characters in animals of all classes, and shall endeavour in each case
to apply the principles explained in the present chapter. The lowest
classes will detain us for a very short time, but the higher animals,
especially birds, must be treated at considerable length. It should be
borne in mind that for reasons already assigned, I intend to give only a
few illustrative instances of the innumerable structures by the aid of
which the male finds the female, or, when found, holds her. On the other
hand, all structures and instincts by which the male conquers other
males, and by which he allures or excites the female, will be fully
discussed, as these are in many ways the most interesting.
_Supplement on the proportional numbers of the two sexes in animals
belonging to various classes._
As no one, as far as I can discover, has paid attention to the relative
numbers of the two sexes throughout the animal kingdom, I will here give
such materials as I have been able to collect, although they are
extremely imperfect. They consist in only a few instances of actual
enumeration, and the numbers are not very large. As the proportions are
known with certainty on a large scale in the case of man alone, I will
first give them, as a standard of comparison.
_Man._--In England during ten years (from 1857 to 1866) 707,120 children
on an annual average have been born alive, in the proportion of 104.5
males to 100 females. But in 1857 the male births throughout England
were as 105.2, and in 1865 as 104.0 to 100. Looking to separate
districts, in Buckinghamshire (where on an average 5000 children are
annually born) the _mean_ proportion of male to female births, during
the whole period of the above ten years, was as 102.8 to 100; whilst in
N. Wales (where the average annual births are 12,873) it was as high as
106.2 to 100. Taking a still smaller district, viz., Rutlandshire (where
the annual births average only 739), in 1864 the male births were as
114.6, and in 1862 as 97.0 to 100; but even in this small district the
average of the 7385 births during the whole ten years was as 104.5 to
100; that is in the same ratio as throughout England.[367] The
proportions are sometimes slightly disturbed by unknown causes; thus
Prof. Faye states "that in some districts of Norway there has been
during a decennial period a steady deficiency of boys, whilst in others
the opposite condition has existed." In France during forty-four years
the male to the female births have been as 106.2 to 100; but during this
period it has occurred five times in one department, and six times in
another, that the female births have exceeded the males. In Russia the
average proportion is as high as 108.9 to 100.[368] It is a singular
fact that with Jews the proportion of male births is decidedly larger
than with Christians: thus in Prussia the proportion is as 113, in
Breslau as 114, and in Livonia as 120 to 100; the Christian births in
these countries being the same as usual, for instance, in Livonia as 104
to 100.[369] It is a still more singular fact that in different nations,
under different conditions and climates, in Naples, Prussia, Westphalia,
France and England, the excess of male over female births is less when
they are illegitimate than when legitimate.[370]
In various parts of Europe, according to Prof. Faye and other authors,
"a still greater preponderance of males would be met with, if death
struck both sexes in equal proportion in the womb and during birth. But
the fact is, that for every 100 still-born females, we have in several
countries from 134.6 to 144.9 still-born males." Moreover during the
first four or five years of life more male children die than females;
"for example in England, during the first year, 126 boys die
for every 100 girls,--a proportion which in France is still more
unfavourable."[371] As a consequence of this excess in the death-rate of
male children, and of the exposure of men when adult to various dangers,
and of their tendency to emigrate, the females in all old-settled
countries, where statistical records have been kept,[372] are found to
preponderate considerably over the males.
It has often been supposed that the relative ages of the parents
determine the sex of the offspring; and Prof. Leuckart[373] has advanced
what he considers sufficient evidence, with respect to man and certain
domesticated animals, to shew that this is one important factor in the
result. So again the period of impregnation has been thought to be the
efficient cause; but recent observations discountenance this belief.
Again, with mankind polygamy has been supposed to lead to the birth of a
greater proportion of female infants; but Dr. J. Campbell[374] carefully
attended to this subject in the harems of Siam, and he concludes that
the proportion of male to female births is the same as from monogamous
unions. Hardly any animal has been rendered so highly polygamous as our
English race-horses, and we shall immediately see that their male and
female offspring are almost exactly equal in number.
_Horses._--Mr. Tegetmeier has been so kind as to tabulate for
me from the 'Racing Calendar' the births of race-horses during
a period of twenty-one years, viz. from 1846 to 1867; 1849
being omitted, as no returns were that year published. The
total births have been 25,560,[375] consisting of 12,763 males
and 12,797 females, or in the proportion of 99.7 males to 100
females. As these numbers are tolerably large, and as they are
drawn from all parts of England, during several years, we may
with much confidence conclude that with the domestic horse, or
at least with the race-horse, the two sexes are produced in
almost equal numbers. The fluctuations in the proportions
during successive years are closely like those which occur with
mankind, when a small and thinly-populated area is considered:
thus in 1856 the male horses were as 107.1, and in 1867 as only
92.6 to 100 females. In the tabulated returns the proportions
vary in cycles, for the males exceeded the females during six
successive years; and the females exceeded the males during two
periods each of four years: this, however, may be accidental;
at least I can detect nothing of the kind with man in the
decennial table in the Registrar's Report for 1866. I may add
that certain, mares, and this holds good with certain cows and
with women, tend to produce more of one sex than of the other;
Mr. Wright of Yeldersley House, informs me that one of his Arab
mares, though put seven times to different horses, produced
seven fillies.
_Dogs._--During a period of twelve years, from 1857 to 1868,
the births of a large number of greyhounds, throughout England,
have been sent to the 'Field' newspaper; and I am again
indebted to Mr. Tegetmeier for carefully tabulating the
results. The recorded, births have been 6878, consisting of
3605 males and 3273 females, that is, in the proportion of
110.1 males to 100 females. The greatest fluctuations occurred
in 1864, when the proportion was as 95.3 males, and in 1867, as
116.3 males to 100 females. The above average proportion of
110.1 to 100 is probably nearly correct in the case of the
greyhound, but whether it would hold with other domesticated
breeds is in some degree doubtful. Mr. Cupples has enquired
from several great breeders of dogs, and finds that all without
exception believe that females are produced in excess; he
suggests that this belief may have arisen from females being
less valued and the consequent disappointment producing a
stronger impression on the mind.
_Sheep._--The sexes of sheep are not ascertained by
agriculturists until several months after birth, at the period
when the males are castrated; so that the following returns do
not give the proportions at birth. Moreover, I find that
several great breeders in Scotland, who annually raise some
thousand sheep, are firmly convinced that a larger proportion
of males than of females die during the first one or two years;
therefore the proportion of males would be somewhat greater at
birth than at the age of castration. This is a remarkable
coincidence with what occurs, as we have seen, with mankind,
and both cases probably depend on some common cause. I have
received returns from four gentlemen in England who have bred
lowland sheep, chiefly Leicesters, during the last ten or
sixteen years; they amount altogether to 8965 births,
consisting of 4407 males and 4558 females; that is in the
proportion of 96.7 males to 100 females. With respect to
Cheviot and black-faced sheep bred in Scotland, I have received
returns from six breeders, two of them on a large scale,
chiefly for the years 1867-1869, but some of the returns
extending back to 1862. The total number recorded amounts to
50,685, consisting of 25,071 males and 25,614 females, or in
the proportion of 97.9 males to 100 females. If we take the
English and Scotch returns together, the total number amounts
to 59,650, consisting of 29,478 males and 30,172 females, or
as 97.7 to 100. So that with sheep at the age of castration the
females are certainly in excess of the males; but whether this
would hold good at birth is doubtful, owing to the greater
liability in the males to early death.[376]
Of _Cattle_ I have received returns from nine gentlemen of 982
births, too few to be trusted; these consisted of 477
bull-calves and 505 cow-calves; _i.e._ in the proportion of
94.4 males to 100 females. The Rev. W. D. Fox informs me that
in 1867 out of 34 calves born on a farm in Derbyshire only one
was a bull. Mr. Harrison Weir writes to me that he has enquired
from several breeders of _Pigs_, and most of them estimate the
male to the female births as about 7 to 6. This same gentleman
has bred _Rabbits_ for many years, and has noticed that a far
greater number of bucks are produced than does.
Of mammalia in a state of nature I have been able to learn very
little. In regard to the common rat, I have received
conflicting statements. Mr. R. Elliot of Laighwood, informs me
that a rat-catcher assured him that he had always found the
males in great excess, even with the young in the nest. In
consequence of this, Mr. Elliot himself subsequently examined
some hundred old ones, and found the statement true. Mr. F.
