396 Annals Entomological Society of America [Vol. IX, 



and become very small, although they persist faintly to the 

 end of the region. The circular muscles also decrease so 

 markedly as to be almost negligible. The Malpighian vessels, 

 which have been almost circular in cross sections, now become 

 elongated along the diameter perpendicular to the axis of the 

 canal, and tend to crowd the longitudinal muscles inside. 

 This tendency becomes more and more pronounced, and some 

 little distance before the end of the colon, the intestine is 

 surrounded by a practically continuous layer of Malpighian 

 tubes, with the small longitudinal muscles intercalated between 

 them. The vessels do not extend along the wall of the rectum, 

 but seem to terminate blindly at the posterior end of the colon. 



It is very difficult to make out the exact course of the 

 vessels in the walls of the colon in a dissected specimen. They 

 seem to extend posteriorly as six parallel tubes, at first of 

 slight diameter and almost straight; but, as was noted in the 

 preceding paragraph, as the diameter increases, they become 

 more and more wavy, with larger and larger folds. It is this 

 character which makes the vessels appear so elongated in cross 

 sections. Toward the end of the colon, these undulatory 

 folds are so large that those of one series almost touch those 

 of the adjacent series, and thus they almost completely sur-. 

 round the wall of the colon. The vessels seem to branch 

 irregularly, the tubes terminating bHndly and separately in 

 irregular ramifications, just anterior to the strong circular 

 muscles which appear abruptly, and mark externally the begin- 

 ning of the rectum. The tubes in the wall of the colon have an 

 extremely abundant tracheal supply, a fact which makes dis- 

 section difficult, and makes it much harder to trace them out. 



The distribution and ramification of one of the Malpighian 

 trunks in the wall of the colon is shown diagrammatically 

 in Figure 4. It should be noted that the strong circular muscles 

 which mark the beginning of the rectum are pecuHar to that 

 region, and there are no muscles overlying the Malpighian 

 vessels in the wall of the colon. 



We may summarize, then, the distribution of the Malpighian 

 tubes in the larva of Haltica himarginata as follows, beginning 

 at their distal ends. Six Malpighian vessels extend parallel 

 to one another, running cephalad in the wall of the colon. 

 They unite at the anterior end of this region to form two com- 

 mon trunks, which leaving the wall of the intestine, split up 



