158 THE INTEGUMENTAL SKELETON OE THE IMAGO. 



and the three thoracic segments are always represented by 

 three metameres, both in the larva and in the embryo. 

 Although a ventral groove appears in the somites of the 

 abdomen in the larva of the Muscidas (see Fig. 4, a and d), 

 nevertheless, each thoracic segment in the imago is developed 

 from two pairs of imaginal discs only. 



I am also unable to see that the structure of the thorax of 

 the imago is more easy of interpretation by the assumption 

 that each segment consists of two united rings. Yet I cannot 

 regard the supposition as unsupported, although I am unable 

 to accept it as proven. 



The Ventral Appendages of the thorax, or legs, are divided into 

 a series of joints or segments. The basal joint is known as the 

 coxa, or hip ; the second is the femur, or thigh* ; the third the 

 tibia, or shank ; and the terminal joints, usually five in number, 

 form the tarsus, or foot. 



The bifurcate form of the ventral appendages already al- 

 luded to appears to be a common phenomenon in all the more 

 generalised forms of arthropods. At first sight, although 

 apparent enough in the maxillae, this condition seems to be 

 entirely absent in the thoracic legs of insects. It is usual to 

 consider that one of the divisions of the limb, that corre- 

 sponding with the expodite in the Crustacea, is suppressed. 



During the evolution of the thoracic limbs, in the imago of 

 the Blow-fly, from the lower thoracic imaginal discs, they are, 

 however, bifurcate in what I shall term the third stage of 

 development. At this stage the coxa, femur, and tibia are repre- 

 sented by an elongated sac (Fig. 34, .i, and 0, ex), which is united 

 at its open extremity with both the thoracic wall and the five- 

 jointed tarsus. A longitudinal septum and two constrictions 

 afterwards appear and separate the femur and tibia. It 

 would seem, therefore, that the femur and tibia are developed 

 from the outer limb of a bifurcated appendage, and the coxa 

 forms a basal joint common to both parts of the limb. 



This remarkable phenomenon was first observed by Weis- 

 mann [2, p. 167], but it appears to have attracted little or no 

 attention since. 



* I regard the trochanter as part of the femur. 