Buckland has bred a large number of white rats, and he also
believes that the males greatly exceed the females. In regard
to Moles, it is said that "the males are much more numerous
than the females;"[377] and as the catching of these animals is
a special occupation, the statement may perhaps be trusted. Sir
A. Smith, in describing an antelope of S. Africa[378] (_Kobus
ellipsiprymnus_), remarks, that in the herds of this and other
species, the males are few in number compared with the females:
the natives believe that they are born in this proportion;
others believe that the younger males are expelled from the
herds, and Sir A. Smith says, that though he has himself never
seen herds consisting of young males alone, others affirm that
this does occur. It appears probable that the young males when
expelled from the herd, would be likely to fell a prey to the
many beasts of prey of the country.
BIRDS.
With respect to the _Fowl_, I have received only one account,
namely, that out of 1001 chickens of a highly-bred stock of
Cochins, reared during eight years by Mr. Stretch, 487 proved
males and 514 females: _i.e._ as 94.7 to 100. In regard to
domestic pigeons there is good evidence that the males are
produced in excess, or that their lives are longer; for these
birds invariably pair, and single males, as Mr. Tegetmeier
informs me, can always be purchased cheaper than females.
Usually the two birds reared from the two eggs laid in the same
nest consist of a male and female; but Mr. Harrison Weir, who
has been so large a breeder, says that he has often bred two
cocks from the same nest, and seldom two hens; moreover the hen
is generally the weaker of the two, and more liable to perish.
With respect to birds in a state of nature, Mr. Gould and
others[379] are convinced that the males are generally the more
numerous; and as the young males of many species resemble the
females, the latter would naturally appear to be the most
numerous. Large numbers of pheasants are reared by Mr. Baker of
Leadenhall from eggs laid by wild birds, and he informs Mr.
Jenner Weir that four or five males to one female are generally
produced. An experienced observer remarks[380] that in
Scandinavia the broods of the capercailzie and black-cock
contain more males than females; and that with the Dal-ripa (a
kind of ptarmigan) more males than females attend the _leks_ or
places of courtship; but this latter circumstance is accounted
for by some observers by a greater number of hen birds being
killed by vermin. From various facts given by White of
Selbourne,[381] it seems clear that the males of the partridge
must be in considerable excess in the south of England; and I
have been assured that this is the case in Scotland. Mr. Weir
on enquiring from the dealers who receive at certain seasons
large numbers of ruffs (_Machetes pugnax_) was told that the
males are much the most numerous. This same naturalist has also
enquired for me from the bird-catchers, who annually catch an
astonishing number of various small species alive for the
London market, and he was unhesitatingly answered by an old and
trustworthy man, that with the chaffinch the males are in large
excess; he thought as high as 2 males to 1 female, or at least
as high as 5 to 3.[382] The males of the blackbird, he likewise
maintained, were by far the most numerous, whether caught by
traps or by netting at night. These statements may apparently
be trusted, because the same man said that the sexes are about
equal with the lark, the twite (_Linaria montana_), and
goldfinch. On the other hand he is certain that with the common
linnet, the females preponderate greatly, but unequally during
different years; during some years he has found the females to
the males as four to one. It should, however, be borne in mind,
that the chief season for catching birds does not begin till
September, so that with some species partial migrations may
have begun, and the flocks at this period often consist of hens
alone. Mr. Salvin paid particular attention to the sexes of the
humming-birds in Central America, and he is convinced that with
most of the species the males are in excess; thus one year he
procured 204 specimens belonging to ten species, and these
consisted of 166 males and of 38 females. With two other
species the females were in excess: but the proportions
apparently vary either during different seasons or in different
localities; for on one occasion the males of _Campylopterus
hemileucurus_ were to the females as five to two, and on
another occasion[383] in exactly the reversed ratio. As bearing
on this latter point, I may add, that Mr. Powys found in Corfu
and Epirus the sexes of the chaffinch keeping apart, and "the
females by far the most numerous;" whilst in Palestine Mr.
Tristram found "the male flocks appearing greatly to exceed the
female in number."[384] So again with the _Quiscalus major_,
Mr. G. Taylor[385] says, that in Florida there were "very few
females in proportion to the males," whilst in Honduras the
proportion was the other way, the species there having the
character of a polygamist.
FISH.
With Fish the proportional numbers of the sexes can be
ascertained only by catching them in the adult or nearly adult
state; and there are many difficulties in arriving at any just
conclusion.[386] Infertile females might readily be mistaken
for males, as Dr. Guenther has remarked to me in regard to
trout. With some species the males are believed to die soon
after fertilising the ova. With many species the males are of
much smaller size than the females, so that a large number of
males would escape from the same net by which the females were
caught. M. Carbonnier,[387] who has especially attended to the
natural history of the pike (_Esox lucius_) states that many
males, owing to their small size, are devoured by the larger
females; and he believes that the males of almost all fish are
exposed from the same cause to greater danger than the females.
Nevertheless in the few cases in which the proportional numbers
have been actually observed, the males appear to be largely in
excess. Thus Mr. R. Buist, the superintendent of the
Stormontfield experiments, says that in 1865, out of 70 salmon
first landed for the purpose of obtaining the ova, upwards of
60 were males. In 1867 he again "calls attention to the vast
disproportion of the males to the females. We had at the outset
at least ten males to one female." Afterwards sufficient
females for obtaining ova were procured. He adds, "from the
great proportion of the males, they are constantly fighting and
tearing each other on the spawning-beds."[388] This
disproportion, no doubt, can be accounted for in part, but
whether wholly is very doubtful, by the males ascending the
rivers before the females. Mr. F. Buckland remarks in regard to
trout, that "it is a curious fact that the males preponderate
very largely in number over the females. It _invariably_
happens that when the first rush of fish is made to the net,
there will be at least seven or eight males to one female found
captive. I cannot quite account for this; either the males are
more numerous than the females, or the latter seek safety by
concealment rather than flight." He then adds, that by
carefully searching the banks, sufficient females for obtaining
ova can be found.[389] Mr. H. Lee informs me that out of 212
trout, taken for this purpose in Lord Portsmouth's park, 150
were males and 62 females.
With the Cyprinidae the males likewise seem to be in excess; but
several members of this Family, viz., the carp, tench, bream
and minnow, appear regularly to follow the practice, rare in
the animal kingdom, of polyandry; for the female whilst
spawning is always attended by two males, one on each side, and
in the case of the bream by three or four males. This fact is
so well known, that it is always recommended to stock a pond
with two male tenches to one female, or at least with three
males to two females. With the minnow, an excellent observer
states, that on the spawning-beds the males are ten times as
numerous as the females; when a female comes amongst the males,
"she is immediately pressed closely by a male on each side; and
when they have been in that situation for a time, are
superseded by other two males."[390]
INSECTS.
In this class, the Lepidoptera alone afford the means of
judging of the proportional numbers of the sexes; for they have
been collected with special care by many good observers, and
have been largely bred from the egg or caterpillar state. I had
hoped that some breeders of silk-moths might have kept an exact
record, but after writing to France and Italy, and consulting
various treatises, I cannot find that this has ever been done.
The general opinion appears to be that the sexes are nearly
equal, but in Italy as I hear from Professor Canestrini, many
breeders are convinced that the females are produced in excess.
The same naturalist, however, informs me, that in the two
yearly broods of the Ailanthus silk-moth (_Bombyx cynthia_),
the males greatly preponderate in the first, whilst in the
second the two sexes are nearly equal, or the females rather in
excess.
In regard to Butterflies in a state of nature, several
observers have been much struck by the apparently enormous
preponderance of the males.[391] Thus Mr. Bates,[392] in
speaking of the species, no less than about a hundred in
number, which inhabit the Upper Amazons, says that the males
are much more numerous than the females, even in the proportion
of a hundred to one. In North America, Edwards, who had great
experience, estimates in the genus Papilio the males to the
females as four to one; and Mr. Walsh, who informed me of this
statement, says that with _P. turnus_ this is certainly the
case. In South Africa, Mr. R. Trimen found the males in excess
in 19 species;[393] and in one of these, which swarms in open
places, he estimated the number of males as fifty to one
female. With another species, in which the males are numerous
in certain localities, he collected during seven years only
five females. In the island of Bourbon, M. Maillard states that
the males of one species of Papilio are twenty times as
numerous as the females.[394] Mr. Trimen informs me that as far
as he has himself seen, or heard from others, it is rare for
the females of any butterfly to exceed in number the males; but
this is perhaps the case with three South African species. Mr.
Wallace[395] states that the females of _Ornithoptera
croesus_, in the Malay archipelago, are more common and more
easily caught than the males; but this is a rare butterfly. I
may here add, that in Hyperythra, a genus of moths, Guenee
says, that from four to five females are sent in collections
from India for one male.
