[Illustration: LARVA OF PLATYPSYLLUS CASTORIS--DORSAL VIEW.]
[Illustration: PLATYPSYLLUS CASTORIS.]
Where the characters of the image have been so often described, it is
unnecessary to refer to them in detail, and I will only call attention to
the more striking structural features and to some omissions by, or
differences between, previous authors. A glance at the illustrations which
I have prepared will show the prevailing characteristics of this
interesting creature, its general ovoid and flattened form, and more
particularly the flattened semicircular head. Dorsally, we notice the
rather prominent occiput fringed behind with short and broad depressed
spines or teeth which form a sort of comb, the prothorax trapezoidal and
but very slightly curved, with side margins strongly grooved. There is a
very distinct scutellem, and the two elytra are rounded at the tip and
without venation. Hind wings and eyes are both wanting. The abdomen shows
five segments, each with a row of depressed bristles.
On the ventral surface we find among the more curious characteristics,
first the antennae; these were originally described by Westwood as
three-jointed, the club being annulated. Le Conte could not distinctly
make out the number of annular joints upon this club, though he thought he
detected seven, which made nine joints to the whole antenna. The club is
received in the deep cup-shaped excavation of the second joint. Horn
thought he detected a division of the second joint, and resolved but six
segments in the club, making also nine joints to the whole antenna, but in
a somewhat different fashion from Le Conte. Westwood's figure shows eight
annuli to the club. He failed to find any trace of the mandibles, but Le
Conte described them as small, flat, subquadrate, with the inner side
deeply crenulate, and resembling those of _Corylophus_; the stipes well
developed, and biarticulate. Horn could not entirely make out the
mandibles as described by Le Conte, and rather concluded that what Le
Conte described is really one of the granules which occur behind the
labrum. He considered that the piece could hardly be even an aborted
mandible, because of its diminutive size.
[Illustration: YOUNG LARVA.]
What all authors have agreed in calling the mentum is very noticeable,
being large and broad, and trilobed behind. The maxillae are strong, with
complicated stipes and with two flat, thin lobes, the inner one smaller
than the outer and rounded at the tip, both lobes being ciliate. The
maxillary palpi are four-jointed, the labial palpi three-jointed. The
prosternum is very large, subtriangular, concealing the insertion of the
coxae, and extending over the front part of the mesosternum, as does this
over the front of the metasternum. Six ventral segments of the abdomen are
visible behind the posterior coxae, which conceal two and the base of a
third. The coxae are flat and not at all prominent. The legs are
characterized by broad and flattened tibiae and femora, and the strong
spines with which they are armed. The tarsi are five-jointed, the front
and middle pair with a row of claviform membraneous appendages each side,
which Le Conte found only in the male.
American entomologists have been satisfied to follow Le Conte and Horn as
to the position of Platypsyllus. Yet with such diversity of opinion on the
subject among high European authorities, the importance of a knowledge of
the adolescent states has been recognized, as the character of either the
larva or pupa would settle the question.
During a stay at West Point, Neb., in October, 1886, I learned from one of
my agents, Mr. Lawrence Bruner, that there was a beaver in a creek not far
from that point, and I at once made arrangements for him to trap the
beaver, and to look particularly for living specimens of Platypsyllus on
the skin, and especially the earlier stages. He succeeded in capturing the
beaver and sent me some fifteen specimens of the larva and also some
imagos, but neither eggs nor pupae were found. A glance at the larva
satisfied me at once of its coleopterous nature; but as we have, waiting
to be worked up and published, an _embarras de richesses entomologiques_
in the collections of the National Museum, and as circumstances largely
decide the precedence, I should probably not have called attention to this
larva for some time, had it not been that at the last monthly meeting of
the Entomological Society of Washington, Dr. Horn, who was present,
announced the finding, the present spring, by one of his correspondents,
of this very larva, and exhibited a specimen. Some points about it, and
especially the position of the spiracles, being yet rather obscure in his
mind, he requested me to examine my material, which I have thus been led
to do. I have made a figure of this larva which will sufficiently indicate
its nature.
The general form of the trophi, and particularly the anal cerci, fully
settle the disputed point, and remove this insect completely from the
Mallophaga (none of which possess them), and confirm its position in the
Clavicorn series of the Coleoptera. Yet in the larva, as in the imago, the
effects of its parasitic life are shown in certain modifications which
approach the running section of the Mallophaga. Without going into details
I may say that, besides its general and more decided coleopterological
features, this larva is distinguished by the shortness and stoutness of
its legs, by the size and stoutness of the antennae, by the stiff and long
depressed hairs on the dorsal and more particularly on the ventral
surface, and by the dorsal position of the abdominal spiracles, all
characters approaching the Mallophaga. The first pair of spiracles is
lateral, and may be said to be mesothoracic, being placed on the
mesothoracic joint, but on a distinct fold. The eight abdominal spiracles
are placed on the sides of the dorsum, and in this respect recall the
parasitic triungulin of the meloid larvae. The mandibles are barely
corneous, and they are more elongate and curved in the younger than in the
older larva, while the legs are also relatively stouter, more curved, and
with a much longer and sharper claw in the younger larva, which seems well
fitted for grasping the hairs of its host.
