NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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PART II.
I. OBSERVATIONS ON THE CHARACTERS AND HABITS OF ANOPHELES
MOSQUITOES AND THEIR LARVÆ.
A
T an early stage in our investigations it became evident that the number of
species of anopheles mosquitoes that exist in India was much larger than
had been anticipated, and that the descriptions of the anopheles which had been
recorded, both from India and from other countries, were so inadequate as to
render their identification difficult, if not, in many cases, impossible.
Until recently, however, this was a matter of small importance from a
medical point of view, because the fact that some species of anopheles might
be more important than others in connection with the transmission of malaria
had not been generally recognised by observers, the majority of whom had
therefore been content with a study of the anopheles genus as a whole, without
recognising the importance of a study of each species separately.
Our observations, however, have shown that the different species of ano-
pheles differ very markedly not only in their habits and breeding places, but
also in their relation to the prevalence of malaria in different places, so much
so, indeed, that whereas the presence of one species appears to be almost
always associated with a considerable amount of malaria, the presence of another
species appears to exert no influence whatever on the prevalence of the disease.
For these reasons, if for no other, it is certainly of the greatest practical
importance to be able to identify the different species of anopheles correctly,
and to study the habits, life history, and pathological significance of each
species separately.
Unfortunately, however, the classification of anopheles is at present in an
early stage, and the difficulties of identification of any particular species are
enhanced by the fact that a considerable number of anopheles mosquitoes have
been lately described as new species, which differ in only very minute respects
from existing species.
It is doubtful whether all these so called new species are really distinct,
and it becomes necessary to enquire whether the characters by which
e n t o m o l - o g i s t s s e p a r a t e c l o s e l y a l l i e d s p e c i e s a r e s u f f i c i e n t l y c o n s t a n t t o b e r e l i e d u p o n ,
or whether the large number of species which differ but slightly from each
other are in reality only variations of one or two existing species.
A. Culicifacies (Giles), for example, may be taken as a type of the group
which I have called the group of "small brown anopheles with unhanded legs."
The five following mosquitoes in India which resemble A. Culicifacies closely
have been described as new and distinct species :—A. Fluviatilis (re-named
A. Christophersi by Mr. Theobold), A. Listonii (Giles), A. Listoni (Liston),
A. Indica (Theobald), and A. Feyporiensis, as well as the African species
E 2
E 2
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A. Funestus, A. Minimus, A. Rhodesiensis, and the Italian species A. Su p e r p i c t us
(Grassi).
We thus have at least nine closely allied species of this one group which,
although considered by entomologists to be quite distinct, are nevertheless
very difficult to identify and to separate from each other. The points relied
upon by Mr. Theobald* for separating all the above mosquitoes from each other
as distinct species are as follow :—
(1) Differences in the characters of the wing markings.
(2) Differences in the relative position of the transverse veins of the wings.
(3) Differences in the character of the male genitalia and of the fore
ungues of the male.
(1) The wing markings of the nine species mentioned above are admitted
to be almost identical, and in forming an estimate of the value to be attached
to slight differences in wing markings we must remember that if only a small
number of any one species of anopheles are examined, considerable variations
in the wing markings will be found. Indeed, it is quite possible that in some
species at any rate a seasonal variation in these markings may occur. These
variations have not previously been taken note of by entomologists in England,
owing to the limited supply of specimens to which they have access, and as
several of the so-called new species (e.g., A. Indica, and A. Minimus) have been
described from only a single specimen, it might easily happen that such a spe-
cimen was a variation from the type of an already described species.
Some of the variations that occur in wing markings will be mentioned in
the notes on the different species, and on the whole I am of opinion that such
slight differences as are recorded between the wing markings of the species
enumerated above, are of little or no value as a means of separating them into
distinct species.