When this subject of the proportional numbers of the sexes of
insects was brought before the Entomological Society,[396] it
was generally admitted that the males of most Lepidoptera, in
the adult or imago state, are caught in greater numbers than
the females; but this fact was attributed by various observers
to the more retiring habits of the females, and to the males
emerging earlier from the cocoon. This latter circumstance is
well known to occur with most Lepidoptera, as well as with
other insects. So that, as M. Personnat remarks, the males of
the domesticated _Bombyx yamamai_, are lost at the beginning of
the season, and the females at the end, from the want of
mates.[397] I cannot however persuade myself that these causes
suffice to explain the great excess of males in the cases,
above given, of butterflies which are extremely common in their
native countries. Mr. Stainton, who has paid such close
attention during many years to the smaller moths, informs me
that when he collected them in the imago state, he thought that
the males were ten times as numerous as the females, but that
since he has reared them on a large scale from the caterpillar
state, he is convinced that the females are the most numerous.
Several entomologists concur in this view. Mr. Doubleday,
however, and some others, take an opposite view, and are
convinced that they have reared from the egg and caterpillar
states a larger proportion of males than of females.
Besides the more active habits of the males, their earlier
emergence from the cocoon, and their frequenting in some cases
more open stations, other causes may be assigned for an
apparent or real difference in the proportional numbers of the
sexes of Lepidoptera, when captured in the imago state, and
when reared from the egg or caterpillar state. It is believed
by many breeders in Italy, as I hear from Professor Canestrini,
that the female caterpillar of the silk-moth suffers more from
the recent disease than the male; and Dr. Staudinger informs me
that in rearing Lepidoptera more females die in the cocoon than
males. With many species the female caterpillar is larger than
the male, and a collector would naturally choose the finest
specimens, and thus unintentionally collect a larger number of
females. Three collectors have told me that this was their
practice; but Dr. Wallace is sure that most collectors take all
the specimens which they can find of the rarer kinds, which
alone are worth the trouble of rearing. Birds when surrounded
by caterpillars would probably devour the largest; and
Professor Canestrini informs me that in Italy some breeders
believe, though on insufficient evidence, that in the first
brood of the Ailanthus silk-moth, the wasps destroy a larger
number of the female than of the male caterpillars. Dr. Wallace
further remarks that female caterpillars, from being larger
than the males, require more time for their development and
consume more food and moisture; and thus they would be exposed
during a longer time to danger from ichneumons, birds, &c., and
in times of scarcity would perish in greater numbers. Hence it
appears quite possible that, in a state of nature, fewer female
Lepidoptera may reach maturity than males; and for our special
object we are concerned with the numbers at maturity, when the
sexes are ready to propagate their kind.
The manner in which the males of certain moths congregate in
extraordinary numbers round a single female, apparently
indicates a great excess of males, though this fact may perhaps
be accounted for by the earlier emergence of the males from
their cocoons. Mr. Stainton informs me that from twelve to
twenty males may often be seen congregated round a female
_Elachista rufocinerea_. It is well known that if a virgin
_Lasiocampa quercus_ or _Saturnia carpini_ be exposed in a
cage, vast numbers of males collect round her, and if confined
in a room will even come down the chimney to her. Mr.
Doubleday believes that he has seen from fifty to a hundred
males of both these species attracted in the course of a single
day by a female under confinement. Mr. Trimen exposed in the
Isle of Wight a box in which a female of the Lasiocampa had
been confined on the previous day, and five males soon
endeavoured to gain admittance. M. Verreaux, in Australia,
having placed the female of a small Bombyx in a box in his
pocket, was followed by a crowd of males, so that about 200
entered the house with him.[398]
Mr. Doubleday has called my attention to Dr. Staudinger's[399]
list of Lepidoptera, which gives the prices of the males and
females of 300 species or well-marked varieties of
(Rhopalocera) butterflies. The prices for both sexes of the
very common species are of course the same; but with 114 of the
rarer species they differ; the males being in all cases,
excepting one, the cheapest. On an average of the prices of the
113 species, the price of the male to that of the female is as
100 to 149; and this apparently indicates that inversely the
males exceed the females in number in the same proportion.
About 2000 species or varieties of moths (Heterocera) are
catalogued, those with wingless females being here excluded on
account of the difference in habits of the two sexes: of these
2000 species, 141 differ in price according to sex, the males
of 130 being cheaper, and the males of only 11 being dearer
than the females. The average price of the males of the 130
species, to that of the females, is as 100 to 143. With respect
to the butterflies in this priced list, Mr. Doubleday thinks
(and no man in England has had more experience), that there is
nothing in the habits of the species which can account for the
difference in the prices of the two sexes, and that it can be
accounted for only by an excess in the numbers of the males.
But I am bound to add that Dr. Staudinger himself, as he
informs me, is of a different opinion. He thinks that the less
active habits of the females and the earlier emergence of the
males will account for his collectors securing a larger number
of males than of females, and consequently for the lower prices
of the former With respect to specimens reared from the
caterpillar-state, Dr. Staudinger believes, as previously
stated, that a greater number of females than of males die
under confinement in the cocoons. He adds that with certain
species one sex seems to preponderate over the other during
certain years.
Of direct observations on the sexes of Lepidoptera, reared
either from eggs or caterpillars, I have received only the few
following cases:--
--------------------------------------------------------------------------------------------------------------------------------------
Males. | Females.
The Rev. J. Hellins[400] of Exeter reared, during |
1868, imagos of 73 species, which consisted of 153 | 137
|
Mr. Albert Jones of Eltham reared, during 1868, |
imagos of 9 species, which, consisted of 159 | 126
|
During 1869 he reared imagos from 4 species, |
consisting of 114 | 112
|
Mr. Buckler of Emsworth, Hants, during 1869, |
reared imagos from 74 species, consisting of 180 | 169
|
Dr. Wallace of Colchester reared from one brood |
of Bombyx cynthia 52 | 48
|
Dr. Wallace raised, from cocoons of Bombyx |
Pernyi sent from China, during 1869 224 | 123
|
Dr. Wallace raised, during 1868 and 1869, |
from two lots of cocoons of Bombyx yamamai 52 | 46
--------------+------------
Total 934 | 761
------------------------------------------------------------------------------------------------------------------+------------------
So that in these eight lots of cocoons and eggs, males were
produced in excess. Taken together the proportion of males is
as 122.7 to 100 females. But the numbers are hardly large
enough to be trustworthy.
On the whole, from the above various sources of evidence, all
pointing to the same direction, I infer that with most species
of Lepidoptera, the males in the imago state generally exceed
the females in number, whatever the proportions may be at their
first emergence from the egg.
With reference to the other Orders of insects, I have been able
to collect very little reliable information. With the
stag-beetle (_Lucanus cervus_) "the males appear to be much
more numerous than the females;" but when, as Cornelius
remarked during 1867, an unusual number of these beetles
appeared in one part of Germany, the females appeared to exceed
the males as six so one. With one of the Elateridae, the males
are said to be much more numerous than the females, and "two or
three are often found united with one female;"[401] so that here
polyandry seems to prevail. With Siagonium (Staphylinidae), in
which the males are furnished with horns, "the females are far
more numerous than the opposite sex." Mr. Janson stated at the
Entomological Society that the females of the bark-feeding
_Tomicus villosus_ are so common as to be a plague, whilst the
males are so rare as to be hardly known. In other Orders, from
unknown causes, but apparently in some instances owing to
parthenogenesis, the males of certain species have never been
discovered or are excessively rare, as with several of the
Cynipidae.[402] In all the gall-making Cynipidae known to Mr.
Walsh, the females are four or five times as numerous as the
males; and so it is, as he informs me, with the gall-making
Cecidomyiiae (Diptera). With some common species of Saw-flies
(Tenthredinae) Mr. F. Smith has reared hundreds of specimens
from larvae of all sizes, but has never reared a single male: on
the other hand Curtis says,[403] that with certain species
(Athalia), bred by him, the males to the females were as six to
one; whilst exactly the reverse occurred with the mature
insects of the same species caught in the fields. With the
Neuroptera, Mr. Walsh states that in many, but by no means in
all, the species of the Odonatous groups (Ephemerina), there is
a great overplus of males: in the genus Hetaerina, also, the
males are generally at least four times as numerous as the
females. In certain species in the genus Gomphus the males are
equally numerous, whilst in two other species, the females are
twice or thrice as numerous as the males. In some European
species of Psocus thousands of females may be collected without
a single male, whilst with other species of the same genus both
sexes are common.[404] In England, Mr. MacLachlan has captured
hundreds of the female _Apatania muliebris_, but has never seen
the male; and of _Boreus hyemalis_ only four or five males have
been here seen.[405] With most of these species (excepting, as
I have heard, with the Tenthredinae) there is no reason to
suppose that the females are subject to parthenogenesis; and
thus we see how ignorant we are on the causes of the apparent
discrepancy in the proportional numbers of the two sexes.
