VI EXTERNAL STRUCTURE 3 I 3 



posterior direction. A broad, but short phragma descends from 

 the hind margin of the metascuteUum into tlie interior of the 

 body. It should be noted that though the metanotum is forced, 

 as it were, backwards by the great extension of the mesonotum in 

 the middle line of the body, yet at the sides the metanotum creeps 

 forward so as to keep the points of attachment of the hind wings 

 near to those of the front wings. In many forms of Hesperiidae, 

 Sphingidae, Noctuidae, etc. the true structure of the metanotum 

 is further concealed by the back of the mesoscutellum reposing 

 on, and covering it. 



Difference of opinion exists as to the thoracic Spiracles ; there 

 is one conspicuous enough in the membrane behind the pronotum, 

 and it is tliought by some writers that no other exists. West- 

 wood and Scudder, however, speak of a mesothoracic spiracle, and 

 Dr. Chapman considers that one exists. Minot describes^ a 

 structure behind the anterior wing, and thinks it may be an 

 imperfect spiracle, and we have .found a similar stigma in 

 Saturnia pavonia. At the back of the thorax there is on each 

 side in some Lepidoptera (Noctuidae, Arctia, etc.), a curious large 

 cavity formed by a projection backwards from the sides of the 

 metasternum, and a corresponding development of the pleura of 

 the first abdominal segment. Minot and others have suggested 

 that this may be an organ of hearing. 



The Abdomen differs according to the sex. In the female 

 seven segments are conspicuous dorsally, but only six ventrally, 

 because the first segment is entirely membranous beneath, and 

 is concealed between the second abdominal ventral plate and 

 the posterior coxae. Besides these segments there are at the 

 hind end two others smaller, more or less completely with- 

 drawn into the body, and in certain cases forming an ovipositor. 

 These nine segments are usually considered to constitute the 

 abdomen ; but according to Peytoureau,^ a tenth dorsal plate is 

 represented on either side of the anal orifice, though there is no 

 trace of a corresponding ventral plate. In the male the segments, 

 externally conspicuous, are one more than in the female. According 

 to the authority quoted,^ this sex has also truly ten abdominal 

 segments, the ninth segment being withdrawn to a greater or 



^ Fourth Rep. U.S. Entom. Commission, 1885, p. 49. 



- C.R. Ac. Sci. Paris, cxviii. 1894, p. 360 ; and his Tliesis, Bordeaux, 1895. 



3 C.B. Ac. Sci. Paris, cxviii. 1894, p. 542. 



