The mesopleural suture is clearly visible here. The mesopleuron is the large lateral thoracic plate above the coxa of the middle leg. Note that the mesopleural suture is roughly horizontal and goes across nearly the entire mesopleuron...making it appear as if it consists of two pieces (i.e. an upper and lower piece)...compare with Fig. 1B from Townes(1)(1957).]

The mesopleural suture appears somewhat curved here...but that's largely due to the somewhat "downward" viewing angle here, together w/ the anterior convexity of the underlying sclerite...note the suture appears more-or-less straight in the 5th image of this series, where the viewing angle provides a (more transverse) profile: