and Life History of Psychoda seocpunctata. 309 



The relations of the component parts of the mcsothorax 

 are peculiar, as will be seen by comparison with the cor- 

 responding structures in Chironomus (Fig. 14). In that 

 type the dorsal region of the mesothorax consists of a 

 large shield-shaped piece, the scutum ; a transverse, semi- 

 cylindrical ridge, arching across the back, the scutellum ; 

 and behind this a large postscutellum, to which the 

 hinder ends of the longitudinal mesothoracic muscles are 

 attached.* 



In Psychoda the same parts are found with the following 

 points of difference : — 



(1) The scutum extends forwards above the head, so 

 that the head and prothorax are depressed to the ventral 

 side, and the face looks downwards. 



(2) In Chironomus the post-scutellum has extended 

 backwards so far as to obliterate the dorsal part of the 

 metathorax. In Psychoda the post-scutellum extends back 

 quite as far as in Chironomus, but underlies the metathorax 

 and the dorsal part of the first and second abdominal 

 segments. It thus appears in longitudinal section as an 

 enormous dorsal invagination of the cuticle, running back- 

 wards and downwards into the body as far as the hinder 

 end of the second abdominal segment. I am informed 

 by Mr. T. H. Taylor that a somewhat similar state of 

 things is found in Simulium, 



The dorsal part of the metathorax is a narrow strip, 

 arching across the back immediately behind the scutellum. 

 At the sides of the metathorax are the club-shaped 

 halteres, or rudimentary second pair of wings, and the 

 metathoracic spiracles. 



The mesothorax is chiefly occupied by the muscles of 

 flight, which are of extraordinary size,- 1 the longitudinal 

 muscles, for instance, being half as long as the body. 



There are two pairs of spiracles. The anterior, which is 

 mesothoracic,f is formed at the place where the tracheal 

 extension from the pupal respiratory appendage enters the 

 imaginal body. Just at this point the cuticle of the fly 

 thickens into a partial ring, while immediately distal to it 



* Miall and Hammond (1900). 



t Voss (1905, p. 739) concludes from his study of Gryllus 

 domcsticus that the thoracic spiracles are probably prothoracic and 

 mesothoracic. Miall and Hammond, on the other hand, find that 

 they are mesothoracic and metathoracic in Chironomus, while Taylor 

 (1902) comes to the same conclusion in his paper on Simulium, 



