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Psyche 5:3-12, 1888.

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PSYCHE.
ON THE PRESENT STATE OF OUR KNOWLEDGE CONCERN- ING CONTAGIOUS INSECT DISEASES.
BY STEPHEN ALFRED FORBES, CHAMPAIGN, ILL. [Annual address of the retiring president of the Cambridge Entomological Club, 14 January 1887.]* It seems to have been from the begin-
ning characteristic of the Cambridge
Entomological Club that,without under-
valuing taxonomic work, our members
have chosen for themselves the field
of biological entomology. Not con-
tent with the mere orderly arrangement
of the facts of insect structure in the
form of a comparative anatomy or of a
classification, we have been especially
interested, as a rule, I think, in the
attempt to philosophize such facts; to
trace them to their causes and to follow them to their effects. Sympathizing
heartily with this tradition of the so-
ciety, I have selected as the principal
topic of my address a subject requiring
us to consider the insect as a living
organism, in active vital relation to the living organic world,- a subject which
frees us in great measure from the
technical harness of a classification,
and must even lead us quite beyond the
borders of entomological science. It
is one of those outlying subjects which
come within the range of a frontier
patrol, interested in the foreign rela-
tions of insect life, whether those of
* For bibliography accompanying this address see the BIBLIOGRAPHICAL RECORD, p. 15.
peaceful commerce or of depredation
and defence. Topics of this sort are
the food relations of insects and, allied to this, the captivating subject of their relations to flowers. Here also belongs
the complex subject or group of sub-
jects included under economic ento-
mology ; and here comes, of course, the
special topic of this address, -that of
contagious insect disease.
Contagious disease, wherever it has
been traced to its origin, has proved to be a phenomenon of parasitism ; and
is included, consequently, under the
general head of the interactions of or-
ganisms.
Rejecting the many cases of parasi-
tism which have no very serious effect
on the insect host, whether because the
parasites are in their nature insignifi- cant, or because - as in the case of the termites and many wood-eating spe-
cies -the organism seems to have
adjusted itself to continuous and
extraordinary parasitism ; and further
excluding - since the outlines of our
subject must at best be arbitrary -
parasitism by other insects, I shall
limit myself to cases of true disease of an epidemic and uncommonly destruc-
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PSYCHE.
[ January-February IW.
tive character which have been
traced
to fungus or protozoan parasites as
their causes.
Within these limits, I will under-
take to summarize briefly existing
knowledge concerning the leading
forms of contagious insect disease, with such references to foreign literature as may be necessary, and with a fuller
analysis of our scanty American contri-
butions.
Of the protozoan diseases of insects,
fdbrine of the silkworm is the best
known example, - an affection which
has for the insect world a
character so
deadly as quite to overshadow any form
of animal parasitism known among
human kind. It is a plague rapidly
and easily conveyed by contamination
of the food, and exceedingly liable to
hereditary transmission through infec-
tion of the forming egg in the ovary, -
differing in this latter respect from any other insect affection known to me.
First clearly distinguished about
thirty years ago, it has been thoroughly studied in most of its relations, and is now described, as it occurs in the silk- worm, in every general work on silk
culture, a very intelligent summary of
its characters being given, for example, in Maillot's Lemons sw Ze ver d sole du
mUrier. The best detailed description
which I have seen of its symptoms and
histology is that by Quatrefages in his
Etudes sur les maladies actue7Zes du
ver d soie* (p. 229-306), to be read,
*Memoires de ~~cadehie des sciences de Z'lnstitut imp>ial de France, tome 30.
however, in connection with Pasteur's
critical remarks in his Etudes szw Zes
maladies des vers bi soie (v. I, p. gg-
106) .
