ENDO-SKELETON OF THORAX 73 



in the dissection of the Tabanid flies. It consists of a wide arched 

 sheet of chitin, arising from the upper part of the meso-epimera on 



each side, and also from the dorsal wall of the 

 , , . , . , 1,1 Mesophragma 



thorax ; this extends inwards and backwards through 



the metathorax to reach as far as the posterior limit of the latter, 

 the chitin from the two sides meeting in the middle line to form one 

 continuous sheet, which thus cuts off the upper portion of the thoracic 

 cavity from the abdomen, and leaves only a narrow interval. All the 

 structures which pass from the thorax to the abdomen have to pass 

 through this aperture. The purpose of this diaphragm is to afford 

 additional surface for the attachment of the powerful muscles which 

 provide the motive power for flight. 



The metathorax has an apodeme similar to that of the mesothorax, 

 lying upon the metasternum. It consists, however, of only one sheet 

 of chitin on each side. The sheets are pointed and wing-like, and 

 are supported by strong ribs of chitin, which converge posteriorly to- 

 wards one another, and again diverge to form a pair of stout hooked 

 processes. The structure lies immediately behind the third pair of legs. 



The three structures in the middle ventral line are sometimes term^ 

 ed the ante-, medi-, and post-furca, respectively. Similar structures 

 are found in most insects. The mesophragma is specially well devel- 

 oped in the Tabanidae, but some arrangement of the kind is found 

 in all Diptera. 



On the internal surface of the thoracic region there are to be found 

 some interesting sclerites which control the movements of the wing. 

 In Tabanus (Plate XIV, fig. 2) there are two distinct 



sets, an anterior and a posterior. The anterior set 



r Wing base 



consists of three conical sclerites attached to one another 

 by a fibrous cord running between their broad upper ends, and stretch- 

 ing from the dorsal plate, just anterior to the fissure separating the 

 praescutum from the scutum, to the free part of the upper border of 

 the meso-epimeron. When the muscles attached to these sclerites 

 contract the longitudinal fissure across which they stretch is narrowed and 

 the vertical diameter of the thorax reduced. The posterior set consists 

 of five very irregularly shaped sclerites, which are so articulated with one 

 another, and which have their movements so co-ordinated, that when 

 the wing is folded the squama is folded on the rest of the wing> 

 while when it is extended the whole of the wing is brought into one 

 plane. The three upper sclerites are continuous with the bases of the 

 three;, principal veins of the wing, and the fourth with the squama; 

 10 