In the other Classes of the Articulata I have been able to
collect still less information. With Spiders, Mr. Blackwall,
who has carefully attended to this class during many years,
writes to me that the males from their more erratic habits are
more commonly seen, and therefore appear to be the more
numerous. This is actually the case with a few species; but he
mentions several species in six genera, in which the females
appear to be much more numerous than the males.[406] The small
size of the males in comparison with the females, which is
sometimes carried to an extreme degree, and their widely
different appearance, may account in some instances for their
rarity in collections.[407]
Some of the lower Crustaceans are able to propagate their kind
asexually, and this will account for the extreme rarity of the
males. With some other forms (as with Tanais and Cypris) there
is reason to believe, as Fritz Mueller informs me, that the male
is much shorter-lived than the female, which, supposing the two
sexes to be at first equal in number, would explain the
scarcity of the males. On the other hand this same naturalist
has invariably taken, on the shores of Brazil, far more males
than females of the Diastylidae and of Cypridina; thus with a
species in the latter genus, 63 specimens caught the same day,
included 57 males; but he suggests that this preponderance may
be due to some unknown difference in the habits of the two
sexes. With one of the higher Brazilian crabs, namely a
Gelasimus, Fritz Mueller found the males to be more numerous
than the females. The reverse seems to be the case, according
to the large experience of Mr. C. Spence Bate, with six common
British crabs, the names of which he has given me.
_On the Power of Natural Selection to regulate the proportional Numbers
of the Sexes, and General Fertility._--In some peculiar cases, an excess
in the number of one sex over the other might be a great advantage to a
species, as with the sterile females of social insects, or with those
animals in which more than one male is requisite to fertilise the
female, as with certain cirripedes and perhaps certain fishes. An
inequality between the sexes in these cases might have been acquired
through natural selection, but from their rarity they need not here be
further considered. In all ordinary cases an inequality would be no
advantage or disadvantage to certain individuals more than to others;
and therefore it could hardly have resulted from natural selection. We
must attribute the inequality to the direct action of those unknown
conditions, which with mankind lead to the males being born in a
somewhat larger excess in certain countries than in others, or which
cause the proportion between the sexes to differ slightly in legitimate
and illegitimate births.
Let us now take the case of a species producing from the unknown causes
just alluded to, an excess of one sex--we will say of males--these being
superfluous and useless, or nearly useless. Could the sexes be equalised
through natural selection? We may feel sure, from all characters being
variable, that certain pairs would produce a somewhat less excess of
males over females than other pairs. The former, supposing the actual
number of the offspring to remain constant, would necessarily produce
more females, and would therefore be more productive. On the doctrine of
chances a greater number of the offspring of the more productive pairs
would survive; and these would inherit a tendency to procreate fewer
males and more females. Thus a tendency towards the equalisation of the
sexes would be brought about. But our supposed species would by this
process be rendered, as just remarked, more productive; and this would
in many cases be far from an advantage; for whenever the limit to the
numbers which exist, depends, not on destruction by enemies, but on the
amount of food, increased fertility will lead to severer competition and
to most of the survivors being badly fed. In this case, if the sexes
were equalised by an increase in the number of the females, a
simultaneous decrease in the total number of the offspring would be
beneficial, or even necessary, for the existence of the species; and
this, I believe, could be effected through natural selection in the
manner hereafter to be described. The same train of reasoning is
applicable in the above, as well as in the following case, if we assume
that females instead of males are produced in excess, for such females
from not uniting with males would be superfluous and useless. So it
would be with polygamous species, if we assume the excess of females to
be inordinately great.
An excess of either sex, we will again say of the males, could, however,
apparently be eliminated through natural selection in another and
indirect manner, namely by an actual diminution of the males, without
any increase of the females, and consequently without any increase in
the productiveness of the species. From the variability of all
characters, we may feel assured that some pairs, inhabiting any
locality, would produce a rather smaller excess of superfluous males,
but an equal number of productive females. When the offspring from the
more and the less male-productive parents were all mingled together,
none would have any direct advantage over the others; but those that
produced few superfluous males would have one great indirect advantage,
namely that their ova or embryos would probably be larger and finer, or
their young better nurtured in the womb and afterwards. We see this
principle illustrated with plants; as those which bear a vast number of
seed produce small ones; whilst those which bear comparatively few
seeds, often produce large ones well-stocked with nutriment for the use
of the seedlings.[408] Hence the offspring of the parents which had
wasted least force in producing superfluous males would be the most
likely to survive, and would inherit the same tendency not to produce
superfluous males, whilst retaining their full fertility in the
production of females. So it would be with the converse case of the
female sex. Any slight excess, however, of either sex could hardly be
checked in so indirect a manner. Nor indeed has a considerable
inequality between the sexes been always prevented, as we have seen in
some of the cases given in the previous discussion. In these cases the
unknown causes which determine the sex of the embryo, and which under
certain conditions lead to the production of one sex in excess over the
other, have not been mastered by the survival of those varieties which
were subjected to the least waste of organised matter and force by the
production of superfluous individuals of either sex. Nevertheless we may
conclude that natural selection will always tend, though sometimes
inefficiently, to equalise the relative numbers of the two sexes.
Having said this much on the equalisation of the sexes, it may be well
to add a few remarks on the regulation through natural selection of the
ordinary fertility of species. Mr. Herbert Spencer has shewn in an able
discussion[409] that with all organisms a ratio exists between what he
calls individuation and genesis; whence it follows that beings which
consume much matter or force in their growth, complicated structure or
activity, or which produce ova and embryos of large size, or which
expend much energy in nurturing their young, cannot be so productive as
beings of an opposite nature. Mr. Spencer further shews that minor
differences in fertility will be regulated through natural selection.
Thus the fertility of each species will tend to increase, from the more
fertile pairs producing a larger number of offspring, and these from
their mere number will have the best chance of surviving, and will
transmit their tendency to greater fertility. The only check to a
continued augmentation of fertility in each organism seems to be either
the expenditure of more power and the greater risks run by the parents
that produce a more numerous progeny, or the contingency of very
numerous eggs and young being produced of smaller size, or less
vigorous, or subsequently not so well nurtured. To strike a balance in
any case between the disadvantages which follow from the production of a
numerous progeny, and the advantages (such as the escape of at least
some individuals from various dangers) is quite beyond our power of
judgment.
When an organism has once been rendered extremely fertile, how its
fertility can be reduced through natural selection is not so clear as
how this capacity was first acquired. Yet it is obvious that if
individuals of a species, from a decrease of their natural enemies, were
habitually reared in larger numbers than could be supported, all the
members would suffer. Nevertheless the offspring from the less fertile
parents would have no direct advantage over the offspring from the more
fertile parents, when all were mingled together in the same district.
All the individuals would mutually tend to starve each other. The
offspring indeed of the less fertile parents would lie under one great
disadvantage, for from the simple fact of being produced in smaller
numbers, they would be the most liable to extermination. Indirectly,
however, they would partake of one great advantage; for under the
supposed condition of severe competition, when all were pressed for
food, it is extremely probable that those individuals which from some
variation in their constitution produced fewer eggs or young, would
produce them of greater size or vigour; and the adults reared from such
eggs or young would manifestly have the best chance of surviving, and
would inherit a tendency towards lessened fertility. The parents,
moreover, which had to nourish or provide for fewer offspring would
themselves be exposed to a less severe strain in the struggle for
existence, and would have a better chance of surviving. By these steps,
and by no others as far as I can see, natural selection under the above
conditions of severe competition for food, would lead to the formation
of a new race less fertile, but better adapted for survival, than the
parent-race.
CHAPTER IX.
SECONDARY SEXUAL CHARACTERS IN THE LOWER CLASSES OF THE ANIMAL KINGDOM.
These characters absent in the lowest classes--Brilliant
colours--Mollusca--Annelids--Crustacea, secondary sexual
characters strongly developed; dimorphism; colour; characters
not acquired before maturity--Spiders, sexual colours of;
stridulation by the males--Myriapoda.
In the lowest classes the two sexes are not rarely united in the same
individual, and therefore secondary sexual characters cannot be
developed. In many cases in which the two sexes are separate, both are
permanently attached to some support, and the one cannot search or
struggle for the other. Moreover it is almost certain that these animals
have too imperfect senses and much too low mental powers to feel mutual
rivalry, or to appreciate each other's beauty or other attractions.
