 group of terminal foot structures is called the unguis or tingula by 

 Schiodte, the prcrtarsus by de Meijere, the articiilaris by MacGilHv- 

 ray, and the Krallcnglicd by Arnhart. Since de Meijere (1901) has 

 given the most comprehensive description of the parts in question, 

 his name for them, prccfarsus, is adopted in the Americanized form 

 of " pretarsus " in the present paper, though not without regret that 

 de Meijere did not invent a more fitting term. 



In its simplest form the terminal part of the insect leg consists 

 of a small, claw-like segment similar to the dactylopodite of a 

 crustacean or chilopod limb (fig. 32 B, Dae). A pretarsus of this 

 kind occurs in adult Protura (fig. 41 A), in some Collembola, in the 

 larvae of mos^ beetles (fig. 43), and in the larvae of Lepidoptera 

 (figs. 32 A, 41 B) and Tenthredinidse (fig. 16 B). A one-clawed 