Its most evident symptoms are, ex-
ternally, the peculiar black specking
ofthe skin, from which it derives its
name, and, internally, the appearance
of similar black spots on the organs
generally; and, in the blood, of the
peculiar spores of parasites (iicorpus-
cles" of Cornalia) to be mentioned
later. Its characteristic pathological
features are
(I) the more or less ex-
tensive disorganization of the gastric
epithelium, within whose cells the
parasites begin their development ;
and '(2) the general invasion of nearly
all the internal tissues by these para-
sites and their spores, which also be-
come abundant in the blood.
At death
the body has a certain elasticity quite
in contrast with the flaccid condition of larvae dead with other forms of con-
tagious disease.
After death it mum-
mifies without decay, and without that
efflorescence of spores especially char- acteristic of muscardine and allied
diseases.
The food of healthy insects may
become infected by the discharges of
diseased larvae, or even, at a consider- erable distance, by the dust of their
excrement.
The "germs" of the dis-
ease may also be introduced by means
of accidental punctures of the skin, as' larvae crawl over each other with claws
soiled with their spore-laden excre-
ment.
Concerning the characteristic para-




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January-February 1888.1 PSTcHE. 5
sites of this disease, an unusual number of conflicting views have been held by
successive writers. Leydig was the first to suggest their affinity with the psoro- sperms of fishes in 1857, but they were
afterwards claimed by botanists and de-
scribed, once as an alga (Panhistofhy-
ton ovatum), by Lebert, and again by
Naegeli, as one of the Schizomycetes
or bacteria (Nosema bornbycis). Even
in so recent and authoritative a work as that by Cornil and Babes, Les &act&
ries, et Zew r6le dans Z'anatomie et
Z7h is to logie
9 a th o logip es des ma la -
dies infectieuses, published in 1885,
this view of Naegeli is taken, and the
spores are classed as bacteria. But,
since the thoro~~h-~oin~ researches of
Balbiani on their life-history, continued from 1867 to 1883, I think that there
can be no longer a reasonable doubt of
their animal nature, or of their agree-
ment in general characters with those
forms now commonly inclu-ded under
the head of s$orozoa,- a parasitic sub-
division of the protozoa, of which Gre-
garina is perhaps the best known type.
The fullest and most satisfactory
account of their very simple life-history is that given by Balbiani three years
ago, in his discussion of the Mcro-
s$oridia in the JournaZ de microgra-
$hie (1883, v. 7, p. 313-323, and p.
404-41 I). It may be thus briefly sum-
marized : -
The minute oval spores, colorless,
highly refractile, homogeneous in ap-
pearance, 4 P long by 2 p. wide, when
swallowed with the food, penetrate in
some way unexplained the cuticle of
the alimentary canal, and, in the cells
of its epithelium, open at one end and
emit their contents, each in a form of
an amoeboid speck of protoplasm.
This grows to a spherical body and,
by a process of internal segmentation
common to the s$orozoa, is soon con-
verted into a mass of spores, each like
the original. These spores every-
where undergo a like development, and
load all the tissues with their products, slowly and gradually arresting all the
functions oflife. Their vitality is tem- porary-Pasteur's experiments showing
that they will not germinate five weeks
after drying out- and the disease is
consequently maintained only by virtue
of its hereditary character.
This microsporidion, or an extremely
similiar one, produces an epizootic dis- ease also in the oak silk-worm (Atta-
cus fernyi'} , in France, in that species, however, being unable to penetrate
beyond the epithelial layer of the intes- tine, and hence not appearing in the
blood or in the tissues at large. Other
forms of microsporidia have been found
in Coccus hes$eridum; in Tipula
@atensis; in Zygaena fiZipendula;
in two orthopterous insects (Decticus
griseus and Gryllus camfestris); in
Emus ole% a coleopterous species ;
in the arachnid JSpeira diadem ; in
the entomostraca, Polyphemus fedicu-
Zus, Simoce~halus vetulus and Chy-
dorm sphaericus ; in the genital tubes
of a nematoid worm ; and even, accord-
ing to Vlacovich, in a colubrine snake
( Coluber carbonarius).
