118 ON THE ANATOMY Ofl THE 



ance. The rapid development of the discs ruptures the capsules 

 which contain them ; but during their development, before the 

 rapture of the capsule, they present the appearance of spiral 

 folds and concentric rings the optic and superior thoracic 

 discs having the former, and the inferior thoracic discs having 

 the latter kind of folds. ^The antennal discs present the appear- 

 ance of concentric rings at their anterior extremity. 



The unfolding of the concentric ring-like folds is exceedingly 

 curious, the internal ring being pushed out first, the rest 

 following in succession, These form the joints of the limbs 

 and antenna 



The development and unfolding of the inferior thoracic discs is repre- 

 sented in Plate VL, figs. 1, 3, and 4. At first the several discs are 

 separated entirely from each other, but as they are developed and enlarged, 

 the discs belonging to each pair become closely united by the common 

 origin of their capsules (fig. 3). The concentric rings each form a tarsal 

 joint, and other rings which are developed later, form the tibia, femur, and 

 ooxa> After the rupture of the capsule, the disc presents the appearance 

 represented in fig. 4. 



The superior thoracic discs, after rupturing their capsules, form sacs* In 

 the interior of which the wings and halteres are developed. 



The pupa covering of the thorax is probably formed entirely from these 

 discs ; but according to my observations the inferior meso- and meta- 

 thoracic discs take no part in the formation of the proper thoracic walls* 

 These seem to be formed entirely from the inferior pro-thoracic and 

 superior meso- aud metathoracic discs. If such is the case, my views con- 

 cerning the number of thoracic segments would acquire confirmation. The 

 pro-thorax being formed from the pro-thoracic discs ; the anterior spiracle, 

 which opens between the pro-thorax and meso-thorax, would belong to the 

 next thoracic segment, undeveloped in the imago, but forming the sixth 

 segment in the larva, and bearing, its anterior spiracles, whilst the 

 superior meso- and meta-thoracic may belong to segments which alternate 

 with the inferior thoracic discs. 



The antennal and optic discs form the pupa covering of the head, and the 

 optic discs present a strongly-marked spiral fold before the rupture of 

 their capauJ.es. 



