The electron microscope can be used to advantage in the investigation of a physiological problem, where the reactions are differentiated at the cellular rather than at the tissue level. This set of electron micrographs shows some of the morphological arrangements accompanying various excretory functions in Tenebrio larvae.

The Tenebrio larva has a cryptonephridial arrangement of the Mlalpighian tubes, in that the distal end of each tube is applied to the surface of the rectum and covered by a heavy membrane, in which periodic “Windows”, the leptophragmata, occur where the membrane is very thin or replaced by a different structure. The free part of the tubes can be differentiated into physiological zones by means of their succinic dehydrogenase activity, which is high in places and low in others along the tube showing no gross morphological differences. The hindgut also contributes to the excretory work of the iinsect. Thc cryptonephridial part of the tube is unique in ultra-structure among cells so far studied. The membrane structure is very extensive, and mitochondria very numerous. Membrane structures bearing no resemblance to ordinary reticulum are common, especially near the leptophragma. The leptophragma is a complex structure, made up of parts from both the Malpighian tube cell and surrounding membrane. Its appearance strongly suggests that it forms a region of easy transport for water and solutes between lumen of tube and body cavity.