Hence in these classes, such as the Protozoa, Coelenterata,
Echinodermata, Scolecida, true secondary sexual characters do not occur;
and this fact agrees with the belief that such characters in the higher
classes have been acquired through sexual selection, which depends on
the will, desires, and choice of either sex. Nevertheless some few
apparent exceptions occur; thus, as I hear from Dr. Baird, the males of
certain Entozoa, or internal parasitic worms, differ slightly in colour
from the females; but we have no reason to suppose that such differences
have been augmented through sexual selection.
Many of the lower animals, whether hermaphrodites or with the sexes
separate, are ornamented with the most brilliant tints, or are shaded
and striped in an elegant manner. This is the case with many corals and
sea-anemonies (Actiniae), with some jelly-fish (Medusae, Porpita, &c.),
with some Planariae, Ascidians, numerous Star-fishes, Echini, &c.; but we
may conclude from the reasons already indicated, namely the union of the
two sexes in some of these animals, the permanently affixed condition of
others, and the low mental powers of all, that such colours do not serve
as a sexual attraction, and have not been acquired through sexual
selection. With the higher animals the case is very different; for with
them when one sex is much more brilliantly or conspicuously coloured
than the other, and there is no difference in the habits of the two
sexes which will account for this difference, we have reason to believe
in the influence of sexual selection; and this belief is strongly
confirmed when the more ornamented individuals, which are almost always
the males, display their attractions before the other sex. We may also
extend this conclusion to both sexes, when coloured alike, if their
colours are plainly analogous to those of one sex alone in certain other
species of the same group.
How, then, are we to account for the beautiful or even gorgeous colours
of many animals in the lowest classes? It appears very doubtful whether
such colours usually serve as a protection; but we are extremely liable
to err in regard to characters of all kinds in relation to protection,
as will be admitted by every one who has read Mr. Wallace's excellent
essay on this subject. It would not, for instance, at first occur to any
one that the perfect transparency of the Medusae, or jelly-fishes, was of
the highest service to them as a protection; but when we are reminded
by Haeckel that not only the medusae but many floating mollusca,
crustaceans, and even small oceanic fishes partake of this same
glass-like structure, we can hardly doubt that they thus escape the
notice of pelagic birds and other enemies.
Notwithstanding our ignorance how far colour in many cases serves as a
protection, the most probable view in regard to the splendid tints of
many of the lowest animals seems to be that their colours are the direct
result either of the chemical nature or the minute structure of their
tissues, independently of any benefit thus derived. Hardly any colour is
finer than that of arterial blood; but there is no reason to suppose
that the colour of the blood is in itself any advantage; and though it
adds to the beauty of the maiden's cheek, no one will pretend that it
has been acquired for this purpose. So again with many animals,
especially the lower ones, the bile is richly coloured; thus the extreme
beauty of the Eolidae (naked sea-slugs) is chiefly due, as I am informed
by Mr. Hancock, to the biliary glands seen through the translucent
integuments; this beauty being probably of no service to these animals.
The tints of the decaying leaves in an American forest are described by
every one as gorgeous; yet no one supposes that these tints are of the
least advantage to the trees. Bearing in mind how many substances
closely analogous to natural organic compounds have been recently formed
by chemists, and which exhibit the most splendid colours, it would have
been a strange fact if substances similarly coloured had not often
originated, independently of any useful end being thus gained, in the
complex laboratory of living organisms.
_The sub-kingdom of the Mollusca._--Throughout this great division
(taken in its largest acceptation) of the animal kingdom, secondary
sexual characters, such as we are here considering, never, as far as I
can discover, occur. Nor could they be expected in the three lowest
classes, namely in the Ascidians, Polyzoa, and Brachiopods (constituting
the Molluscoida of Huxley), for most of these animals are permanently
affixed to a support or have their sexes united in the same individual.
In the Lamellibranchiata, or bivalve shells, hermaphroditism is not
rare. In the next higher class of the Gasteropoda, or marine univalve
shells, the sexes are either united or separate. But in this latter case
the males never possess special organs for finding, securing, or
charming the females, or for fighting with other males. The sole
external difference between the sexes consists, as I am informed by Mr.
Gwyn Jeffreys, in the shell sometimes differing a little in form; for
instance, the shell of the male periwinkle (_Littorina littorea_) is
narrower and has a more elongated spire than that of the female. But
differences of this nature, it may be presumed, are directly connected
with the act of reproduction or with the development of the ova.
The Gasteropoda, though capable of locomotion and furnished with
imperfect eyes, do not appear to be endowed with sufficient mental
powers for the members of the same sex to struggle together in rivalry,
and thus to acquire secondary sexual characters. Nevertheless with the
pulmoniferous gasteropods, or land-snails, the pairing is preceded by
courtship; for these animals, though hermaphrodites, are compelled by
their structure to pair together. Agassiz remarks,[410] "Quiconque a eu
l'occasion d'observer les amours des limacons, ne saurait mettre en
doute la seduction deployee dans les mouvements et les allures qui
preparent et accomplissent le double embrassement de ces
hermaphrodites." These animals appear also susceptible of some degree of
permanent attachment: an accurate observer, Mr. Lonsdale, informs me
that he placed a pair of land-shells (_Helix pomatia_), one of which was
weakly, into a small and ill-provided garden. After a short time the
strong and healthy individual disappeared, and was traced by its track
of slime over a wall into an adjoining well-stocked garden. Mr. Lonsdale
concluded that it had deserted its sickly mate; but after an absence of
twenty-four hours it returned, and apparently communicated the result of
its successful exploration, for both then started along the same track
and disappeared over the wall.
Even in the highest class of the Mollusca, namely the Cephalopoda or
cuttle-fishes, in which the sexes are separate, secondary sexual
characters of the kind which we are here considering, do not, as far as
I can discover, occur. This is a surprising circumstance, as these
animals possess highly-developed sense-organs and have considerable
mental powers, as will be admitted by every one who has watched their
artful endeavours to escape from an enemy.[411] Certain Cephalopoda,
however, are characterised by one extraordinary sexual character,
namely, that the male element collects within one of the arms or
tentacles, which is then cast off, and, clinging by its sucking-discs to
the female, lives for a time an independent life. So completely does the
cast-off arm resemble a separate animal, that it was described by Cuvier
as a parasitic worm under the name of Hectocotyle. But this marvellous
structure may be classed as a primary rather than as a secondary sexual
character.
Although with the Mollusca sexual selection does not seem to have come
into play; yet many univalve and bivalve shells, such as volutes, cones,
scallops, &c., are beautifully coloured and shaped. The colours do not
appear in most cases to be of any use as a protection; they are probably
the direct result, as in the lowest classes, of the nature of the
tissues; the patterns and the sculpture of the shell depending on its
manner of growth. The amount of light seems to a certain extent to be
influential; for although, as repeatedly stated by Mr. Gwyn Jeffreys,
the shells of some species living at a profound depth are brightly
coloured, yet we generally see the lower surfaces and the parts covered
by the mantle less highly coloured than the upper and exposed
surfaces.[412] In some cases, as with shells living amongst corals or
brightly-tinted sea-weeds, the bright colours may serve as a protection.
But many of the nudibranch mollusca, or sea-slugs, are as beautifully
coloured as any shells, as may be seen in Messrs. Alder and Hancock's
magnificent work; and from information kindly given me by Mr. Hancock,
it is extremely doubtful whether these colours usually serve as a
protection. With some species this may be the case, as with one which
lives on the green leaves of algae, and is itself bright-green. But many
brightly-coloured, white or otherwise conspicuous species, do not seek
concealment; whilst again some equally conspicuous species, as well as
other dull-coloured kinds, live under stones and in dark recesses. So
that with these nudibranch molluscs, colour apparently does not stand in
any close relation to the nature of the places which they inhabit.
These naked sea-slugs are hermaphrodites, yet they pair together, as do
land-snails, many of which have extremely pretty shells. It is
conceivable that two hermaphrodites, attracted by each others' greater
beauty, might unite and leave offspring which would inherit their
parents' greater beauty. But with such lowly-organised creatures this is
extremely improbable. Nor is it at all obvious how the offspring from
the more beautiful pairs of hermaphrodites would have any advantage, so
as to increase in numbers, over the offspring of the less beautiful,
unless indeed vigour and beauty generally coincided. We have not here a
number of males becoming mature before the females, and the more
beautiful ones selected by the more vigorous females. If, indeed,
brilliant colours were beneficial to an hermaphrodite animal in relation
to its general habits of life, the more brightly-tinted individuals
would succeed best and would increase in number; but this would be a
case of natural and not of sexual selection.
