THE THORACIC EXO-SKELETON. 



159 



If we compare the thoracic leg of an insect with that of a 

 crayfish, we recognise in the five joints of the insect's tarsus 

 the representatives of the five joints of the crustacean limb 

 known as the basipodite, ischiopodite, meropodite, carpopodite, 

 and propodite, whilst the claw-like dactylopodite is represented 

 in insects by two tarsal claws. In the crustacean the basal 

 coxopodite corresponds with the coxa of the insect, whilst the 

 exopodite of the generalised Crustacea corresponds with the 

 femoro-tibial portion of the limb in the insect. 



The bladder-like femoro-tibial rudiment closely resembles 



FIG. 34. Five stages in the development of the leg in the nymph, showing the 

 manner in which the femoro-tibal joints are formed from the exopodite. /, The 

 rudiment of a limb in the second stage of development. 2, The same in the 

 third stage, j, 4, and j", Three successive stages of the same : c, coxopodite ; 

 ex, exopodite ; en, endopodite ; s, sternum ; c ', coxa ; f t femur ; /, tibia ; / to j", 

 tarsal joints. 



the exopodite of some Crustacea, and the rudimentary limb of 

 the fly-nymph takes us back to the primary bifurcate condition 

 still retained in the thoracic limbs of the generalised Crustacea. 



This much is certain, the five tarsal joints in the Blow-fly 

 are all differentiated distinctly before any trace of the segmen- 

 tation of the femoro-tibial portion of the limb is apparent, and 

 this is developed from a process which closely resembles the 

 exopodite of the crustacean limb. 



The Dorsal Appendages of the thorax, or wings, are highly 



