the remarkable cases or sacs the larvæ inhabit, the Germans call
_sack--träger_, before noticed[771], have been supposed to have the
faculty of producing fertile eggs without any sexual intercourse;
and various observers, after taking great pains, appeared to have
satisfactorily proved the fact; so that some doubted whether these
insects produced any males at all[772]. The enigma was at length
explained by the accurate Von Scheven. At first his experiments were
attended with the same result as those of his predecessors; but upon
making them more carefully, and separating what he conceived to be
the female from the male pupæ, he ascertained not only the existence
of a _female_ in the species he examined (_Psyche vestita_), but that
when thus secluded she laid _barren_ eggs; evidently proving that
in the contrary instances above alluded to, an unperceived sexual
intercourse must have taken place[773]. Though he thus ascertained that
these insects do not in this respect deviate from the general rule,
he remarked or confirmed several facts in their economy sufficiently
anomalous and striking;--as that the female is not only without wings,
but with scarcely any feature of a _moth_, much more closely resembling
a _caterpillar_; and that in ordinary circumstances she never attempts
to leave the pupa-case in which she has been disclosed, but that being
there impregnated by the male, she there also, apparently after the
manner of the female _Cocci_, deposits her eggs, which hatching produce
young larvæ that make their way out of the case, and thus seem to
originate without maternal interference[774].
But the most remarkable fact bearing upon this head, though as relating
to a _viviparous_ insect it does not strictly belong to it, is the
impregnation of the female _Aphides_, or plant-lice, before alluded
to[775]. If you take a young female _Aphis_ at the moment of its
birth, and rigorously seclude it from all intercourse with its kind,
only providing it with proper food, it will produce a brood of young
ones: and not only this; but if one of these be treated in the same
way, a similar result will ensue, and so on, at least to the _fifth_
generation!! to which period Bonnet, who first made an accurate
series of observations on this almost miraculous fact, successfully
carried his experiments, till the approach of winter and the want of
proper food forced him to desist[776]; and Lyonet extended it still
further[777]. It is now generally admitted as an incontestible fact,
that female _Aphides_ have the faculty of giving birth to young ones
without having had any intercourse with the other sex. How are we to
explain this most extraordinary fact? Are we to suppose with Bonnet
that these insects are truly androgynous, as strictly uniting both
sexes in one? This supposition, however, is completely overturned by
the circumstance, that there are actually _male_ as well as _female
Aphides_, and that these, as was first observed by Lyonet, are united
towards the close of the summer in the usual manner[778]. The most
likely supposition therefore is, that one conjunction of the sexes
suffices for the impregnation of all the females that in a succession
of generations spring from that union. It is true that at the first
view this supposition appears incredible, contradicting the general
laws and course of nature in the production of animals. But the case
of the hive-bee, stated above, in which a single intercourse with the
male fertilizes all the eggs that are laid for the space of _two_
years, and in the case of a common spider mentioned by Audebert[779],
for _many_ years, shows that the sperm preserves its vivifying powers
unimpaired for a long period, indeed a longer period than is requisite
for the impregnation of all the broods that a female Aphis can produce;
and if immediate contact with the fluid be not necessary, who can say
that this is impossible? It is, however, one of those mysteries of the
CREATOR that human intellect cannot fully penetrate. But this anomaly
in nature is not wholly confined to the Aphides; since Jurine has
ascertained that the same thing takes place with _Daphnia pennata_ Müll
(_Monoculus Pulex_ L.), one of Branchiopod _Crustacea_[780]. It is
worth observing whether the female _Aphides_ in their natural state, I
mean those of the summer or viviparous broods, have intercourse with
the male. I think I have noticed males amongst them; but they seem to
become most numerous in the autumn, preparatory to the impregnation
of the oviparous females. The object of this law of the CREATOR is
probably the more ready multiplication of the species[781].
* * * * *
As to the period of _gestation_, most insects begin to lay their eggs
soon after fecundation has taken place: but in some _Arachnida_, as the
Scorpion, which seems to be both oviparous and ovo-viviparous, nearly
a year intervenes, and the eggs increase to _four_ times the size
which they had attained at that period, before they are extruded[782].
