138 INJURIOUS AND USEFUL INSECTS 



the size of the head. Microscopic preparations of the head 

 of the blow-fly larva may easily be made by cutting off the 

 prothorax with part of the mesothorax attached. Place it in 

 equal parts of glycerine and 5 per cent, potash solution, lay on 

 a large cover, and apply gentle pressure. The preparation 

 gradually increases in transparency, owing to the action of the 

 potash. 



Examine with a lens the hinder end of a larva preserved in 

 spirit. Afterwards cut off the truncated end, turn it up, and 

 examine again. Notice that the hinder end is obliquely trun- 

 cated. It is divisible into a dorsal spiracular field, enclosed 

 by twelve small papillae, and a smaller ventral anal field, in 

 which lies the prominent anus. The pair of large, obscurely 

 jointed processes which lie on either side of the anus, seem to 

 answer to the anal feet of the Chironomus-larva. The posterior 

 spiracles, which lie in the spiracular field, are circular chitinous 

 plates, each enclosing three respiratory slits, which are guarded 

 by a strong chitinous network. 



Next examine the small, fan-shaped anterior spiracles, which 

 project from each side of the prothorax. These are probably 

 little used for respiration. The longitudinal tracheal trunks 

 proceed forwards from the posterior spiracles, and give off 

 numerous branches to all the segments. They are connected 

 in the mesothorax by a transverse trachea, immediately in 

 front of which they receive branches from the anterior 

 spiracles. 



The tracheal system, the nerve-cord, and the heart, can be 

 studied in a fresh larva by subjecting it to slight pressure in a 

 compressorium or live box. The large tracheae will then 

 appear as silvery threads. At the hinder end of the body 

 the pulsation of the heart will be observed. 



The Pupa 



The full-fed larva buries itself, if possible, in the earth. 

 The head becomes permanently retracted, the alimentary 

 canal is emptied, and the body assumes an egg-like shape. 

 At first it can creep about when disturbed, but it soon becomes 

 incapable of movement, owing to the detachment of the 

 muscles from the larval skin. After two or three days the 

 larval skin hardens, and turns successively yellow, red, and 



