1916] Malpighian Vessels of Haltica Bimarginata 393 



apposed to the alimentary canal throughout most of their 

 extent, but are always entirely distinct from its walls (except 

 in the colon at their distal ends, as will be explained later). 

 Following very closely the course of the mid-intestine and 

 appressed to its walls, they extend forward as far as the beginning 

 of the crop (i. e., into the mesothorax). Here they bend back 

 on themselves and extend nearly to the posterior end of the 

 body. In a typical cross-section through the meso- or meta- 

 thorax and the first five abdominal segments, therefore, the 

 mid-intestine is surrounded by eight sections of the Malpighian 

 vessels quite regularly arranged, so that one would naturally 

 interpret them as eight distinct vessels if he did not know the 

 true circumstances of the case. At the constriction between 

 the two parts of the mid-intestine, the Malpighian vessels 

 cease to be associated with the walls of the alimentary canal, 

 and then run posteriorly in the fat body as far as the eighth 

 abdominal segment, where they again double back on them- 

 selves, and run cephalad to the sixth abdominal segment. 

 Here they pass into the walls of the colon in a manner that will 

 presently be described. 



There is a great deal of variation in the length and course 

 of these tubes, and the condition described above is to be 

 regarded as the average condition rather than an exact descrip- 

 tion of the distribution of the tubes as found in any one individ- 

 ual specimen. Sometimes the vessels are much convoluted, 

 at other times they seem to be nearly straight; in some cases 

 they follow closely the windings of the second division of the 

 mid-intestine in their course cephalad, and at other times they 

 do not; often they do not quite reach the anterior end of the 

 mesenteron and rarely they may even extend a little beyond 

 into the prothorax. But the diameter of the vessels and their 

 color, a dull opaque white, are constant characters in this 

 series. 



The second series of vessels arises independently of the first, 

 and some little distance in front. It consists of two isolated 

 Malpighian vessels which enter the alimentary canal at the 

 exact point where the mid-intestinal epithelium ends. Sections 

 through the evagination of these tubes show typical mid- 

 intestinal epithelium, but the sections immediately succeeding 

 show the beginning of the hind-intestinal epitheHum. These 



