EXTERNAL ANATOMY. O 



closely applied to the thorax for this to be visible. In the average 

 Dipteron. the head moves with great freedom upon the neck, and 

 is capable of being turned in all directions. The metathorax is 

 hardly distinguishable from the mesothorax, especially on the 

 upper side. 



On the shoulders, that is, the anterior corners of the thorax, a 

 slight swelling is often present, known as the Jiumerus, or humeral 

 calhis or callosity. These humeral calli are frequently absent, and 

 when present are very often differently coloured from the rest of 

 the thorax. Other calli situated on various parts of the thorax 

 bear significant names. Thepre-alar callus is a small swelling in 

 front of the wing-root, towards the side of the thoracic dorsum, 

 which latter is generally spoken of as the mesonotum. The post- 

 alar calli lie behind the wing ; when they are on the hinder 

 corners of the dorsum they are termed posterior calli. Sometimes 

 1 have used the term post-sutural calli in place of post-alar calli. 



Across the centre of the mesonotum in many Diptera is seen a 

 more or less distinctly impressed line, often somewhat like a very 

 widely opened V, faintest in the middle, and ending on each side 

 a little in front of the base of the wings. This is the transverse 

 suture, and it has a high morphological value, being very con- 

 sistent when present. In the NEMATOCERA it is a strong character 

 of the TiPULiDjE (with one or two comparatively unimportant 

 exceptions). Behind this suture (speaking now principally of 

 TIPULID^E) and between the slight swellings of the post-alar calli 

 is a slight depression, which in the present work is referred to as 

 the post-sutural depression. 



The presutural depression is a small depression at each end of 

 the transverse suture, usually triangular in shape. 



The supra-alar groove is a groove on the mesothorax just above 

 the wing-root, and in many species bristles of taxonomic import- 

 ance are found along its inner margin. 



The scutellum is a projecting posterior lobe of the mesonotum, 

 and a horny irregular ridge-like projection joining the scutellum 

 on each side to the mesonotum is termed the scutellar ridge. It 

 is sometimes, but not usually, conspicuous. . 



Behind and below the scutellum is the metanotum, a smooth 

 and more or less swollen part, attaining its maximum develop- 

 ment in the TIPULIDJB, in which it is usually much more 

 conspicuous than the scutellum. 



The halteres, which in Diptera replace the posterior wings, are 

 small delicate organs consisting of a narrow moderately long 

 stem, ending in an oval club, which occasionally is flattened or 

 spatulate. 



The sides of the thorax in many Diptera are distinctly sub- 

 divided by impressed lines known as pleural sutures, whilst in 

 others such demarcations are not perceptible ; when present they 

 divide the sides into pleural spaces, which in some groups are 

 well defined, whilst in others they are very indefinite. Most 

 authors speak of them in general terms as " the pleurae." These 



