Hypermetry of the tracheal system - where is the limit?

In flying insects, like grasshoppers and scarab beetles, the tracheal system fills a maximum of about 40% of the body core. In a theoretical consideration, we can extrapolate any scaling relationship to maximum values and find limits of body size or mass. If the hypermetry of the tracheal system of darkling beetles system is extrapolated to 40%, we can estimate maximum body sizes:

In this figure the percent of the body core (in black) and the of the leg (in red) are plotted against body length of darkling beetles. As darkling beetles get larger, the body core and the leg cross-sectional area are filled more and more with trachea. The scaling relationship in the leg is more pronounced than in the body core. If 40% is the limit for tracheal investment, beetles could be 320 millimeters (12.6 inches) long according to the scaling relationship in the body core (black arrow). According to scaling relationship in the leg, beetles could be only 70 millimeters (2.8 inches) long (light red arrow).
The tracheal tube at the base of the leg might take up more space than only 40%, possibly 90% without confining the space for other tissues. Then, the maximum body length of the beetles could be about 170 mm (6.7 inches). This body length happens to be the body length of the largest beetle on earth today: Titanus giganteus, a longhorn beetles from tropical America (Brazil, French Guiana). You can watch an amazing video of the titan beetle in this BBC production "Life in the Undergrowth" with David Attenborough.

This assumptions only hold the water, when the scaling relationships don't change in larger insects. It could be, however, that scaling relationships change with size (look at the figure lines B and C). In case of isometry (line B), which means that the geometrical relationship between the structures don't change, there is no limit on body size. If the scaling relationship is somwhere in between A and B (like in C), the limits are not so obvious.
The plan now is to find the largest insects and beetles on earth and investigate their tracheal scaling relationships, a task that the Harrison lab is going to follow over the next few years.