(2) Great stress is laid by Mr. Theobald on the value of the relative posi-
tions of the cross veins of the wings as a means of distinguishing species, and
a reference to the monograph will show that this is the chief character by which
he separates nearly all the species of this group. Thus of A. Funestus he
says, "It is clearly related to A. Rhodesiensis and A. Superpictus, but it can
be readily told by the positions of the cross veins." Of A. Indica he says,
"It is seemingly related to Grassi's A. Superpictus, the wings, etc., being
almost identical......... FromA . F u n e s t u s
i t d i f f e r s i n t h e p o s i t i o n o f t h e c r o s s
veins." Of A. Rhodesiensis he says, "At first sight one would take it for
Grassi's A. Superpictus, but the position of the cross veins at once separates
them." Of A. Minimus he says, "Very like A. Funestus, A. Rhodesiensis, and
A. Superpictus, but from Funestus it can be at once told by the disposition of
* "A Monograph of the Culicidæ of the World,"by F. V. Theobald, Esq., M.A., F.E.S. British
Museum (Natural History), 1901.
29
the cross veins and the darker scaled wings, from Rhodesiensis by the orna-
mented wing fringe and the cross veins, and from Superpictus by its darker hue
and ornamented fringe."
Before, however, it would be possible to place such reliance as this on the
value of this character as a means of distinguishing species, it must be shown
that the relative position of the cross veins to each other in any species is
always constant. If it can be shown that in any species the relative position of
the cross veins is not constant, then of course its value as a distinguishing
character is very much lessened.
In order, therefore, to test this point I examined a number of specimens of two
well known species of anopheles, viz., A. Culicifacies and A. Rossii, to ascertain
whether any marked variations occur in the relative positions of the cross veins.
The results of my examinations are given in the diagrams below (Fig. 8),
and it will be seen that in the same batch of both these species very marked
variations are found, so that, in my opinion, this character does not appear to
be sufficiently constant to enable it to be used for the separation of species.
Fig. 8.—To show variation in the relative positions of the transverse veins of the wing.
a,b, c, d. A. Rossi i (20 examined). | e, f, g. A. Culicifacies (15 exami ned).
The apex of the wing is towards the right side in each figure.
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(3) The other characters upon which Mr. Theobald relies, depend upon
differences in the males. If, however, two species which can be separated from
each other only by the characters of the males, occur in the same locality, we
shall be unable to say to which species any female specimens we may catch
belong, and if only one of the two species happens to be carrying malaria at the
time, we shall be unable to say which species is to blame.
None of the above methods of separating closely allied species therefore
can be considered entirely satisfactory, and for the most part they are liable
to lead us into the error of considering as a new species any particular specimen
which varies slightly from the type of an already known species. A good deal of
our time has been spent on the question of the distinctions between the different
Indian species of anopheles, and it appears probable that our work on the
differences between the eggs and larvæ of different species ofa n o p h e l e s ,
w i l l
point the way to a definite and final system of classification ; for it will, I
think, be admitted by all, that if a definite difference between the eggs or
larvæ of any two species of anopheles can be shown to exist these species must
be certainly distinct—whatever the characters of the adult may be.
I purpose, therefore, in the following notes on the Indian species, to include
as distinct species only those in which a difference between the eggs or larvæ
can be shown to exist.
We have found that, to a certain extent, the various species can be divided
into groups based on the following characteristics :—
(i) the general appearance of the adult insect ;
(ii) the characters of the eggs and larvæ ;
(iii) the habits of the adult insects and their larvæ.
I have therefore arranged the different species in four groups as follows :-
GROUP I.
I. A. Fluviatilis (sp. n.)
2. A. Jeyporiensis (sp. n.)
Indian species.
3. A. Culicifacies (Giles)
4. A. Immaculata (sp. n.)
[5. A. Superpictus (Grassi) ... Italian species.]
[6. A. Funestus (Giles)
... African species.]
and its allies.
Group characteristics.—The adults of this group are characterised by their
small size, by their being generally of a dull brown colour, and by the fact that
their legs are uniformly coloured without any banding at the joints.
The eggs are characterised by their small size, by the upper surface being
very narrow, and by the lateral floats being near together on this surface. The
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larvœ are characterised by possessing a pair of well developed palmate hairs
on all the abdominal segments and on the thorax.
In habits this group is chiefly characterised by the fact that they breed
almost exclusively in running water. They may in fact be called the "stream
breeding mosquitoes." They are further characterised by the very important
fact that their members appear to be the chief malaria-carrying species in India
and Africa.