The time that is required to lay the whole they are to produce,
varies also in _insects_. In this respect they may be divided into
two great classes:--those namely which deposit the whole at _once_,
as _Ephemerina_, _Trichoptera_, &c., and those which deposit them in
_succession_, occupying in this operation a longer or shorter period.
Many in the _first_ class, as the _Trichoptera_ or caseworm-flies,
envelope their eggs in a gelatinous substance[783], which renders their
extrusion in a mass more easy. Of the _second_ class, which includes
by far the greater proportion of insects, some exclude the whole
number in a very short period, others require two or three days or a
week, as the cockroach[784]; and others, as the queen-bee, not less
than two years. The eggs in the ovaries of the last vary infinitely in
size; those that have entered the oviduct have arrived at maturity,
while the rest grow gradually smaller as they approach the capillary
extremity of the tubes, where they become at length invisible to the
highest magnifier[785]. In many insects the eggs seem nearly to have
reached their full growth previously to the exclusion of the female
from the pupa; and this exclusion and the impregnation and laying of
the eggs rapidly succeed each other. One moth (_Hypogymna dispar_),
which is remarkable for the number of eggs she contains, sometimes
deposits them, even before they are fecundated, in the pupa-case[786].
But in other cases the sexual union is not so immediate, and some
time, longer or shorter, is requisite for the due expansion of the
eggs; and the ovaries of the animal swell so much, as often to enlarge
the abdomen to an extraordinary bulk: this is seen in a very common
beetle (_Chrysomela Polygoni_) that feeds upon the knot-grass; but in
no insect is it so striking as in the female of the white ants, whose
wonderful increase of size after impregnation I have related to you on
a former occasion[787].
* * * * *
I shall conclude this subject with a few observations upon
_ovo-viviparous insects_; _supposed neuters_, and _hybrids_, which,
though they do not fall in regularly under any of the foregoing
heads, may very well have a place in this letter.
1. It has already been observed that there are a few
_ovo-viviparous_ insects[788], the young of which exist in the
ovaries at first as eggs, but are hatched within the body of the
mother, and come forth in the living form of a larva and sometimes
even of a pupa. Of the first description are certain _Diptera_, the
_Aphides_, and the _Scorpion_.
Reaumur has described two modes in which the larvæ of the first
are arranged in the matrix of the mother. In some they are heaped
together without much appearance of order, being placed merely
parallel to each other[789]; but in others they are arranged in a
kind of riband--the length of the little animals, which are also
parallel, forming its thickness--rolled up like the mainspring of
a watch[790]. These larvæ in general are not divided into _two_
masses corresponding with the pair of ovaries in other insects,
but form only a single one[791]. You must not suppose that these
little fetuses lie naked in the womb of the mother; each has its
own envelope formed of the finest membrane, which, however, is not
entirely divided from that of those adjoining to it, but appears to
be one tube, which becomes extremely slender between each individual,
so as when drawn out to look like a chain[792]. Reaumur seems to
have thought that in these flies the larvæ were never confined in
any other case or egg[793]; but De Geer sometimes found _eggs_ in
the body of _Sarcophaga carnaria_, though most generally larvæ, from
which he conjectures that it is really _ovo-viviparous_, the eggs
being hatched in the body of the mother[794]. As these flies are
all carnivorous, and their office is to remove putrescent flesh,
you may see at one glance the object of PROVIDENCE in this law
of nature--that no time may be lost, and the animal exercise its
function as soon as it is disclosed from the matrix.
The _Aphides_, so fruitful in singular anomalies, are ovo-viviparous,
as I have before hinted[795], at one period of the year, that is
during the summer, but strictly oviparous at its close. From the
experiments of De Geer, however, upon _Aphis Rosæ_, it would appear
that this faculty is not conferred upon the _same_ individuals, but
only upon those of _different_ generations of the same species; all
the generations being ovo-viviparous except the _last_, which is
oviparous[796]: nor does it appear, as has been sometimes imagined,
that it is common to the whole genus. De Geer observed a species in
the fir, which makes curious galls resembling a fir cone (_Aphis
Abietis_), which appeared never to be ovo-viviparous[797].
