THE SOUNDS PRODUCED BY INSECTS. 377 



ties produce a sound by the friction of the tergum of the pro- 

 thorax upon a process of that of the mesothorax, and the 

 Dung-beetles by rubbing the coxae of the hind-legs against 

 the hinder edge of the third abdominal sternum. Further, 

 sounds are necessarily produced by the extremely rapid vibra- 

 tion of the wings, which characterizes the flight of many in- 

 sects. Landois, however, found that the thorax of a Blue- 

 bottle fly continued to buzz after the separation of the head, 

 the wings, the legs, and the abdomen. The separation of the 

 halteres weakened the sound but slightly. The acoustic ap- 

 paratus, in fact, lies in the immediate neighborhood of the 

 thoracic stigmata. The main trunk of the trachea3 dilates 

 into a hemispherical sac, which opens externally by the stig- 

 matic orifice. The sac presents a hooplike thickening, to 

 which are attached free chitinous folds or processes, and it is 

 to the vibration of these that Landois ascribes the sound. 

 The vocal organ of the Fly would thus appear to be a modi- 

 fication of the occlusor apparatus of the stigmata, just as the 

 oro*an of voice of mammals is a modification of the occlusor 

 apparatus of their respiratory opening. 



In the Cicadce the vocal organs are, according to Lan- 

 dois, the posterior thoracic stigmata. These open into cham- 

 bers, in the walls of which tense membranes are so disposed 

 as to intensify the sound by their resonance. 



As in the Crustacea, so in insects, the central nervous 

 system varies very much in the extent to which its compo- 

 nent ganglia are united together. In most Orthoptera and 

 N~europtera and in many Coleoptera, the thoracic and abdom- 

 inal ganglia remain distinct, and are united by double com- 

 missures as in Blatta. In the Lepidoptera, the thoracic gan- 

 glia have coalesced into two masses, united by double com- 

 missures ; while in the abdomen there are five ganglia, with 

 single or partially separated commissural cords. The concen- 

 tration goes furthest in some Diptera and in the Streps ipter a, 

 in which the thoracic and abdominal ganglia are fused into a 

 common mass. 



A system of stomato-gastric nerves, similar in its general 

 arrangement to that of Blatta, is very generally present. 



A special system of nerves, termed respiratory or trans- 

 verse, is found in very many insects, both in the larval and in 

 the perfect condition. The principal nerves of this system 

 are arranged in pairs on the sternal aspect of the body, and 

 their outer extremities anastomose with branches of the or- 

 dinary peripheral nerves, and are distributed to the muscles 



