THE THORAX OF TABANUS 69 



is reduced to a narrow ring into which the membrane of the neck is 

 inserted, and the metathorax to a ring of only a little greater extent, 

 uniting the thorax to the abdomen ; on the dorsal surface in many forms 

 the whole of the wall belongs to the mesothorax. The sutures between 

 the various sclerites are generally close ones. In many cases the separation 

 is only indicated by a ridge on the internal surface and a narrow groove 

 externally, or sometimes only by a dome-like elevation on the outer 

 wall. The whole thorax forms a chitinous box, opening anteriorly 

 into the neck and posteriorly into the abdomen, and is filled up 

 almost entirely by the powerful muscles of flight. The various 

 structures which pass from the head to the abdomen lie in the ventral 

 portion of the cavity, and occupy a very small amount of the space. 



The difficulty that exists in attaching the correct names to the dif- 

 ferent parts will be readily appreciated when it is stated that there 

 are in existence four drawings of the thorax of the house-fly by 

 entomologists of note, no two of which are named in the same 

 manner. Fortunately the subject is not of great practical importance, 

 and it will suffice if the discussion is confined to the practical 

 points which have to do with the dissection of the fly, and to the 

 nomenclature in so far as the terms are used in systematic classification. 

 With regard to the latter point a good deal of difficulty is met with, for 

 the terms in use by systematists are frequently very vague so far as their 

 reference to the same part in different flies is concerned. 



The distribution of the various sclerites and the alteration in position 

 which they may undergo will be best understood by studying a concrete 

 instance first ; the thorax of Tabanus will serve as an 

 example, as it occupies a more or less intermediate / P | ate X iv fig 2) 

 position between the extremes. In this form the 

 whole of the dorsal wall of the thorax as seen externally is formed 

 by the tergite of the mesothorax, which is subdivided into several 

 portions by more or less distinctly marked transverse fissures. About 

 the middle of the thorax, and just anterior to the wing insertion, 

 there is on each side a short fissure or notch which indents the margin. 

 The part in front of this is the praescutum, and the part behind the 

 scutum ; the separation between the two is indistinct, and only marked 

 by these lateral notches. Behind the scutum, which is somewhat 

 narrowed posteriorly, there is' a stout and conspicuous triangular piece 

 termed the scuteUnm, This projects a little above the first segment 

 of the abdomen, and is distinctly demarcated from the scutum by a 

 transverse fissure. This is all that can be seen on the dorsal surface 