There can no longer be any doubt, therefore, about the true position of
Platypsyllus. The eggs will probably be found attached in some way to the
hairs of the animal they are laid on, much as they are in Mallophaga, and
the pupa is probably formed in the nests of the host, and not upon the
skin, which will explain the reason for its not occurring with the larva
and imago upon the beaver, either in the case of my specimens or those of
Dr. Horn.
The greatest resemblance of Platypsyllus in the imago state to the
Mallophaga is found in the spinous comb on the hind border of the occiput,
the arrangement of the spines on the abdomen, and the superficial antennal
structure, but particularly in the broad trilobed mentum. All of the other
characteristics are readily referable to the Coleoptera, though, as Le
Conte pointed out, they are composite, recalling in the antennae the
Grynidae, in the pronotum the Silphidae, in the mesosternum Limulodes, in
the elytra the Staphilindae, in the legs the Anisotomidae, and in the
mandibles the Corylophidae. The scutellum and the five-jointed tarsi at
once remove it from Mallophaga, and it is a wonder that Le Conte and Horn
have not more fully insisted on this fact. The trophi are very
complicated, and there are various details of structure not noticed or not
mentioned by any of the writers upon the subject hitherto.
I have been led to very carefully examine the imago, and the more closely
I have done so, the more completely I realize the accuracy of Le Conte's
original work. The mandibles are visible or not, according as they are
exposed or withdrawn, and their existence may depend on the sex, as, so
far as my material justifies conclusion, they are visible in the male
only. Where found they correspond to Le Conte's description. Even in the
larva they are weak and of doubtful service in mastication, while in the
imago they are, as is also the labrum, quite rudimentary, which fact
hardly justifies us, however, in arguing their non-existence.
As confirmatory of the affinities of Platypsyllus, as here proved, it may
be mentioned that _Leptinus testaceous_ Mull., the only species of its
genus, is known to be parasitic on mice, as it has been found upon them in
Philadelphia by Dr. Jno. A. Ryder, and I have taken it in the nests of a
common field mouse near Washington. But still more interesting is the fact
that _Leptinillus validus_ Horn (also the only species of its genus) is an
associate parasite of Platypsyllus on the beaver, a number of both having
been taken by one of my agents, Mr. A. Koebele, in San Francisco, from
beaver skins brought from Alaska.
[Illustration: LARVA OF PLATYPSYLLUS CASTORIS.]
In reference to the classificatory value that should be attached to an
aberrant type like this, I have already expressed my opinion in a paper on
Megathymus, a Lepidopteron that connects in many ways the two great
divisions of butterflies and moths, published in the Transactions of the
Academy of Sciences of St. Louis, volume iii., 1876, and will take the
liberty of reading a few passages therefrom:
"Between all classificatory divisions, from variety to kingdom,
the separating lines we draw get more and more broken in
proportion as our knowledge of forms, past and present,
increases. Every step in advance toward a true conception of the
relations of animals brings the different groups closer
together, until at last we perceive an almost continuous chain.
Even the older naturalists had an appreciation of this fact.
Linnaeus' noted dictum, '_Natura saltus non facit_' implies it;
and Kirby and Spence justly observe that 'it appears to be the
opinion of most modern physiologists that the series of
affinities in nature is a concatenation or continuous series;
and that though an hiatus is here and there observable, this has
been caused either by the annihilation of some original group or
species, or that the objects required to fill it up are still in
existence but have not yet been discovered.'"
"Modern naturalists find in this more or less gradual blending
their strongest arguments in favor of community of descent; and
speculation as to the origin, or outcome rather, in the near
present or remote past of existing forms is naturally and very
generally indulged, even by those who a few years back were more
inclined to ridicule than accept Darwinian doctrine. Shall we
then say that the old divisions must be discarded because not
absolute? As well might we argue for the abolition of the four
seasons because they differ with the latitude, or because they
gradually blend into each other. Entomologists will always speak
of moths and butterflies, howsoever arbitrary the groups may
come to be looked upon, or however numerous the intermediate
gradations."
"Families should, I think, be made as comprehensive as possible,
and not unduly multiplied; and in considering aberrant forms,
the objects of classification are best subserved by retaining
them in whatever division can claim the balance of characters.
It is better to widen than to restrict in the higher groups. Le
Conte does better service in bringing Platypsylla among the
Coleoptera than does Westwood in creating a new order--
Achreioptera--for it. Phylloxera, in Homoptera, is much more
wisely retained in the Aphididae than made the type of a new
family."
Platypsyllus, therefore, is a good Coleopteron, and in all the characters
in which it so strongly approaches the Mallophaga it offers merely an
illustration of modification due to food habit and environment. In this
particular it is, however, of very great interest as one of the most
striking illustrations we have of variation in similar lines through the
influence of purely external or dynamical conditions, and where genetic
connection and heredity play no part whatever. It is at the same time
interesting because of its synthetic characteristics, being evidently an
ancient type from which we get a very good idea of the connection in the
past of some of the present well-defined orders of insects.
Westwood, though now an octogenarian, may safely be called England's most
eminent entomologist by virtue of the character and volume of the work
which he has accomplished. Dr. Le Conte was, _facile princeps_, America's
leading coleopterist. I do not know that any greater tribute could be
added to the sound judgment and deep knowledge possessed by that late
distinguished member of the Academy than the confirmation of his views as
opposed to the views of Westwood and other European authorities which the
discovery of this larva now gives us.