EXTERNAL ANATOMY OF INSECTS. 533 



female ants, &c. is very elevated, forming an arch, and 

 sloping towards the abdomen. In general it may be ob- 

 served with respect to the remaining Orders, that the 

 form of the trunk merges in that of the whole body, the 

 tendency of which is often to a three-sided figure. 



III. Proportions. — The trunk is usually longer and 

 larger than the head and longer than the abdomen, but 

 not wider : but there are exceptions to both these rules. 

 In Colliicris, Mantis, &c, it is more slender ; and in Atta 

 megacephala and some neuter ants, it is shorter than the 

 head ; in Atractocerus, many Staphylinidce, Phasma, the 

 Libelhdhia, the Lepidoptera, and various Hymenoptera, 

 it is shorter, and in the Mantidce more slender than the 

 abdomen. The greatest disproportion between it and the 

 last part is exhibited by the genus Evania, parasitic 

 upon the Blattce, in which the abdomen appears merely 

 as a minute and insignificant appendage of the trunk. 

 The vertical diameter of this part, almost without ex- 

 ception, is greater than that of either head or abdomen. 

 When we consider that it contains the muscles that move 

 both the organs of flight and the legs, we see clearly the 

 reason why the Creator gave it greater volume. 



IV. Composition. — I lately intimated to you that the 

 trunk, though resolvable into three segments, in most 

 cases properly consists of only two primary ones. Who- 

 ever examines the perfect insects of every Order, except 

 the Aptera a , will find this distinction strongly pointed 



- 1 In Nirmus Anseris, &c. however, in this Order, the same distinc- 

 tion is observable. 



