mSECTA. 



351 



142 



and some along glass and the ceilings of rooms, against gravity : 

 the house-flj achieves this hy virtue of the development of discs 

 upon the under surface of certain expanded joints of the tarsus, 

 which some have supposed to act as suckers, and others contend 

 to be pads exuding an adhesive mucus from the hair follicles of the 

 cushions ; which accordingly require the act of frequent friction 

 or cleansing, as may be seen when a fly rubs its feet against one 

 another. 



The muscular system is, as may be supposed, developed in relation 

 to the several kinds and powers of locomotion indicated by the modi- 

 fications of the extremities. As a necessary corollary of the cylin- 

 drical form and external position of the principal parts of the ske- 

 leton, the joints are for the most part ginglymoid, and restricted to 

 movements in one plane : the muscles of those segments of the limb 

 are consequently simply flexors and extensors. The coxa has three 

 flexors and one extensor ; the trochanter has three extensors (^fig. 

 142, a, 6, c), one flexor (<f), 

 and one abductor (c). The 

 femur has one long penniform 

 flexor (^) and a similar ex- 

 tensor (y). The tibia has one 

 flexor (t), inserted into a long 

 slender process like a tendon, 

 which traverses the joints of 

 the tarsus to be attached to 

 the inferior margin of the claw: 

 the extensor (A) occupies the 

 inferior portion of the tibia : 

 its tendonlike attachment is 

 fixed to the upper margin of 

 the claw. Besides these mus- 

 cles, which are common to the 

 joints of the tarsus, there are 

 two others belonging to the 

 trifid claw of the last joint : 

 the extensor (»i) is short, the 

 flexor (n) is a longer penni- 

 form muscle, and is attached, 



like the long flexor of the MusdesoflegofaChafferCJ/fto/oiitta). 



tarsus, to the inferior part of the claw-joint. The coxjb have a round 

 head inserted into a cup of the mesosternal arc, and the movements 

 of the hip-joint are rotatory ; the head is usually connected with the 

 thorax by a similar joint, which, from the greater freedom of the 



