in Insects, and Us Functions during Flight. 393 



performs very essential functions during flight, while in Tenthredo, 

 Chrysis, Sphex, &c. it is composed of an incomplete ring. 



The internal surface of the prothorax is furnished, in common 

 with the other parts of the body, with nervures affixed to the in- 

 teguments, which serve either as supports, or for the attach- 

 ment of muscles. It also bears several transverse ridges proceed- 

 ing from a common centre on the inferior side, which is sometimes 

 perforated. These have been termed the furcular branches. 



This segment of the thorax takes in general no share in the 

 action of flying, except by its vibrations. In the Coleoptera, 

 Hemiptera, and Orthoptera, it appears to be designed, in some 

 measure, as a counterpoise, during flight, to the abdomen and the 

 portion of the trunk which is situated behind the wings, and thus 

 to facilitate the passage through the air, which is considerably im- 

 peded when these latter parts are so heavy as to destroy the 

 horizontal position. Its principal use is to afford a strong point 

 of attachment to the powerful muscles of the anterior legs, and 

 hence it is requisite that it should be free during flight in the 

 Libellula?, as these insects seize their prey by means of these 

 feet, and devour it while on the wing. 



The organs of flight are always affixed to the two posterior 

 segments of the thorax, which may be termed collectively the 

 wing-bearing trunk, or the two alar segments ; and may be dis- 

 tinguished into the anterior or middle alar segment, (Mesothorax), 

 and the posterior alar segment, (Metathorax.) These segments, 

 corresponding in size to that of the wings which they support, are 

 nearly equal only in the Libellulae. In these insects, in which the 

 muscles of flight to each pair of wings are also equal, these seg- 

 ments are not inclosed the one within the other, and are only 

 united in their pectoral portion where they appear soldered to- 

 gether. This intimate union of the sternal portions of the two 

 alar segments, exists also in the Coleoptera, in the Grylli, Cicada?, 

 and Pentatomae, in the greater number of the Hymenoptera and in 

 the Diptera. 



The Mesothorax is often of considerable size, as in the Hyme- 

 noptera, the Lepidoptera, and the Diptera ; it bears the superior 

 wings and contains the principal muscles of flight which are com- 