_Sub-kingdom of the Vermes or Annulosa_: Class, _Annelida (or
Sea-worms)_.--In this class, although the sexes (when separate)
sometimes differ from each other in characters of such importance that
they have been placed under distinct genera or even families, yet the
differences do not seem of the kind which can be safely attributed to
sexual selection. These animals, like those in the preceding classes,
apparently stand too low in the scale, for the individuals of either sex
to exert any choice in selecting a partner, or for the individuals of
the same sex to struggle together in rivalry.
_Sub-kingdom of the Arthropoda_: Class, _Crustacea_.--In this great
class we first meet with undoubted secondary sexual characters, often
developed in a remarkable manner. Unfortunately the habits of
crustaceans are very imperfectly known, and we cannot explain the uses
of many structures peculiar to one sex. With the lower parasitic species
the males are of small size, and they alone are furnished with perfect
swimming-legs, antennae and sense-organs; the females being destitute of
these organs, with their bodies often consisting of a mere distorted
mass. But these extraordinary differences between the two sexes are no
doubt related to their widely different habits of life, and consequently
do not concern us. In various crustaceans, belonging to distinct
families, the anterior antennae are furnished with peculiar thread-like
bodies, which are believed to act as smelling-organs, and these are much
more numerous in the males than in the females. As the males, without
any unusual development of their olfactory organs, would almost
certainly be able sooner or later to find the females, the increased
number of the smelling-threads has probably been acquired through sexual
selection, by the better provided males having been the most successful
in finding partners and in leaving offspring. Fritz Mueller has described
a remarkable dimorphic species of Tanais, in which the male is
represented by two distinct forms, never graduating into each other. In
the one form the male is furnished with more numerous smelling-threads,
and in the other form with more powerful and more elongated chelae or
pincers which serve to hold the female. Fritz Mueller suggests that these
differences between the two male forms of the same species must have
originated in certain individuals having varied in the number of the
smelling-threads, whilst other individuals varied in the shape and size
of their chelae; so that of the former, those which were best able to
find the female, and of the latter, those which were best able to hold
her when found, have left the greater number of progeny to inherit their
respective advantages.[413]
[Illustration: Fig. 3. Labidocera Darwinii, (from Lubbock).
_a._ Part of right-hand anterior antenna of male, forming a prehensile
organ.
_b._ Posterior pair of thoracic legs of male.
_c._ Ditto of female.]
In some of the lower crustaceans, the right-hand anterior antenna of the
male differs greatly in structure from the left-hand one, the latter
resembling in its simple tapering joints the antennae of the female. In
the male the modified antenna is either swollen in the middle or
angularly bent, or converted (fig. 3) into an elegant, and sometimes
wonderfully complex, prehensile organ.[414] It serves, as I hear from
Sir J. Lubbock, to hold the female, and for this same purpose one of the
two posterior legs (_b_) on the same side of the body is converted into
a forceps. In another family the inferior or posterior antennae are
"curiously zigzagged" in the males alone.
[Illustration: Fig. 4. Anterior part of body of Callianassa (from
Milne-Edwards), showing the unequal and differently-constructed right
and left-hand chelae of the male.
N.B.--The artist by mistake has reversed the drawing, and made the
left-hand chela the largest.]
[Illustration: Fig. 5. Second leg of male Orchestia Tucuratinga (from
Fritz Mueller).]
[Illustration: Fig. 6. Ditto of female.]
In the higher crustaceans the anterior legs form a pair of chelae or
pincers, and these are generally larger in the male than in the female.
In many species the chelae on the opposite sides of the body are of
unequal size, the right-hand one being, as I am informed by Mr. C.
Spence Bate, generally, though not invariably, the largest. This
inequality is often much greater in the male than in the female. The two
chelae also often differ in structure (figs. 4, 5, and 6), the smaller one
resembling those of the female. What advantage is gained by their
inequality in size on the opposite sides of the body, and by the
inequality being much greater in the male than in the female; and why,
when they are of equal size, both are often much larger in the male than
in the female, is not known. The chelae are sometimes of such length and
size that they cannot possibly be used, as I hear from Mr. Spence Bate,
for carrying food to the mouth. In the males of certain freshwater
prawns (Palaemon) the right leg is actually longer than the whole
body.[415] It is probable that the great size of one leg with its chelae
may aid the male in fighting with his rivals; but this use will not
account for their inequality in the female on the opposite sides
of the body. In Gelasimus, according to a statement quoted by
Milne-Edwards,[416] the male and female live in the same burrow, which
is worth notice, as shewing that they pair, and the male closes the
mouth of the burrow with one of its chelae, which is enormously
developed; so that here it indirectly serves as a means of defence.
Their main use, however, probably is to seize and to secure the female,
and this in some instances, as with Gammarus, is known to be the case.
The sexes, however, of the common shore-crab (_Carcinus maenas_), as Mr.
Spence Bate informs me, unite directly after the female has moulted her
hard shell, and when she is so soft that she would be injured if seized
by the strong pincers of the male; but as she is caught and carried
about by the male previously to the act of moulting, she could then be
seized with impunity.
Fritz Mueller states that certain species of Melita are distinguished
from all other amphipods by the females having "the coxal lamellae of the
penultimate pair of feet produced into hook-like processes, of which the
males lay hold with the hands of the first pair." The development of
these hook-like processes probably resulted from those females which
were the most securely held during the act of reproduction, having left
the largest number of offspring. Another Brazilian amphipod (_Orchestia
Darwinii_, fig. 7) is described by Fritz Mueller, as presenting a case of
dimorphism, like that of Tanais; for there are two male forms, which
differ in the structure of their chelae.[417] As chelae of either shape
would certainly have sufficed to hold the female, for both are now used
for this purpose, the two male forms probably originated, by some having
varied in one manner and some in another; both forms having derived
certain special, but nearly equal advantages, from their differently
shaped organs.
It is not known that male crustaceans fight together for the possession
of the females, but this is probable; for with most animals when the
male is larger than the female, he seems to have acquired his greater
size by having conquered during many generations other males. Now Mr.
Spence Bate informs me that in most of the crustacean orders, especially
in the highest or the Brachyura, the male is larger than the female; the
parasitic genera, however, in which the sexes follow different habits of
life, and most of the Entomostraca must be excepted. The chelae of many
crustaceans are weapons well adapted for fighting. Thus a Devil-crab
(_Portunus puber_) was seen by a son of Mr. Bate fighting with a
_Carcinus maenas_, and the latter was soon thrown on its back, and had
every limb torn from its body. When several males of a Brazilian
Gelasimus, a species furnished with immense pincers, were placed
together by Fritz Mueller in a glass vessel, they mutilated and killed
each other. Mr. Bate put a large male _Carcinus maenas_ into a pan of
water, inhabited by a female paired with a smaller male; the latter was
soon dispossessed, but, as Mr. Bate adds, "if they fought, the victory
was a bloodless one, for I saw no wounds." This same naturalist
separated a male sand-skipper (so common on our sea-shores), _Gammarus
marinus_, from its female, both of which were imprisoned in the same
vessel with many individuals of the same species. The female being thus
divorced joined her comrades. After an interval the male was again put
into the same vessel and he then, after swimming about for a time,
dashed into the crowd, and without any fighting at once took away his
wife. This fact shews that in the Amphipoda, an order low in the scale,
the males and females recognise each other, and are mutually attached.
[Illustration: Fig. 7. Orchestia Darwinii (from Fritz Mueller), showing
the differently-constructed chelae of the two male forms.]
The mental powers of the Crustacea are probably higher than might have
been expected. Any one who has tried to catch one of the shore-crabs, so
numerous on many tropical coasts, will have perceived how wary and alert
they are. There is a large crab (_Birgus latro_), found on coral
islands, which makes at the bottom of a deep burrow a thick bed of the
picked fibres of the cocoa-nut. It feeds on the fallen fruit of this
tree by tearing off the husk, fibre by fibre; and it always begins at
that end where the three eye-like depressions are situated. It then
breaks through one of these eyes by hammering with its heavy front
pincers, and turning round, extracts the albuminous core with its narrow
posterior pincers. But these actions are probably instinctive, so that
they would be performed as well by a young as by an old animal. The
following case, however, can hardly be so considered: a trustworthy
naturalist, Mr. Gardner,[418] whilst watching a shore-crab (Gelasimus)
making its burrow, threw some shells towards the hole. One rolled in,
and three other shells remained within a few inches of the mouth. In
about five minutes the crab brought out the shell which had fallen in,
and carried it away to the distance of a foot; it then saw the three
other shells lying near, and evidently thinking that they might likewise
roll in, carried them to the spot where it had laid the first. It would,
I think, be difficult to distinguish this act from one performed by man
by the aid of reason.
