THE THORAX OF TABANUS 71 



The sclerites of the lateral wall are comparatively simple, and can 

 be homologized with the primitive type. At the anterior end there 

 are two dome-shaped elevations, one behind the other, conspicuous on 

 the external surface on account of the prominent tufts of hairs which 

 they bear. The most anterior of these is continuous with the dorsal 

 wall, and lies just below the pronotum on each side ; it may be taken to 

 represent the epimeron, or dorso-lateral pleural plate. The episternum, 

 or ventro-lateral plate, lies immediately behind it, and is continuous 

 with the upper anterior angle of the mesosternum. Between it and 

 the three divisions of the prosternum there is a comparatively wide 

 interval, in which the first leg of each side is articulated ; it is on 

 account of this loose articulation that the fore legs have in all Diptera 

 a wider range of movement than the other pairs. Behind these sclerites 

 and above the mesosternum there is the episternum of the mesothorax, 

 a large plate similar in shape to the mesosternum, but smaller. The 

 epimeron, if the term may be used here, does not conform to the type 

 with the same exactness. It is represented by a most irregularly shaped 

 plate which extends from the insertion of the wing to the middle line 

 ventrally, and connects the mesothorax with the metathorax. In its 

 middle third it is firmly united to the upper half of the posterior border 

 of the mesosternum, but above and below this part it is attached to 

 the adjacent sclerites only by a membrane, leaving a wide interval be- 

 low in which the second leg is inserted. Behind its somewhat irreg- 

 ular posterior border there is a narrow plate representing the fused 

 pleural plates of the metathorax ; these articulate posteriorly with the 

 dorsal plate of the first segment of the abdomen, and are continuous 

 above with the metanotum. 



It will be noted from the above description that the wall of the 

 thorax is formed from a large number of plates of different sizes and 

 shapes, which are for the most part fitted closely to one another. They 

 are, in fact, closely fused together to form a chitinous box, and cannot 

 be separated by dissection. But at certain places the union is less 

 firm, and expansion of the thoracic wall can and does take place. 

 The most important of these regions is that between the mesonotum 

 and the lateral wall, where there is a wide longitudinal fissure extend- 

 ing from the level of the epimeron of the prothorax to the insertion 

 of the wing, and passing downwards between the posterior border 

 of the meso-episternum and the anterior border of the upper third of 

 the meso-epimeron. By virtue of this membraneous space the dorsal 

 wall of the thorax is capable of a certain amount of movement on 