I. A. Fluviatilis, sp. n.
Synonyms—A. Christophersi (Theobald), A. Listoni (Liston), Fig. 9.
Fig. 9.—Wing of A. Fluviatilis.
This mosquito has been re-named A. Christophersi (though not as yet
described) by Mr. Theobald. It has also been accurately described by Cap-
- t a i n G l e n - L i s t o n , I . M . S . , i n t h eI
n d i a n M e d i c a l G a z e t t e
( O c t . 1 9 0 1 ) u n d e r
the name A. Listoni. The description of A. Listonii (Giles) in Colonel Giles'
Hand-book of Mosquitoes* does not, I think, apply to this species, and his des--
cription is possibly from a specimen of A. Culicifacies.
It is a small mosquito of a generally brown colour. The palpi are marked
with three white bands—one broad band which includes the tips of the palps
and two narrow bands nearer the base.
The legs are unbanded.
The wing markings are detailed in the diagram, and it is therefore not
necessary to enumerate the number of black and white spots on each vein here.
The following points should be specially noted : (1) The third longitudinal vein
is white scaled nearly throughout its length. (2) Including the white apical
spot there are six white patches on the wing fringe opposite the terminations of
the longitudinal veins, except the sixth.
The wing markings in this species may, however, vary a good deal, and it
may even be possible to find specimens in which the third longitudinal vein is
black scaled almost throughout, thus making it difficult to distinguish this
species from A. Culicifacies.
* A Hand-book of the Gnats or Mosquitoes, 2nd edition, 1902. John Bale Sons and Danielsson.
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One specimen which I received from Sukchar, which might possibly
deserve to be considered a new species, has two equal broad white bands and
one narrow one on the palpi, but in the absence of more specimens and of
its larvæ it is, for the present, best placed as a variety only.
The adults can be distinguished from the other members of this group by
a careful comparison of the wing markings (see figures).
Characters of larva.—These are shown in the drawing (Fig. 10). The
"Frontal hairs" are simple and unbranched. The "Palmate hairs" occur
on all the abdominal segments and very prominently on the thorax. The ter--
minal filament of each leaflet is short.
The "head pattern "is shown in the figure.
Fig. 10.—Larval characters of A. Fluviafilis.
Habitat and observations.—A. Fluviatilis was first found by us in the Duars
where it was present in fair numbers in the native huts on the tea plantations.
We have also found it in the Central Provinces (Nagpur) and in the Jeypur
State. It has also been found in the Berars (Liston) and in the Goa Pro--
vince (Aitken), so it appears to be fairly widely distributed in India. It does
not, I think, occur in the Punjab, but it has lately been found in Kashmir.
We have usually found its larvæ in clear streamlets with grassy edges.
This species is the carrier of malaria in the Duars (6 . 25% infected with
sporozoites at the time of examination). Experimentally we have proved that
the parasites of simple Tertian, Quartan, and malignant Tertian will develop in it.
2. A. Jeyporiensis, sp. n.
Fig. II.—Wing of A. Jeyporiensis.
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This species resembles the preceding one very closely in the adult stage,
but the differences of the larvæ at once separates them as distinct species.
The markings of the palpi are the same as those of A. Fluviatilis and the
legs are unbanded. The wing markings are shown in the figure, and it will be
seen that they differ in certain points from those of A. Fluviatilis, thus
A. Feyporiensis has three black scaled areas on the sixth longitudinal vein
and seven pale patches on the wing fringe (including the apical one), whereas
A. Fluviatilis has only two dark spots on the sixth longitudinal vein and only
six white patches on the wing fringe.
Characters of the larva.—These are shown in the figure (Fig. 12.). Both
the median and external frontal hairs are thickly branched, whereas those of
A. Fluviatilis and A. Culicifacies are simple.
F i g . 1 2 . — L a r v a l c h a r a c t e r s o f
A .
F e y p o r i e n s i s .
Habitat.—The Jeypur State, the Central Provinces (Nagpur).
3. A. Culicifacies (Giles).
Fig. 13.—Wing of A. Culicifacies.
This mosquito has probably been described by Mr. Theobald under the
name A. Indica. Colonel Giles' description of A. Listonii is also possibly taken
from a specimen of this species.