With regard to _scorpions_, it does not seem clear that they are
_always_ ovo-viviparous: M. Dufour twice found in the midst of the
eggs nearly mature, a young scorpion which appeared to him at large
in the cavity of the abdomen; it was so large that it was difficult
to comprehend how it could possibly be excluded from the animal,
without an extraordinary operation[798]. The _pupiparous_ insects
(_Hippobosca_, &c.) have been sufficiently noticed before[799].
2. I have already in several of my former letters stated to you
what the modern doctrine of physiologists is with respect to certain
individuals, usually forming the most numerous part of the community
with insects living in society, that were formerly supposed to be
_neuters_, or as to their sex neither male nor female--that they
are in almost every instance a kind of abortive females, fed with
a different and less stimulating food than that appropriated to
those whose ovaries are to be developed, and in consequence in most
instances incapable of conception[800]. Upon these sterile females,
you also heard, devolve in general the principal labours of their
respective colonies, showing the beneficent design of PROVIDENCE
in exempting them from sexual cares and desires, and meriting for
them the more appropriate name, now generally used, of _workers_.
The differences in the structure of the female bee and the workers
were also then accounted for; and similar reasoning may be had
recourse to with regard to those of ants, in which the worker and
the female differ still more materially. My reason for introducing
this subject here, is to observe to you that I have some grounds for
thinking that this system extends further than is usually supposed,
and that to each species in some _Coleopterous_ and other genera
there are certain individuals intermediate between the male and
female; this I seem to have observed more especially in _Copris_ and
_Onthophagus_. For in almost every British species in my cabinet of
these genera I possess such an individual, distinguished particularly
by having a horn on the head longer than that of the female, but much
shorter than that of the male. I once observed a pair of _Pentatoma
oleracea_, a very pretty bug, _in coitu_, both sexes being ornamented
with _white_ spots, and by them stood a third distinguished from them
by _red_ ones. I do not, however, build on this circumstance, though
singular; but mention it merely that you may keep it in your eye. It
would be curious should it turn up, that, to answer some particular
end of PROVIDENCE, in some tribes of insects there are two kinds of
_males_, as in the gregarious ones two descriptions of _females_.
I am, &c.
FOOTNOTES:
[703] Herold _Schmetterl._ tab. expl. vii.
[704] Herold _Schmetterl._ _t._ iv. _f._ 1. _x._ &c. PLATE XXX. FIG.
12. _d._
[705] _De Bombyc._ 36.
[706] _Ibid._ _t._ xii. _f._ 1. I. and, _f._ 2. O. M.
[707] _Philos. Trans._ 1792. 186.
[708] Swammerdam, in dissecting the female of _Oryctes nasicornis_,
discovered a blind vessel opening into the vagina, and at the other
or inner extremity not terminated by any secretory tube, containing
a yellowish matter, that seems analogous to the organ mentioned in
the text; and in the hive-bee he found a similar organ covered with
air-vessels, which he supposes to be connected with the _Colleterium_
(see above, p. 132.), and which he states to contain a slimy matter.
_Bibl. Nat._ i. 151. b. _t._ xxx. _f._ 10. _g._ 204. b. _t._ xxix.
_f._ 3. _t._ Perhaps likewise the organ discovered by M. L. Dufour
in _Scolia_,--which he imagines to belong to the poison-secretor,
and which he describes as a sac consisting of a double tunic, the
exterior one muscular and the interior membranous, and filled with
a blueish-green gelatinous matter (_N. Dict. d'Hist. Nat._ xxx.
388.)--may be a _spermatheca_.
[709] _De Insector. Genital._ 17.
