1918] Alimentary Canal of Altica Larva 303 



alternating with the six Malpighian vessels, the whole sur- 

 rounded by a nucleated peritoneum, which is a double layer, 

 since it represents the investing sheath of the vessels. The 

 two sheets separate at the vessels, one layer passing inside and 

 the other outside of the tubes, but both sheets pass outside 

 of the longitudinal muscles. 



Normally the two trunks do not join the colon op])osite 

 one another as would be expected; at least, this is the case in 

 every series which the writer has examined. The ventral 

 trunk occupies what may be termed the normal position, and 

 when it redivides into three vessels, the central vessel remains 

 in position and each of the two lateral ones migrates around 

 the adjacent muscle fibre. The dorsal trunk, however, does 

 not enter exactly opposite (i. e., w^ith three muscles inter- 

 vening), but slightly to one side of the canal (i. e., with but 

 two muscles intervening). Therefore when it redivides, it is 

 the innermost vessel which remains in place; the outermost 

 not only must slip outside the adjacent muscle fibre, but must 

 also pass the next one beyond, so that in its migration it slips 

 outside of two muscles before it is in position ; the middle vessel 

 slips outside a single fibre, the one originally adjacent to the 

 outermost vessel, and then it is in place. Thus is brought 

 about the regular alternation of muscle fibre and Malpighian 

 vessels, w^hich obtained in every instance which the writer has 

 observed. 



In the anterior part of the colon the cross-sections of the 

 Malpighian vessels and of the longitudinal muscles are about 

 equal in diameter. But as they pass caudad, the Malpighian 

 vessels gradually increase in size, while the longitudinal muscles 

 become weaker, and two-thirds caudad the Malpighian tubes 

 are twice as large as the muscles. At this point the longitudinal 

 muscles begin to diminish in size rapidly, and become very 

 small, although they persist faintly to the end of the region. 

 The circular muscles also decrease so markedly in size 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 a little distance 

 before the end of the colon the intestine is surrounded . by a 

 practically continuous layer of Malpighian tubes, with the 