The palpi resemble those of the other members of this group. The legs may
F
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be described as unbanded, although the apices of some of the segments may
occasionally show small white markings.
The wing markings are shown in the diagram. It should be noted that the
third longitudinal vein is black scaled throughout its length, and that, includ
- i n g t h e w h i t e a p i c a l s p o t , t h e r e a r e o n l y t h r e e w h i t e p a t c h e s o n t h e w i n g f r i n g e .
Characters of the larva (Fig. 14).—Both the median and external frontal
hairs are simple and unbranched. There is a pair of palmate hairs on all
the abdominal segments and on the thorax, but the thoracic pair are not so well
developed as in A. Fluviatilis or A. Feyporiensis, and the terminal filament
of each leaflet is longer than in either of those larvæ.
The head pattern also differs from that of both the above larvæ (see
figure).
Fig. 14.—Larval characters of A. Culicifacies.
Habitat and observations.—This mosquito is widely distributed in India.
We have found it in the Punjab (Lahore, Mian Mir, etc.), Madras and several
places on the East Coast (Ennur, Armagaon, etc.), the Central Provinces (Nag--
pur, etc.) and in the Jeypur State. It has also been recorded from the Berars
(Ellichpur) and from other parts of India.
In the Punjab it appears to have no special seasonal prevalence, and its
larvæ can be found in the irrigation canals throughout the year. The adults
are found in the houses as soon as the weather become swarm enough for the
larvæ to develop. Its prevalence in fact coincides rather with the changes
which occur in the flow or stoppage of the irrigation canals (in which it breeds
almost exclusively in the Punjab), than with any particular season.
At Ennur, near Madras, I found it breeding chiefly in the clear pools by the
side of the railway "borrow pits," and it has also been found breeding in
pools in the bed of a river.
The adults are peculiar in that they do not adopt the characteristic attitude
of other anopheles when resting on a wall or roof, but sit more or less like a
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culex with the body parallel to the wall and the back hunched (the proboscis,
head and body not being in one line). For this reason it is easily mistaken
for a culex. We have proved this species to be the malaria carrier in Mian
Mir and in Ennur (Madras). Experimentally also we have shown that the
parasites of the three varieties of malarial fever can readily develop in this
species.
4. A. Immaculata, sp. n.
This is a small mosquito resembling in general characters the other mem-
bers of this group, except that its wings are entirely unspotted. It is apparently
a rare species, for we only obtained one specimen from Ennur. Several speci-
mens, however, have since been obtained by Mr. Aitken from the Goa Province.
The legs are unbanded but are darker coloured towards the tarsi.
We have not yet obtained the larva of this species, but I think it is
rightly placed in this group, on the grounds of its general appearance and
unbanded legs.
GROUP II.
1. A. Barbirostris (Van der Wul p).
2. A. Sinensis and its allied species A. Nigerrimus (Giles).
3. A. Gigas (Giles).
Group characteristics.—The adults of this group are characterised by their
large size and by their being very dark in colour. It may therefore be called
the group of large black anopheles. The eggs are characterised by the fact
that the lower surface is marked off by light coloured lines into polygonal
areas, by the fact that their upper surface is very narrow, and that the floats
do not reach the margin of the upper surface.
The larvœ are characterised by the fact that the external pair of frontal
hairs are developed into large and prominent " cocades" and the antennæ
possess a large branching hair on their inner surface which is absent in the
larvæ of the other groups.
In habits this group may well be termed the group of "wild" mosquitoes.
The adults are very seldom found in houses, and the larvæ are usually found
in deep natural weed-grown pools in the jungle or under trees.
I. A. Barbirostris (Van der Wulp).
This mosquito is characterised by its very large size and black colour.
Its wing veins are covered for the most part with dense black scales, interrupted
on the costa by two small white spots. There is almost always one pale patch
on the wing fringe opposite the lower branch of the fifth longitudinal vein. The
F 2
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thick black densely-scaled palpi which are unbanded will serve to distinguish
this species from any other of the Indian anopheles.