[710] I allude to those organs above described (p. 132.) for the
secretion of matter for varnishing the eggs or lubricating the
oviduct. It seems most probable, if the fecundation of the eggs takes
place gradually, that upon their passing into the oviduct, a special
reservoir should be appropriated to the reception of the male sperm,
adapted to maintaining in due activity the vivifying principle, or
_aura seminalis_.
[711] Herold _Schmett._ _t._ iv. _f._ 2. _m n._
[712] Treviran. _Arachnid._ 36. _t._ iv. _f._ 32. _aa._ Marcel de
Serres in _Mém. du Mus._ 1819. 89.
[713] Marcel de Serres, _Mém. du Mus._ 1819. 115.
[714] Rifferschw. _De Genital. Ins._ 11.
[715] Marcel de Serres in _Mém. du Mus._ 1819. 109. PLATE XXX. FIG.
12. _a._
[716] Rifferschw. _ubi supr._ 23--. Swamm. _Bibl. Nat._ _t._ xlii.
_f._ 8. _a, f, g, h._
[717] _Ibid._ i. 104. _t._ xv. _f._ 3. ii. 62. _t._ xii. _f._ 8.
Treviran. _Arachnid._ _t._ iv. _f._ 32.
[718] Reaum. iv. 391.
[719] Posselt _Anat. der Ins._ _t._ i. _f._ 28, 29.
[720] _N. Dict, d'Hist. Nat._ xxx. 387--. Swamm. _ubi supr._ ii. 23.
_t._ xxxv. _f._ 3.
[721] _Ibid._ i. 203.
[722] PLATE XXII. FIG. 2.
[723] Swamm. _ubi supr._ i. 151. Gaede _Anat. der Ins._ _t._ ii. _f._ 3.
[724] Swamm. i. 203.
[725] Gaede _Anat. der Ins._ 20. _t._ i. _f._ 9.
[726] _Ibid._ 25, 28. _t._ ii. _f._ 10.
[727] _Ibid._ 32.
[728] Swamm. ii. 74.
[729] _Ibid._ 203. _t._ xix. _f._ 3.
[730] Reaum. iv. 391--.
[731] Swamm. _t._ xliii. _f._ 19.
[732] Gaede 22.
[733] Swamm. _Bibl. Nat._ i. 203.
[734] _Ibid._
[735] Rifferschw. 11--.
[736] Swamm. _t._ xlii. _f._ 8. Gaede, _t._ i. _f._ 3. _cc._
[737] Herold _Schmett._ _t._ v. _f._ 10. 12.
[738] PLATE XXX. FIG. 12.
[739] PLATE XXII. _f._ 2. _b._
[740] Swamm. _t._ xix. _f._ 4. _b._
[741] _Ibid._ _f._ 3.
[742] VOL. I. p. 355--.
[743] De Geer iv. 127. _t._ iv. _f._ 17.
[744] De Geer iv. 143. _t._ v. _f._ 15.
[745] VOL. I. p. 357.
[746] De Geer. v. 62. _t._ iii. _f._ 12.
[747] PLATE XV. FIG. 18.
[748] Stoll _Sauterel._ _t._ xxii. b. _f._ 87, &c.
[749] De Geer iii. 418. _t._ xxi. _f._ 10, 11. Latr. _Gen. Crust. et
Ins._ iii. 98.
[750] Stoll _ubi supr._ _t._ xiii. a. _f._ 51.
[751] This insect, which connects _Conocephalus_, _Acrida_, &c. with
_Locusta_, is also distinguished by antennæ at first filiform and
then setaceous.
[752] De Geer iii. _t._ xxiv. _f._ 1, 12.
[753] _Ibid._ 176. _t._ xi. _f._ 19.
[754] VOL. II. p. 397--.
[755] Reaum. v. 177--.
[756] _Ibid._ vi. 435. _t._ xl. _f._ 6, 7.
[757] _Natural History of the Slug-worm_, 12--. _f._ 12, 13.