With respect to colour which so often differs in the two sexes of
animals belonging to the higher classes, Mr. Spence Bate does not know
of any well-marked instances with our British crustaceans. In some
cases, however, the male and female differ slightly in tint, but Mr.
Bate thinks not more than may be accounted for by their different habits
of life, such as by the male wandering more about and being thus more
exposed to the light. In a curious Bornean crab, which inhabits sponges,
Mr. Bate could always distinguish the sexes by the male not having the
epidermis so much rubbed off. Dr. Power tried to distinguish by colour
the sexes of the species which inhabit the Mauritius, but always failed,
except with one species of Squilla, probably the _S. stylifera_, the
male of which is described as being "of a beautiful blueish-green," with
some of the appendages cherry-red, whilst the female is clouded with
brown and grey, "with the red about her much less vivid than in the
male."[419] In this case, we may suspect the agency of sexual selection.
With Saphirina (an oceanic genus of Entomostraca, and therefore low in
the scale) the males are furnished with minute shields or cell-like
bodies, which exhibit beautiful changing colours; these being absent in
the females, and in the case of one species in both sexes.[420] It
would, however, be extremely rash to conclude that these curious organs
serve merely to attract the females. In the female of a Brazilian
species of Gelasimus, the whole body, as I am informed by Fritz Mueller,
is of a nearly uniform greyish-brown. In the male the posterior part of
the cephalo-thorax is pure white, with the anterior part of a rich
green, shading into dark brown; and it is remarkable that these colours
are liable to change in the course of a few minutes--the white becoming
dirty grey or even black, the green "losing much of its brilliancy." The
males apparently are much more numerous than the females. It deserves
especial notice that they do not acquire their bright colours until they
become mature. They differ also from the females in the larger size of
their chelae. In some species of the genus, probably in all, the sexes
pair and inhabit the same burrow. They are also, as we have seen, highly
intelligent animals. From these various considerations it seems highly
probable that the male in this species has become gaily ornamented in
order to attract or excite the female.
It has just been stated that the male Gelasimus does not acquire his
conspicuous colours until mature and nearly ready to breed. This seems
the general rule in the whole class with the many remarkable differences
in structure between the two sexes. We shall hereafter find the same law
prevailing throughout the great sub-kingdom of the Vertebrata, and in
all cases it is eminently distinctive of characters which have been
acquired through sexual selection. Fritz Mueller[421] gives some
striking instances of this law; thus the male sand-hopper (Orchestia)
does not acquire his large claspers, which are very differently
constructed from those of the female, until nearly full-grown; whilst
young his claspers resemble those of the female. Thus, again, the male
Brachyscelus possesses, like all other amphipods, a pair of posterior
antennae; the female, and this is a most extraordinary circumstance, is
destitute of them, and so is the male as long as he remains immature.
Class, _Arachnida_ (Spiders).--The males are often darker, but sometimes
lighter than the females, as may be seen in Mr. Blackwall's magnificent
work.[422] In some species the sexes differ conspicuously from each
other in colour; thus the female of _Sparassus smaragdulus_ is
dullish-green; whilst the adult male has the abdomen of a fine yellow,
with three longitudinal stripes of rich red. In some species of Thomisus
the two sexes closely resemble each other; in others they differ much;
thus in _T. citreus_ the legs and body of the female are pale-yellow or
green, whilst the front legs of the male are reddish-brown: in _T.
floricolens_, the legs of the female are pale-green, those of the male
being ringed in a conspicuous manner with various tints. Numerous
analogous cases could be given in the genera Epeira, Nephila,
Philodromus, Theridion, Linyphia, &c. It is often difficult to say which
of the two sexes departs most from the ordinary coloration of the genus
to which the species belong; but Mr. Blackwall thinks that, as a
general rule, it is the male. Both sexes whilst young, as I am informed
by the same author, usually resemble each other; and both often undergo
great changes in colour during their successive moults before arriving
at maturity. In other cases the male alone appears to change colour.
Thus the male of the above-mentioned brightly-coloured Sparassus at
first resembles the female and acquires his peculiar tints only when
nearly adult. Spiders are possessed of acute senses, and exhibit much
intelligence. The females often shew, as is well known, the strongest
affection for their eggs, which they carry about enveloped in a silken
web. On the whole it appears probable that well-marked differences in
colour between the sexes have generally resulted from sexual selection,
either on the male or female side. But doubts may be entertained on this
head from the extreme variability in colour of some species, for
instance of _Theridion lineatum_, the sexes of which differ when adult;
this great variability indicates that their colours have not been
subjected to any form of selection.
Mr. Blackwall does not remember to have seen the males of any species
fighting together for the possession of the female. Nor, judging from
analogy, is this probable; for the males are generally much smaller than
the females, sometimes to an extraordinary degree.[423] Had the males
been in the habit of fighting together, they would, it is probable, have
gradually acquired greater size and strength. Mr. Blackwall has
sometimes seen two or more males on the same web with a single female;
but their courtship is too tedious and prolonged an affair to be easily
observed. The male is extremely cautious in making his advances, as the
female carries her coyness to a dangerous pitch. De Geer saw a male that
"in the midst of his preparatory caresses was seized by the object of
his attentions, enveloped by her in a web and then devoured, a sight
which, as he adds, filled him with horror and indignation."[424]
Westring has made the interesting discovery that the males of several
species of Theridion[425] have the power of making a stridulating sound
(like that made by many beetles and other insects, but feebler), whilst
the females are quite mute. The apparatus consists of a serrated ridge
at the base of the abdomen, against which the hard hinder part of the
thorax is rubbed; and of this structure not a trace could be detected in
the females. From the analogy of the Orthoptera and Homoptera, to be
described in the next chapter, we may feel almost sure that the
stridulation serves, as Westring remarks, either to call or to excite
the female; and this is the first case in the ascending scale of the
animal kingdom, known to me, of sounds emitted for this purpose.
Class, _Myriapoda_.--In neither of the two orders in this class,
including the millipedes and centipedes, can I find any well-marked
instances of sexual differences such as more particularly concern us. In
_Glomeris limbata_, however, and perhaps in some few other species, the
males differ slightly in colour from the females; but this Glomeris is a
highly variable species. In the males of the Diplopoda, the legs
belonging to one of the anterior segments of the body, or to the
posterior segment, are modified into prehensile hooks which serve to
secure the female. In some species of Iulus the tarsi of the male are
furnished with membranous suckers for the same purpose. It is a much
more unusual circumstance, as we shall see when we treat of Insects,
that it is the female in Lithobius which is furnished with prehensile
appendages at the extremity of the body for holding the male.[426]
CHAPTER X.
SECONDARY SEXUAL CHARACTERS OF INSECTS.
Diversified structures possessed by the males for seizing the
females--Differences between the sexes, of which the meaning is
not understood--Difference in size between the
sexes--Thysanura--Diptera--Hemiptera--Homoptera, musical powers
possessed by the males alone--Orthoptera, musical instruments
of the males, much diversified in structure; pugnacity;
colours--Neuroptera, sexual differences in colour--Hymenoptera,
pugnacity and colours--Coleoptera, colours; furnished with
great horns, apparently as an ornament; battles; stridulating
organs generally common to both sexes.
In the immense class of insects the sexes sometimes differ in their
organs for locomotion, and often in their sense-organs, as in the
pectinated and beautifully plumose antennae of the males of many species.
In one of the Ephemerae, namely Chloeon, the male has great pillared
eyes, of which the female is entirely destitute.[427] The ocelli are
absent in the females of certain other insects, as in the Mutillidae,
which are likewise destitute of wings. But we are chiefly concerned with
structures by which one male is enabled to conquer another, either in
battle or courtship, through his strength, pugnacity, ornaments, or
music. The innumerable contrivances, therefore, by which the male is
able to seize the female, may be briefly passed over. Besides the
complex structures at the apex of the abdomen, which ought perhaps to be
ranked as primary organs,[428] "it is astonishing," as Mr. B. D.
Walsh[429] has remarked, "how many different organs are worked in by
nature, for the seemingly insignificant object of enabling the male to
grasp the female firmly." The mandibles or jaws are sometimes used for
this purpose; thus the male _Corydalis cornutus_ (a neuropterous insect
in some degree allied to the Dragon-flies, &c.) has immense curved jaws,
many times longer than those of the female; and they are smooth instead
of being toothed, by which means he is enabled to seize her without
injury.[430] One of the stag-beetles of North America (_Lucanus
elaphus_) uses his jaws, which are much larger than those of the female,
for the same purpose, but probably likewise for fighting. In one of the
sand-wasps (_Ammophila_) the jaws in the two sexes are closely alike,
but are used for widely different purposes; the males, as Professor
Westwood observes, "are exceedingly ardent, seizing their partners round
the neck with their sickle-shaped jaws;"[431] whilst the females use
these organs for burrowing in sand-banks and making their nests.