Characters of the larva.—Its larvæ are easily identified by the naked
eye, frbm their large size, black colour, and the fact that when resting at the
surface, they are nearly always bent more or less in the shape of an S. Their
microscopical characters are shown in the figure. The external frontal hairs
are converted into a large tuft which is easily seen lying over the so-called
whorl-organs or " shaving brushes," and on the inner side of each antenna is a
large branching hair.
Fig. 15.—Larval characters of A. Barbirostris.
Habitat and observations.—We have found this mosquito in several places
in the plains of Bengal, in the Central Provinces, and in the Punjab. It occurs
also in Assam and in the United Provinces as well as in several places in the
south of India.
Its larvæ are found almost exclusively in deep dark pools with much acqua-
tic vegetation, and they can only be caught separately one at a time by dipping
with a cup or tin. In Lahore they occur in the lily ponds in the public gardens.
Experimentally we have proved that the zygotes and sporozoites of the
human malaria parasites can develope in this species, but it is doubtful whether
it ever acts as a carrier in nature owing to its rarity in houses.
2. A. Sinensis, sub.-species Nigerrimus.
Fig. 16.—Wing of A Nigcrrim us.
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This mosquito may be considered as a type of several species of anopheles
which are regarded by Mr. Theobald as distinct, viz. :—
A. Sinensis (Wiedmann), A. Indiensis (Theobald), A. Annularis (Van
der Wu1p), A. Psuedopictus (Grassi).
Colonel Giles, however, regards all these as one moderately variable species,
and with this view I am inclined to agree.
The palpi of A. Nigerrimus are marked with four white bands, the ter-
minal of which includes the tip. The tibiæ and tarsal segments of the legs
are apically white banded.
The chief wing markings are shown in the figure, but a few additional white
spots occur on some of the longitudinal veins.
The characters of the larva resemble those of the larva of A. Barbirostris in
the possession of " cocades " and of the branching hair on the antenna. Pal-
mate hairs are borne by the second, third, fourth, fifth, sixth, and seventh
abdominal segments. Their general shape resembles that of the palmate hairs
of A. Barbirostris (Fig. 15).
Habitat and observations.—This is a fairly common species in the out-
skirts of Calcutta and in other places in the plains of Bengal. It also occurs
in Travancore (Southern India) and in the Madras Presidency (outskirts of,
Madras).
We have not carried out any feeding experiments on favourable cases of
malaria with this species.
I have shown that the embryos of Filaria Sanguinis Hominis can develop
in this species, as well as in A. Rossii, and this observation has been lately
confirmed as regards A. Nigerrimus, in Calcutta.
At Jalpaiguri (Bengal) we found its larvæ chiefly in deep natural pools
on swampy ground at some distance from houses.
3. A. Gigas (Giles).
We have not yet met with this new species which is, however, probably
related to the above mosquitoes. Colonel Giles states that it has up to the
present been found only at an elevation of 5,000 feet in Conoor (Madras Presi-
dency).
GROUP III.
1. A. Fuliginosus (Giles). 3, A. Theobaldi (Giles).
2. A.Jamesii (Theobald).
4. A. Lindesayii (Giles).
Group characters.—The mosquitoes of this group may be considered as
intermediate between those of Group 11 and those of Group IV.
The adults are of medium size and vary in colour from very dark (A.
Jamesii, A. Lindesayii) to a light grey or brown (A. Fuliginosus, A. Theobaldi).
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The larvae are characterised by the fact that although the external frontal
hairs are very distinctly branched (except A. Lindesayii), they are not developed
into the actual cocades which characterise the larvæ of GroupI I .
T h e y a r e ,
in fact, intermediate between the larvæ of Group II and Group IV in which the
frontal hairs are simple and unbranched.
In habits the members of this group are intermediate between the typically
"wild " mosquitoes of Group II, and the typically "domestic" mosquitoes of
Group IV.
They are all occasionally found in houses in small numbers, and excep-
tionally A. Fuliginosus may be found in stables and out-houses in large
numbers.
The breeding places of A. Fuliginosus and A. Jamesii are similar to those
of Group II, except that they appear to prefer shallower, clearer water with
m o r e s u n l i g h t .
I. A. Fuliginosus (Giles).