That epizootic attacks are not more




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6 PS2'-c'HEf [ January-February 1888.
frequently caused by them, is doubtless
due to the sparse distribution and the
isolated occurrence of many -of these
species.
Balbiani gives us in his latest work
on this subject the interesting and im-
portant information that he
has often
succeeded in conveying $&brine to
other insects by treating their food
with the dejections of affected silk-
worms. Bombyx neustria he found
more susceptible than the silkworm it-
self, but another bombycid, Li$aris
chrysowhoea, proved wholly i-efi-ac-
tory, seemingly because the cutic~~lar
lining' of the intestine is there unusually thick. Dipterous maggots, larvae of
ants, and the meal worm (Tenebrio
molitor) were also used by him in sim-
ilar experiments, but quite without
result.
I have myself, this year, attempted
to convey $dbrine of the silkworm to
various other species, obtaining my
material for infection from pupae a few
days dead, reared by myself from
worms conspicuously diseased. Larvae
of Telea $olypJzemus, the fall web
worm (Hy/Jzantria textor'), the com-
mon cabbage worm (Pieris rqae),
the caterpillars of the thistle butterfly (Pyrameis cardut}, various species of
cutworms (noctuidae), and both adult
and larval Dowhora, were infected,
sometimes by way of the food, sometimes
by puncturing- the skin, but in every
case without positive success. I ob-
tained, it is true, one curious
my specimens of Melolontha, Pieris,
and Telea all developed unmistakably
the characteristic specks and spots of
@brine subsequent to infection, but the
most critical and protracted search of
their fluids and tissues failed to discover the slightest evidence of parasitism,-a
fact which I could only explain on the
hypothesis that the marks on the skin
were due to the direct action of the
material ingested or injected from the
silkworm, and not to any morbific sub-
stance elaborated within the bodies of
the insects experimented upon.
You will perhaps allow me to add
an item upon the possible economic
applications of this disease. There is
not the slightest probability that the
@oroma can be artificially cultivated
outside the bodies of the animals which
they may infest ; neither have we yet
any sufficient proof that forms nor-
mally occurring in one species will
multiply or permanently maintain them-
selves in any other. We are conse-
quently limited, practically, to artificial measures for developing and accelerat-
ing this disease wherever it may be
found, and to more careful and ex-
tended experiments for its transfer
from the silkworm to related noxious
species.
The American literature of $brine
is an absolute blank, not a single item
of new information concerning it hav-
ing been published on this side of the
water, nor a single observation of its
occurrence in this country in any other
form than in the common silkworm hav-
ing been placed on record, as far as I
can find.
The notable fungous diseases of in-




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January-Febuary 18%.
sects are readily divisible into two
principal groups ; schizomycoses, pro-
duced by bacteria, and hyphomycoses,
due to fungi which form a more or less
evident mycelium of cylindrical threads
(Hy$homycetes and Pyrenomycetes) .
These are roughly distinguishable in two important particulars : (I) The bacteria invade the body from within, by way
of the alimentary canal ; and the thread fungi penetrate from without through
the skin or spiracles ; (2) Death from
a schizomycosis is followed by
rapid
decay, which soon reduces the tissues
to a putrid fluid ; while after death
from a hyphomycosis the often flaccid
body hardens and mummifies without
decay, usually swelling to more than
its usual size, and frequently becoming
covered with a flour-like efflorescence
of spores or spore-like bodies. These
last characters distinguish the hypho-
mycoses from the ~M~z'nes,-the body
mummifying in the latter, but shrivel-
ing at the same time and never covering
itself with spores, unless with those of a common mould of -post modem devel-
opment. Further, the $brine mummy
contains only the minute oval spores of
the parasite, while that of a hyphomy-
cosis contains either a mass of mycelial threads or large thick-walled, spherical spores,-the lasting spores of an &to-
mofhthora or, possibly, both spores
and mycelium together.
