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 Muscidae (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 maxilla;, 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 expedite 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, ~, and -/, 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. 



* 1 regard the trochanter as part of the femur. 



