NO. I INSECT THORAX — SNODGRASS 83 



ventral surface is a large basal plate, the imgnitractor (B, Utr), 

 which is partially invaginated into the end of the tarsus {Tar), 

 where the flexor, or retractor, " tendon " of the claws {x) is 

 attached to its proximal end. The unguitractor may consist of two 

 pieces (C, Utr), or sometimes there is a second plate, the planta, 

 distal to the unguitractor at the base of the arolium, and there may 

 be lateral plates, the auxilicE (/), at the bases of the claws. In the 

 honeybee, a transverse chitinous band lies beyond the planta in the 

 ventral wall of the arolium. 



All parts of the pretarsus are subject to much variation. The claws 

 sometimes have each two points ; sometimes the claws are of unequal 

 size, one claw becoming reduced and occasionally obliterated, the 

 result being a one-clawed foot. Again, both claws become very small, 

 and both may be lacking. In the Physopoda, the claws are minute, 

 and the foot consists principally of the large bladder-like arolium. 

 In some of the Thysanura there is a third, median claw between the 

 two lateral claws, well developed though small in Lcpisma (fig. 44, 

 C, D, Dae) rudimentary in Japyx (fig. 44 A, p). This median claw 

 is probably not analogous to the lateral claws, being more likely a 

 remnant of the primitive dactylopodite. The pretarsus of first-stage 

 larvae of Meloid beetles, the triungulins, also apparently has three 

 claws, a large median one and two slender lateral ones ; but there is 

 some doubt as to the nature of the lateral claws of the triungulin 

 foot, Boving (1924) pointing out that they are claw-like setae rather 

 than true claws. In a lampyrid larva with three claws on each foot, 

 the lateral claws appear to be outgrowths of the dactylopodite, since 

 they have no articulations with the tarsus. 



The arolium {Ar) varies in size and in form from a small simple 

 lobe to a large complex appendage ; or again it may be rudimentary 

 or entirely lacking (fig. 36 C). In the Diptera there are two lateral 

 ventral foot lobes, the pulviUi (fig. 36 D, E, Pv), which arise from 

 the auxilite (/), one beneath the base of each claw. The arolium is 

 rudimentary or absent in most Diptera ; only in the Tii)uli(ke, accord- 

 ing to de Meijere, is it well developed, in other families a median 

 process, the cinpodiitiii {Bnip), or " ]irocessus plantaris " of de 

 Meijere, is commonly developed from the distal end of the ungui- 

 tractor ])late. The empodiuni may be spinelike (fig. 36, U, E), or 

 lobelike and similar to the lateral pulvilli. 



MUSCLES AND MECHANISM OF THE LEG 



Though the legs of dififerent insects are adapted in their structure 

 to a great variety of uses, their motions are made according to the 