[758] Valisn. _Esperienz. &c. Musca dé Rosai._ Reaum. v. 100--. De
Geer ii. 916--. The last writer thought he saw in the back of the saw
itself a longitudinal cavity (918), which applied to the groove would
form an open canal.
[759] PLATE XVI. FIG. 1.
[760] Ibid.
[761] Reaum. v. 347. _t._ xlix. _f._ 10. _d, f._
[762] See above, VOL. III. 390. a.
[763] See above, VOL. I. 448--.
[764] De Geer ii. 835. _t._ xxviii. _f._ 20, 21. PLATE XV. FIG. 22.
This figure was drawn by a friend--the organ seems more exerted than
in De Geer's. I cannot make out the little appendage at the end.
[765] PLATE XVI. FIG. 2, 3.
[766] Reaum. v. 19--. _t._ iii. _f._ 3-6.
[767] _Arachnid._ 40.
[768] Huber _Nouvel. Observ._ i. 106.
[769] Swamm. _Bibl. Nat._ _t._ xix. _f._ 2.
[770] _Philos. Trans._ 1797. 80.
[771] VOL. I. p. 461.
[772] Compare Reaum. iii. 153. Pallas _Act. Nat. Cur._ 1767. iii.
430. _Wien. Verzeich._ 292.
[773] _Naturfor_ St^k. xx. 59--.
[774] It does not appear to be clearly decided whether the eggs
are extruded from the female, or whether dying immediately after
fecundation they are hatched within her body. As the young larvæ
certainly are hatched in the pupa (not merely within the exterior
case of bits of grass, &c., which includes it) which the body of
the insect must fill, it does not seem easy to conceive how she can
find room for oviposition; and yet Von Scheven expressly says that
one female of _Ps. vestita_--which being kept from all access to the
male actually left the pupa-case and wandered about the glass which
contained them--laid unfruitful eggs.
[775] VOL. I. p. 32, 175.
[776] Bonnet i. 19--.
[777] Reaum. vi. 551.
[778] Reaum. vi. 552.
[779] _N. Dict. d'Hist. Nat._ ii. 284.
[780] _N. Dict. d'Hist. Nat._ ix. 125. Bonnet and Jurine both found
that the female Aphides and Branchiopods that were fertile without
the usual intercourse of the sexes were less fruitful than their
mother, and those of the last generation less so than the first.
Latr. _Hist. Nat. des Crust. et Ins._ xi. 292.
[781] See more on the subject of fecundation, VOL. II. p. 154--. 169--.
[782] _N. Dict. d'Hist. Nat._ xxx. 426.
[783] VOL. III. p. 68.
[784] De Geer iii. 533.
[785] Swamm. i. 203. b. _t._ xix. _f._ 3.
[786] Reaum. ii. 66.
[787] VOL. II. p. 36.
[788] VOL. III. p. 64--.
[789] PLATE XXII. FIG. 4.
[790] Ibid. FIG. 3.
[791] Reaum. iv. 414.
[792] _Ibid._ _t._ xxviii. _f._ 14, 15.
[793] _Ibid._ 404.
[794] De Geer vi. 63--.
[795] VOL. I. p. 175.
[796] De Geer iii. 70--.
[797] _Ibid._ 128.
[798] _N. Dict. d'Hist. Nat._ xxx. 426--.
[799] VOL. III. p. 64--.
[800] VOL. II. p. 50, 110--, 118--, 125--, 130--. The neuters of the
_Termites_, however, (p. 33.) seem to be a distinct sex, if I may so
speak--and to merit that name.
LETTER XLIII.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS, CONCLUDED._
MOTION.
We have seen upon a former occasion the great variety of movements
that insects can perform, and of the _external_ organs with which
they perform them[801]: but we are now to consider the _internal_
apparatus, by the immediate action of which they take place--their
system of _muscles_. When we reflect upon the wonderful velocity,
their size considered, with which many insects move, and the
unparalleled degree of muscular force that many exert[802], we feel
no small degree of curiosity to know something of that part of their
internal structure that produces these almost incredible effects. I
shall in the present letter endeavour in some degree to gratify that
curiosity, and give you an account of the _muscles_ of these little
animals,--first considering them in _general_; and then, as far as my
information goes, adverting to those in _particular_ that move the
different parts and organs of an insect's body.
