MESOTHORACIC STRUCTURES 57 



(Eps^) above the base of the front coxa (CxJ, and which is connected by a 

 transverse arm with the prosternum. (The last is not visible in side view.) 

 The third prothoracic plate (Epm^) lies behind and above the other two and is 

 much larger than either of them. It extends posteriorly almost to the first 

 spiracle (ISp) and sends a narrow strip downward to the precoxal membrane. 

 It might be questioned whether this plate does not really belong to the meso- 

 thorax, but a little comparative study of other flies will make its morphological 

 relations clear. It will be observed that in Psorophora the anterior coxa is not 

 directly articulated to the side of the prothorax, being connected with it only 

 by a wide membrane. In most other flies, however, it is articulated in normal 

 fashion to the propleurum, at the ventral end of the pleural suture. In the lower 

 flies this suture separates two distinct pleural plates — the episternum and the 

 epimeram. In Tipula the first spiracle is much farther forward than in the 

 mosquito and lies on a line which is clearly the boundary between the prothorax 

 and the mesothorax, the epimerum of the prothorax being in no way expanded 

 or specially developed. In Bibio the first spiracle is farther back than in Tipula 

 and the epimerum is much extended posteriorly. In Psorophora, finally, this 

 character is still more exaggerated, the first spiracle being halfway between the 

 base of the wing and the front of the mesotergum, while the epimerum of the 

 prothorax forms a large lobe reaching posteriorly to the spiracle, though its 

 ventral extremity is reduced to a narrow band close to the posterior edge of the 

 plate in front. The latter (Eps^) we can, therefore, identify as the episternum, 

 and the line (PS^) between the two plates as the pleural suture. The only dis- 

 tinctive character in the prothorax is, therefore, the separation of the coxa from 

 its pleural articulation. 



The plate (eps2) lying below and behind the spiracle is separated only by a 

 weak line from the proepimerum, but for reasons to be given later it is regarded 

 as belonging to the mesopleurum. 



The prosternum consists of a transverse plate lying in front of the bases of the 

 coxge, connected laterally with the episterna, and having a median posterior in- 

 tercoxal extension fused with the anterior sternal part of the mesothorax. 



The cervicum or " microthorax " is represented by the two lateral cervical 

 plates (mi) already mentioned. The two are connected with each other ven- 

 trally by uniting transverse arms. 



The Mesothorax. — This segment, in the mosquito, forms the great bulk of the 

 entire thoracic mass. The anterior end of its tergum projects over the top of the 

 head and the posterior end is separated from the abdomen only by the narrow 

 metatergal bridge (Tg). As in nearly all insects the mesotergum consists of 

 two principal plates, a large anterior one {psc.,, sct„, and scl^) constituting the 

 true notum and carrying the wings, and of a smaller posterior postnotal plate 

 (PN2) lying between the first and the metatergum and connected laterally with 

 the epimera of the mesopleura. The first plate is indistinctly divided into a 

 prescutum (psc^), a scutum (sc^a) ^^^ ^ scutellum (sdz). The second plate 

 constitutes the postnotum, usually termed postscutellum in the higher orders. 

 On account of the posterior location of the wings and the narrowness of their 



