THE DOB8AL VESSEL. 75 



in its wall. The epitreme consists of a ring more or less 

 hardened on one side, from which a kind of handle projects. 

 This handle is connected with the ring differently on its two 

 sides, the junction being vertical on one side and horizontal 

 on the other (Plate IX., fig. 4). A short muscle arises from 

 the peritreme, and is inserted into the handle of the epitreme. 

 The spiracle is closed by depressing the handle, which 

 folds the vertical against the horizontal half of the ring. 

 Immediately within the epitreme the trachea swells out, and 

 gives off numerous large branches, each one as large or larger 

 than the trunk between the epitreme and the peritreme. 



Section X.The Dorsal Vessel 

 PLATE IX., FIG. 1. 



The dorsal vessel of the fly is perhaps the most difficult 

 organ in the insect to make out thoroughly. Its walls consist of 

 three coats at least; an external muscular, an intermediate 

 membranous, and an internal elastic coat. The external and 

 middle coats seem to form only a loose investment around the 

 elastic tunic in the abdomen, and after death when the vessel 

 is empty they appear as a semi-transparent fusiform flattened 

 band beneath the integument. They seem to be stretched 

 laterally by the tracheal vessels which supply them, and pro- 

 bably also by a thin membranous exparsion which connects 

 them with the abdominal parietes. The muscular fibres are 

 striated ; and the majority, if not all, are longitudinal. I am 

 inclined to regard this portion of the organ as homologous to 

 the lateral wings of the dorsal vessel ef the cockchafer. 