I. The muscles of insects may be considered in general as to their
_Origin_; _Substance_ and _Parts_; _Shape_; _Colour_; _Kinds_;
_Attachment_; and _Motions_.
i. _Origin._ The origin of the muscular fibre in the higher animals
is from the _blood_, the globules of which, by their coagulation in a
_series_, appear to form it[803]; and in insects it is derived from
the same universal source of nutrition and accretion, but not till
it has been concreted into the adipose tissue or _epiploon_ before
noticed[804]. In the pupa of the cabbage-butterfly, Herold observed
that this substance first assumed a fine flocky appearance and a
blue-green colour, and that from it so changed were produced tender
bundles of muscular fibres, extending in various directions, the
epiploon itself decreasing in proportion as they were formed[805].
ii. _Substance_ and _Parts_. The muscular fibre in vertebrate animals
appears to consist of globules arranged in a series, and of no larger
diameter than those of the blood,--the mean diameter of which in the
human subject, when measured under the microscope by a micrometer, is
found to be about 1/5000th part of an inch[806]. When Cuvier published
his immortal work in 1805, the powers of any magnifier then constructed
were not sufficient to enable this great physiologist to arrive at
the simple fibre[807]; but Mr. Bauer, by the use of improved glasses,
amongst other discoveries that will immortalize his name, was the first
to detect, under the directions of Sir E. Home, the ultimate thread
of which the muscular bundles are composed[808]. Chemists distinguish
the substance of which we are speaking, by the name of _fibrine_. By
the abundance of azote or nitrogen that enters into its composition,
it possesses a character of animalization more marked than any other
animal substance; and its elements are so approximated in the blood,
that the slightest stagnation causes them to coagulate: and the muscles
are without doubt, in the living subject, the only organs that can
separate this matter from the mass of blood, and appropriate it to
themselves[809]. The _primary_ bundles of muscles are formed of the
simple fibres, and the _secondary_ are the result of an aggregation of
the primary. The smaller bundles are not always exactly parallel to
each other, but must in many cases diverge more or less, to produce
those variations in shape observable in the muscles themselves: there
are intervals therefore between the bundles, which in some animals are
filled by a cellular substance[810]. Probably much of this statement
will apply in most instances to the muscles of _insects_, but we may
conclude that the globules that form them are infinitely smaller[811].
Lyonet has given some interesting observations with regard to those
of the caterpillar of the _Cossus_: he describes them as of a soft
transparent substance, capable of great extension, covered and filled
by silver tubes of the _bronchiæ_, penetrated by the nerves, and
containing oily particles. Each muscle was enveloped in membrane, and
was composed of many parallel bands, consisting of bundles of fibres
enveloped likewise in separate membranes. The fibres themselves, (but
it is doubtful whether he arrived at the ultimate term of muscular
fibre,) in a favourable light and under a good magnifier, appeared
to be twisted spirally[812]. In spiders the muscles seemed to him
to consist of _two_ substances, the one soft and the other hard,
the last forming a kind of stiff twisted filament[813]. A muscle
thus composed of different bundles of fibres may be stated as to its
_parts_, in _insects_, to consist of base, middle, and apex: the
_base_ is that part by which they are fixed to any given point of
the internal surface of the crust, or of one of its processes, which
serves as their fulcrum; the _apex_ is that part by which they are
fixed, either mediately or immediately, to the organ to be moved; and
the _middle_ is the remainder of the muscle. We usually discover in
them no inflation of the middle corresponding with the _belly_ of the
muscles in vertebrate animals; they occasionally, however, terminate in
a _tendon_, as those of the thighs and legs; but these tendons are of
a different nature from the fibrous ones of warm-blooded animals; for
they are hard, elastic, and without apparent fibres: the fleshy ones of
the muscle envelope them, and are inserted in their surface[814].