396 Mr. Bennelt on the Anulomi/ of the Thorax 



thorax ; It is contracted in the Cicadae ; while in the Pentatomae, 

 it is on the contrary the broadest part of the dorsum that is covered 

 by the prothorax. la many of the Hymenoptera, the dorsum is 

 terminated in front by a kind of button which penetrates into the 

 collar, and is freely articulated with it. In the other orders this 

 part does not enter into the prothorax. 



In the Hemiptera, the Hymenoptera, the Lepidoptera, the 

 Diptera, and in some of the Neuroptera, the dorsum of the su- 

 perior wings is much larger than that of the inferior ones ; this 

 latter is reduced almost to nothing in the Diptera in which there 

 exist only rudiments of posterior wings. It is on the contrary the 

 principal one in the Coleoptera and the Orthoptera ; and in the 

 Libellulas, the ivio dorsa are equal. 



It is generally through the intervention of the proper motions 

 of the dorsum, which are very considerable during flight, that the 

 wings or the elytra are moved equally and simultaneously. Thus 

 when it is elevated it carries with it the internal side of the base 

 of the wings with which it is articulated, from which ensues the 

 depression of the external side of the wing ; and when it ap- 

 proaches the sternal portion of the trunk, the contrary takes 

 place, and the wings are raised. During the depression of the 

 "wings, the dorsum is curved from before, backwards, or in such a 

 manner that its anterior extremity is brought nearer to its pos- 

 terior, that its middle is elevated, and its lateral portions re- 

 moved farther from each other. The reverse takes place in the 

 elevation of the wings ; the anterior extremity of the dorsum 

 being removed to a greater distance from the posterior, its middle 

 being depressed, and its sides brought nearer to each other. Thus 

 its bending in one direction produces a diminution of its curve in 

 the direction normally opposed to it j and by the alternations of 

 this motion, assisted by other means, the body is alternately com- 

 pressed and dilated, and the wings are raised and depressed by 

 turns. 



The dorsum is connected behind to the body and the two 

 branches of a semicircular piece, the podorsum or post dorsum, 

 which is susceptible of a spring and performs the oflice of a lever, 

 and which lias therefore been occasionally termed the bascule or 



