RE Snodgrass first published Principles of Insect Morphology in 1935. Several reprints and 70 years later, this text still is useful and interesting for a student of entomology, and is a wonderful reference for any entomologist to have. Yes, it does not have the molecular information that one would find in more recent works such as Chapman's Insects: Structure and Function. At one point Snodgrass refers to the germ cells as "the carriers of inheritance, whatever that may be". Modern understanding of inheritance through DNA was not discovered until the middle part of the 20th century.
However, the drawings are excellent, and our understanding of basic structures of the insects such as the general body plan, the sclerites, muscles, wings, digestive tract, etc, have not changed much since this book was published. The writing and explanation of the figures is also excellent, and Snodgrass' hypotheses for the evolution of the insect ground plan are still in use today.
Snodgrass was a very meticulous, ordered and organized character. His daily habits were regular, he would fill his coffee cup to the same exact line every day at the same time and walk the same 30 some steps down the hallway to his office. This understanding of order and attention to detail shows very well in his drawings. His particular style of work was to reduce figures to the smallest necessary number of lines to convey proper understanding of structures. Those of you who have worked in biology labs before understand that too much detail in a reference figure obscures understanding of the position and structure of the very parts which you are using as reference. By reducing lines, Snodgrass effectively conveyed the exact message he wished to with his drawings, rather than putting us in awe of his artistic work and scrambling to understand what goes where.
Above: An example of Snodgrass' drawing style. Note the lack of stipling except to differentiate membrane from sclerotized regions.
Principles of Insect Morphology is a book every entomologist should have, as a reference text, and simply for the pleasure of its excellence. It is as useful now as it was 70 years ago.
Amazon.com link for Principles of Insect Morphology
Tapes of three lectures by RE Snodgrass with transcripts and drawings