302 Mr. J. A. Dell on the Structure 



longitudinal trachea a number of small branches, which 

 break up into branches of distribution in the neighbour- 

 hood of the pericardium. 



Pupation : External Features of the Pupa. 



After reaching its full size, the larva becomes slu<j£jish 

 and ceases to feed ; the head becomes retracted, so that the 

 part behind the eyes is concealed by the first body- 

 segment ; the last larval skin is then shed, and the pupa 

 emerges. This takes place while the animal is still buried 

 in slime. In a short time the pupa makes its way to the 

 surface, where it lies with the respiratory appendages 

 exposed. This facilitates respiration, and the escape of 

 the fly. 



The pupa (Fig. 7) is, as usual, shorter and thicker than 

 the larva. When it first appears it is white, but the 

 cuticle soon thickens and turns brown. The head is not 

 externally marked off from the rest of the body, but the 

 compound eyes of the fly can be seen through the pupal 

 skin. From the front of the prothorax arise the two 

 respiratory appendages (Prothorakalhorne of De Meijere*). 

 They are rather less than one-third of the length of the 

 body, and transversely wrinkled. Scattered over their 

 surfaces, especially towards the distal end, occur numerous 

 transparent circles, which appear to be holes, but are really 

 thin spots in the cuticle. A shallow transverse furrow 

 separates the prothorax from the mesothorax. The three 

 pairs of leg-sheaths are straight, and reach the beginning 

 of the third abdominal segment, where the tips of the wing- 

 sheaths are also situated. All the exposed abdominal 

 segments, except the eighth or last, are protected by 

 dorsal and ventral plates, whose hinder margins end in 

 rows of backwardly-directed spines. There are also a few 

 scattered spines on both dorsal and ventral surfaces. Each 

 spine is a pointed process of the body-wall. Its apex bears 

 a spike set in a socket. The last segment bears four large 

 spines, two terminal and two subterminal. The spines 

 assist the pupa to move about in the filth in which it 

 is buried, and so keep the respiratory appendages exposed 

 to the air. 



* De Meijere (1902). 



