in Insects, and its Functions during Flight. 397 



bascular appendix. This is intimately united to the dorsum in 

 all insects, excepting some of the Hymenoptera, the Vespae, 

 Bombi, Apes, &c. in which it is nearly free, being attached 

 only by a single small muscle from the anterior and internal ex- 

 tremity of each branch. 



In all insects, with the exception of the Libellulae, the dorsal 

 muscles or the depressors of the wings are attached posteriorly to 

 the concave surface of a kind of transverse septum, which is very 

 convex behind, free in the posterior part of its superior and in- 

 ferior margins, and the sides of which alone are articulated with 

 the pectoral concha, and often (as in the Acrydia, Cicadae, Papi- 

 llones, and Diptera) intimately united with it. This part is 

 termed the costal, or costal septum, on account of its functions in 

 flight, which correspond in some measure with those of the ribs of 

 birds, inasmuch as like them it serves for a posterior attachment 

 to the depressor muscles of the wings and the dilators of the trunk. 

 In all insects, with the exception of some of the Hymenoptera, 

 the costal septum forms the immediate separation between the 

 thorax and the abdomen, and may hence be regarded as a kind of 

 diaphragm. Its inferior portion is in general directed backwards ; 

 but the contrary takes place in many of the Ilemiptera, this por- 

 tion being in them directed forwards and connected by elastic 

 ligaments to apophyses of the sternum. In those Hymenoptera, 

 the abdomen of which is pedicelled, as Ichneumon, Sphex, Vespa, 

 &c. the costal is entirely involved in the interior of the meta- 

 thorax, articulating itself, by its branches only, with the bascule 

 and with the small bones of the base of each wing ; while in al- 

 most all insects with a sessile abdomen the attachment is formed 

 by a very strong and loose ligamentous membrane, extending 

 from the uncovered and free posterior portion of the upper 

 margin of the costal, and which protects the interior of the trunk 

 and is alternately stretched and relaxed during flight. The only 

 exception from this structure is in the Acrydia, in which the 

 costal septum is divided into lobes and is situated entirely within 

 the trunk, having its superior margin anteriorly united intimately 

 and immediately with the bascular appendix, and posteriorly 



