Article beginning on page 561.
Psyche 6:561-564, 1891.

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PSYCHE.
THE PRIMITIVE NUMBER OF MALPIGHIAN VESSELS IN INSECTS.-VII.
BY WILLIAM MORTON WHEELER, PH.D., CHICAGO, ILL. With these conclusions the object of
the present paper is attained. As
stated in the beginning, the homologies
of the Malpighian vessels with other
insect or vermian organs are most
obscure, and if I venture to add some
remarks of a critical nature, it is in the . belief that a discussion of the difficulties attending the establishment of such hom- ologies is not without a certain, albeit negative value.
It is an interesting fact, which has
impressed several observers, that the
fore-gut, and more especially the hind-
gut in many insects, have a very regular hexagonal outline. This is most readily
seen in the more primitive orders, like
the Orthoptera, where it extends even
to the mid-gut which is of entodermal
origin. Minot* was struck with this
peculiarity while studying Melanoplus.
*'I cannot but think," he concludes, ('that it (the curious repetition of the number six) will be ultimately found to have
some hitherto unsuspected meaning.
There are six rows of teeth in the pro-
ventriculus, six diverticula arising from the stomach, and twelve longitudinal
*Histology of the locust (Caloptenus) and the cricket (Anabrus). Second rep. U. S. ent. comm. 1878-79 relat- ing to the Rocky Mt. locust. 1880, pp. 220-221. folds in each diverticulum. There are
twelve (twice six) gastroileal folds,
arranged in twos, each pair appearing
as the double anterior termination of the six ileal folds, which, changing their
character, extend backwards through
the colon; finally in the rectum there
are six rectal glands."
The hexagonal symmetry of the hind-
gut makes its appearance during embry-
onic life, as I have observed in members of the orders Orthoptera, Coleoptera
and Lepidoptera. The Malpighian
vessels arise at the 6 angles, or horns of the proctodaeal wall. There is evi-
dently, therefore, some correlation
between the 6 vessels and the hexagonal
outline of the hind-gut.
Miall and Denny* were, I believe,
the first to offer an explanation for the hexagonal structure of the rectum.
They are of the opinion that "the tend-
ency to produce a 6-banded stoinodaeum
and proctodaeum may possibly be
related to the 6 theoretical elements
{two tergal, two pleural, two sternal)
traceable in the Arthropod exo-skeleton, of which the stomodaeum and procto-
daeum are reflected folds."




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562 PSYCHE. [December 1893.
Mingazzini * too in his study of the
alimentary canal of the phytophagous
Lamellicornia . concludes that the
hexagonal structure of the rectum is
"di grande importanza filogenetico,
perch& si riscontra in quasi tutti gli
ordini d'insetti finora studiati." Min-
gazzina adds as further evidence to Miall and Denny's suggestion that the proc-
todaeum presents 6 bands of longitudinal muscles corresponding to the 6 longitud- inal muscles of a segment.
Miall and Denny's hypothesis is cer-
tainly ingenious, but I fear that it is
untenable. To be admissible it would
require that each of the 6 sides of the
hind-gut should be parallel to one of the 6 exoskeletal elements of the segment
(Fig. I). The median dorsoventral
A
FIG. I.
plane, represented by the line A A,
should cut the prismatic hind-gut into
two symmetrical halves, either of which
-- -
*Ricerche sul canale digerente delle larve dei lamellicorni fitofagi. Mitth. zool. stat. Neapel. bd. ix heft i, 1889, p. 95 at seq.
would contain three entire planes. But
in reality this is not the case, as sections through the embryo show (Fig. 2.) ;
there being in the proctodaeum one
dorsal, one ventral, and on either side
two pleural planes. The median dor-
soventral plane A A cuts the procto-
daeum into two symmetrical halves?
either of which consists of two entire
and two half sides. It is therefore nec- essary to suppose either that the rectum has undergone a rotation of 30' on its.
median longitudinal axis - and this
would produce a torsion of which there
is not the slightest evidence in the
embryo or adult-or that a single side
of the proctodaeum does not correspond
to a segmental element and this is, of
course, equivalent to abandoning the
hypothesis altogether.*
* Miall and Denny's remarks are hardly explicit enough.
I have interpreted their hypothesis as refer- ing to the sides and not to the angles of the hind-gut. On the supposition that each corner, or horn with the adja- cent half sides is regarded as equivalent to an exoskele- tal element, no very serious objection can be raised to the view.




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December 1893.1 PA' EEL!?. 563
The actual position, of the procto-
daeutn 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 (Annelicla 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 inetamere. As the
ninth, tenth and eleventh segments are
etracheate we might hope to fill this
gap with the three pairs of Malpig-
hian vessels, 011 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 ectodennal 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 "blood-tissue" of the Insecta. Psy- che. Feb.-April, 1892.




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564 PS2'-ck!E. [December 1893.
out* that the sexual ducts arise in Xiph- idium as hollow diverticula of the
somites, like the nephridia and sexual
ducts of Peripatus. It is, however, only the mesodermal portion of the nephridia
which arises in this way ; if what stu-
dents of annelid development tell us
prove to be correct, each nephridium
must also have a small proximal piece
derived from the ectoderm. The oeno-
cyte cluster may be this piece disinte-
grated and changed in function. I deem
it probable that the insect nephridium
has passed through the following stages : I. The nephropore (now represented
by the oenocyte pit in the embryo)
became occluded and then obliterated
together with the lumen of the ectoder-
ma1 portion of the nephridium.
2. With further degeneration the
ectodermal and mesodermal anlagen of
the nephridiuin failed to unite during
ontogeny and persisted as separate
structures.
3. These two anlagen took on new
functions - the ectodermal or oenocyte
portion acquiring some function
perhaps analogous to that of the
thymus in vertebrates ; the mesodermal
portion becoming converted partly into
the corpora adiposa and in part into the sexual ducts. +
The Malpighian vessels, if at all com-
parable to nephridia, can represent only the ectodermal (oenocytic) ends. On
the trisegmental hypothesis the vessels
would be equivalent to three pairs of
oenocyte-clusters, which had preserved
their tubular character and excretory
function. But the great difficulty lies
in understanding the process whereby
these three pairs of metameric and
originally integiimental structures could be carried into the anal invagination
and appear ontogenetically as hollow
buds at its inner end. On the other
hand my hypothesis is beset with fewer
difficulties, if we suppose that the three pairs of Malpighian vessels do not rep-
resent three metameres, but only a single one, for it is easy to understand how a
single pair of oenocyte tubules could be carried in and subsequently, in the re-
mote ancestors of existing insects, give rise to three pairs of vessels by a simple process of budding.
These considerations lead me to
reject the trisegmental hypothesis and,
while regarding 6 as the primitive
number of Malpighian vessels in insects, to postulate only a single pair in more
ancestral Articulata.
The University of Chicago.
Sept. 1892.
å´* contribution to insect embryology.
Journ. of
morph. vol. viii, no. I. 1893, p. 116 et seq. f The fat-body retains to this day a nephric function inasmuch as it stores up urates often in considerable quantity. Kowalevsky (Ein beitrag zur kenntnis der exkretionsorgane, Biol. centralbl. bd. ix, 1890, p. 42 et seq.) in a long series of experiments found that carmine, chloride of iron, and litmus were abstracted from the blood by the cells of the pericardial fat-body. In Muscid larvae the cells of the garland-shaped strand have a similar function.




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