December iS 



PSYCHE. 



563 



The actual position of the procto- 

 daeum in the body of the insect leaves 

 no doubt that the Malpighian vessels 

 are paired structures; there being a 

 dorsal, a pleural and a ventral pair. 

 These may be readily resolved into an 

 anterior, median and posterior pair, if 

 we go back to the earliest stage in the 

 formation of the hind-gut and its diver- 

 ticula in such a form as Doryphora. 

 At this time the future long axis of 

 the proctodaeum lies at right angles to 

 the long axis of the ventral plate, so 

 that the vessels budding from its walls 

 may be regarded as arising in three 

 consecutive pairs. 



If we go a step further and look for 

 the meaning of the paired arrangement 

 of the vessels we are at once confronted 

 with difficulties. Two alternatives pre- 

 sent themselves : 



i. The number of vessels is simply 

 correlated with the number of rectal 

 folds, and the regular prismatic character 

 of the rectum requires some other ex- 

 planation than the presence of the 

 vessels, since it recurs also in the fore- 

 gut. According to this alternative the 

 6 Malpighian vessels belong to one 

 segment, the eleventh abdominal, or 

 telson, from which the proctodaeum 

 arises. 



2. Each pair of vessels represents a 

 metameric unit. Hence three separate 

 segments must have contributed to the 

 formation of the proctodaeum. This 

 alternative, if accepted, only leads to 

 further difficulties, since the vessels 

 must either be the serial homologues 

 of paired ectodermal structures be- 



longing to the more typical anterior 

 abdominal segments or such structures 

 no longer exist in the Insecta and must 

 be sought for in the more ancestral 

 Articulata (Annelida presumably). 

 Insect structures which may be homody- 

 namous with Malpighian vessels are: — 



a. The tracheae. These are known 

 to arise as tubular invaginations of the 

 integumental ectoderm pleurad to the 

 appendages, a pair to a segment. 

 They occur only as far back as the 

 eighth abdominal metamere. As the 

 ninth, tenth and eleventh segments are 

 etracheate we might hope to fill this 

 gap with the three pairs of Malpig- 

 hian vessels, on the assumption that 

 these diverticula had in some way 

 come to arise from the proctodaeal 

 instead of the integumental ectoderm. 



b. The oenocyte-clusters. These, as 

 I have shown in a former paper," arise 

 as paired ectodermal cell-masses just 

 caudad to the tracheae. They, too, 

 are wanting in the ninth to eleventh 

 segments. 



The Malpighian vessels resemble the 

 tracheae in being tubular, the oenocytes 

 in the glandular character and large 

 size of the cells. The faint invagina- 

 tion, which still accompanies the forma- 

 tion of an oenocyte-cluster in embryonic 

 Orthoptera and Coleoptera, may be the 

 last trace of the lumen of a tube, the 

 disintegrated walls of which are now 

 represented by the oenocytes. 



c. The nephridia. I have pointed 



♦ Concerning the "bloo Psy- 



che. Feb. -April, 1S9.J. 



