I first encountered the Tillyard plates in the reading room at South Kensington, one damp November afternoon in 1923. The volume had been bound in buckram, its pages foxed at the edges. When I opened to Plate XIV I was stopped cold. Here was not a photograph, but a declaration of what the eye had been trained to see: every crossvein traced with a crowquill, each node labeled in a steady hand.
The Node and the Stigma
Taxonomy keys of the Odonata rely on two invariant landmarks. The nodus, a reinforced crossvein near the wing’s midpoint, serves as the primary hinge distributing torsional load across the membrane. The pterostigma is an opaque cell near the wingtip whose mass dampens flutter at speed. Every species key published between 1910 and 1945 opens with these two points, marked N and Pt in the same cyan-grey ink.