Muscardine, longest-lmown of insect
diseases - often a cause of astounding
destruction both to domesticated and
native species, and by far the most
promising natural agent for the artificial restriction of noxious insects-is caused by a number of fungous forms-Botry-
t$ Isa* Cordycefi-several species
of each-the classification and ontogenet- ic relations of which are not yet wholly settled. Some Botrytis forms have
been unmistakably connected with
some Isarias as an earlier developmen-
tal stage, and other Botrytis forms
have been as clearly connected with
Cordyce* while all the entomochthon-
ous Isarias are classed by Cooke as Cor- dyceps in a vegetative stage ; but, on
the other hand, the longest-knovn
Botrytis-tha t of silkworm m uscardine
-has never been followed, that I can
find, beyond an Isaria stage, and other
species are in doubt.
Hence, as one
consciously beyond the limits of his prop- er territory, I will touch these dubious and contested matters in the lightest
way, endeavoring only to get at and
apply the very important entomological
data which the cryptogamic botanists
have incidentally worked up for us.
Generalizing the life histories of
these fungi and their modes of attack
on the living insect structu~e (which
is for most of them the indispensable
substratum of their later growth), we
may say that they invade their 110s-ts
from without-or, sometimes, by the
spiracles and tracheae - but never, SO
far as known, by way of the alimentary
canal ; that their minute spoies gein~i- nate on the surface and send inward
through the cuticleslender tl21 t a(': v hich grow through the body wall sr.d thtn
separate into small single ce 11s - cyli1;- drical conidia--that these p; sh tvay-
where, growing, dividing 3rd rgain
dividing as they go, derivirg tlit ir i;niri-



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8 PS2TH.L?. [ January-February 1888.
merit from the tissues and the blood, ren- dering the latter distinctly acid, as a
rule ;-and that death slowly supervenes* After this event, these conidia elon-
gate, producing mycelial threads with
which the body soon becomes stiffened
and distended. Then they shoot up-
ward through the skin a forest of little stems, fertile hyphae, which may
branch much or little, according to the
form, often covering the dead insect
with a microscropic pile
like that of
velvet. From these hyphae other spore-
like bodies-spherical conidia-are
variously budded off, borne on the stems and branches singly, in heaps, in neck-
lace strings, forming finally a dense
powdery layer of cell-like particles,
white or greenish, often excessively deli- cate and minute-in the silk-worm
species not more than z p. or 3 p. in
diameter. Here the development may
stop-as it usually seems to do, indeed,
ill the best known form,-the Botrytis
iassiana of the silkworm inuscardine-
the conidia detached germinating else-
'where, if they fall on favorable condi- tions, and directly reproducing this
lowly vegetative stage. Under other
conditions -sometimes on other insects
-(the silkworm fungus on Gastro-pa-
cha rubi, for examplehthe fertile
hyphae, instead of forming an infinites- imal surface pile, spring up in strong
club-shaped tufts, bearing conidia on
their threads-this being the so-called
Isaria stage. Finally, the mycelium
within the dead body of the insect may
thicken, forming one or more compact
masses, from which a strong stipe may
spring up-like that from the mouth of
the white grub (Lachnoste~za fusca) ,
of which all have seen examples, or at
least illustrations-and at the end of
this stipe, immersed in a head more or
less distinct, another form of spores-
ascospores or thecaspores - may be
borne by a more complicated apparatus
of reproduction. This is the final repro- ductive stage -the Cordyce-psÌÔbes
illustrated by our Cordycejs meld
onthae* of the common white grubs.
These ascospores carry the fungus
species over winter; but seem not
always
necessary to this end, as the
spherical conidia of the Botrytis stage
of the silkworm muscardine have been
known to retain their vitality more than a year.
All these reproductive bodies-asco-
spores and conidia - of Cordyce$s,
Botrytis, and Isaria, have germinated