106 THE PHYSIOLOGY OF INSECT SENSES 



tibia (the subgenual organs), the distal tibia, the tarsus-pretarsus. The 

 degree to which the organs are developed varies from Order to Order 

 and species to species. The number of sensilla in a group varies from 

 one to two hundred or three hundred. Each group, with the exception 

 of the subgenual organ, is associated with an articulation of the leg. In 

 the subgenual region there may be one or two concentrations of sen- 

 silla or none at all. The true subgenual organ lies immediately distal to 

 the femur-tibia articulation. When a second group of sensilla occurs, 

 it lies distal to the true subgenual organ. True subgenual organs have 

 not been found in the thysanuran Machilis, the beetle Rliagonycha, or 

 in Diptera, Heteroptera, Homoptera, and Neuropterpodea. Even the 

 more distal organ is absent in Machilis, Rhagonycha, and Diptera 

 (Debaisieux, 1938). 



Subgenual organs are unusually diverse in structure, shape, and 

 numbers of component sensilla. They are most highly developed in 

 Orthoptera, Hymenoptera, and Lepidoptera and take the form of 

 cones, or sails, or fans. They are unique in that they more or less 

 completely occlude the dorsal blood sinus of the leg (Figs. 47, 56). At 

 the same time they lie in close proximity to the main tracheal trunk. 

 They are innervated by a branch of the leg nerve or, in the case of 

 insects possessing a tibial tympanic organ, by a branch of the tympanic 

 nerve and a branch of the leg nerve combined. They are supported 

 proximally by their nerves and distally by accessory cells which form a 

 ligament attached most often to the cuticula of the leg. Standard 

 chordotonal sensilla comprise the sensory elements. In number they 

 range from ten to forty. Extensive descriptions of these organs have 

 been given by Schwabe (1906), Schon (1911), Eggers (1928), and 

 Debaisieux (1935, 1938). 



It has not been easy to ascertain just which of the many sense organs 

 in the legs mediate responses to vibrations of the substrate. The most 

 extensive experiments have been those of Autrum (1942, 1943), 

 Antrum and Schneider (1948), and Schneider (1950). Data are based 

 upon electrophysiological recordings of total activity in leg nerves 

 while the preparation was 'standing' on a metal plate which was set into 

 sinusoidal oscillations of measurable amplitude (Autrum, 1941). 

 Tests were conducted with Orthoptera, Lepidoptera, Hymenoptera, 

 and Coleoptera. In many, the prothoracic legs are the least sensitive; 

 the mesothoracic, the most. 



All of the insects tested fall roughly into two groups as regards 

 sensitivity. In the less-sensitive group are the beetles, Carabus, Silpha, 

 Pterostrichus, and the Hymenoptera, Vespa and Bombus, The more 