[Illustration: Fig. 8. Crabro cribrarius. Upper figure, male: lower
figure, female.]
The tarsi of the front-legs are dilated in many male beetles, or are
furnished with broad cushions of hairs; and in many genera of
water-beetles they are armed with a round flat sucker, so that the male
may adhere to the slippery body of the female. It is a much more unusual
circumstance that the females of some water-beetles (Dytiscus) have
their elytra deeply grooved, and in _Acilius sulcatus_ thickly set with
hairs, as an aid to the male. The females of some other water-beetles
(Hydroporus) have their elytra punctured for the same object.[432] In
the male of _Crabro cribrarius_ (fig. 8.), it is the tibia which is
dilated into a broad horny plate, with minute membraneous dots, giving
to it a singular appearance like that of a riddle.[433] In the male of
Penthe (a genus of beetles) a few of the middle joints of the antennae
are dilated and furnished on the inferior surface with cushions of
hair, exactly like those on the tarsi of the Carabidae, "and obviously
for the same end." In male dragon-flies, "the appendages at the tip of
the tail are modified in an almost infinite variety of curious patterns
to enable them to embrace the neck of the female." Lastly in the males
of many insects, the legs are furnished with peculiar spines, knobs or
spurs; or the whole leg is bowed or thickened, but this is by no means
invariably a sexual character; or one pair, or all three pairs are
elongated, sometimes to an extravagant length.[434]
[Illustration: Fig. 9. Taphroderes distortus (much enlarged). Upper
figure, male; lower figure, female.]
In all the orders, the sexes of many species present differences, of
which the meaning is not understood. One curious case is that of a
beetle (fig. 9), the male of which has the left mandible much enlarged;
so that the mouth is greatly distorted. In another Carabidous beetle,
the Eurygnathus,[435] we have the unique case, as far as known to Mr.
Wollaston, of the head of the female being much broader and larger,
though in a variable degree, than that of the male. Any number of such
cases could be given. They abound in the Lepidoptera: one of the most
extraordinary is that certain male butterflies have their fore-legs more
or less atrophied, with the tibiae and tarsi reduced to mere rudimentary
knobs. The wings, also, in the two sexes often differ in neuration,[436]
and sometimes considerably in outline, as in the _Aricoris epitus_,
which was shown to me in the British Museum by Mr. A. Butler. The males
of certain South American butterflies have tufts of hair on the margins
of the wings, and horny excrescences on the discs of the posterior
pair.[437] In several British butterflies, the males alone, as shewn by
Mr. Wonfor, are in parts clothed with peculiar scales.
The purpose of the luminosity in the female glow-worm is likewise not
understood; for it is very doubtful whether the primary use of the light
is to guide the male to the female. It is no serious objection to this
latter belief that the males emit a feeble light; for secondary sexual
characters proper to one sex are often developed in a slight degree in
the other sex. It is a more valid objection that the larvae shine, and in
some species brilliantly: Fritz Mueller informs me that the most luminous
insect which he ever beheld in Brazil, was the larva of some beetle.
Both sexes of certain luminous species of Elater emit light. Kirby and
Spence suspect that the phosphorescence serves to frighten and drive
away enemies.
_Difference in Size between the Sexes._--With insects of all kinds the
males are commonly smaller than the females;[438] and this difference
can often be detected even in the larval state. So considerable is the
difference between the male and female cocoons of the silk-moth
(_Bombyx mori_), that in France they are separated by a particular mode
of weighing.[439] In the lower classes of the animal kingdom, the
greater size of the females seems generally to depend on their
developing an enormous number of ova; and this may to a certain extent
hold good with insects. But Dr. Wallace has suggested a much more
probable explanation. He finds, after carefully attending to the
development of the caterpillars of _Bombyx cynthia_ and _yamamai_, and
especially of some dwarfed caterpillars reared from a second brood on
unnatural food, "that in proportion as the individual moth is finer, so
is the time required for its metamorphosis longer; and for this reason
the female, which is the larger and heavier insect, from having to carry
her numerous eggs, will be preceded by the male, which is smaller and
has less to mature."[440] Now as most insects are short-lived, and as
they are exposed to many dangers, it would manifestly be advantageous to
the female to be impregnated as soon as possible. This end would be
gained by the males being first matured in large numbers ready for the
advent of the females; and this again would naturally follow, as Mr. A.
E. Wallace has remarked,[441] through natural selection; for the smaller
males would be first matured, and thus would procreate a large number of
offspring which would inherit the reduced size of their male parents,
whilst the larger males from being matured later would leave fewer
offspring.
There are, however, exceptions to the rule of male insects being smaller
than the females; and some of these exceptions are intelligible. Size
and strength would be an advantage to the males, which fight for the
possession of the female; and in these cases the males, as with the
stag-beetle (Lucanus), are larger than the females. There are, however,
other beetles which are not known to fight together, of which the males
exceed the females in size; and the meaning of this fact is not known;
but in some of these cases, as with the huge Dynastes and Megasoma, we
can at least see that there would be no necessity for the males to be
smaller than the females, in order to be matured before them, for these
beetles are not short-lived, and there would be ample time for the
pairing of the sexes. So, again, male dragon-flies (Libellulidae) are
sometimes sensibly larger, and never smaller, than the females;[442] and
they do not, as Mr. MacLachlan believes, generally pair with the
females, until a week or fortnight has elapsed, and until they have
assumed their proper masculine colours. But the most curious case,
shewing on what complex and easily-overlooked relations, so trifling a
character as a difference in size between the sexes may depend, is that
of the aculeate Hymenoptera; for Mr. F. Smith informs me that throughout
nearly the whole of this large group the males, in accordance with the
general rule, are smaller than the females and emerge about a week
before them; but amongst the Bees, the males of _Apis mellifica_,
_Anthidium manicatum_ and _Anthophora acervorum_, and amongst the
Fossores, the males of the _Methoca ichneumonides_, are larger than the
females. The explanation of this anomaly is that a marriage-flight is
absolutely necessary with these species, and the males require great
strength and size in order to carry the females through the air.
Increased size has here been acquired in opposition to the usual
relation between size and the period of development, for the males,
though larger, emerge before the smaller females.
We will now review the several Orders, selecting such facts as more
particularly concern us. The Lepidoptera (Butterflies and Moths) will be
retained for a separate chapter.
Order, _Thysanura_.--The members of this Order are lowly organised for
their class. They are wingless, dull-coloured, minute insects, with
ugly, almost misshapen heads and bodies. The sexes do not differ; but
they offer one interesting fact, by showing that the males pay sedulous
court to their females even low down in the animal scale. Sir J.
Lubbock[443] in describing the _Smynthurus luteus_, says: "it is very
amusing to see these little creatures coquetting together. The male,
which is much smaller than the female, runs round her, and they butt one
another, standing face to face, and moving backward and forward like two
playful lambs. Then the female pretends to run away and the male runs
after her with a queer appearance of anger, gets in front and stands
facing her again; then she turns coyly round, but he, quicker and more
active, scuttles round too, and seems to whip her with his antennae; then
for a bit they stand face to face, play with their antennae, and seem to
be all in all to one another."
Order, _Diptera_ (Flies).--The sexes differ little in colour. The
greatest difference, known to Mr. F. Walker, is in the genus Bibio, in
which the males are blackish or quite black, and the females obscure
brownish-orange. The genus Elaphomyia, discovered by Mr. Wallace[444] in
New Guinea, is highly remarkable, as the males are furnished with horns,
of which the females are quite destitute. The horns spring from beneath
the eyes, and curiously resemble those of stags, being either branched
or palmated. They equal in length the whole of the body in one of the
species. They might be thought to serve for fighting, but as in one
species they are of a beautiful pink colour, edged with black, with a
pale central stripe, and as these insects have altogether a very elegant
appearance, it is perhaps more probable that the horns serve as
ornaments. That the males of some Diptera fight together is certain; for
Prof. Westwood[445] has several times seen this with some species of
Tipula or Harry-long-legs. Many observers believe that when gnats
(Culicidae) dance in the air in a body, alternately rising and falling,
the males are courting the females. The mental faculties of the Diptera
are probably fairly well developed, for their nervous system is more
highly developed than in most other Orders of insects.[446]
Order, _Hemiptera_ (Field-Bugs).--Mr. J. W. Douglas, who has
particularly attended to the British species, has kindly given me an
account of their sexual differences. The males of some species are
furnished with wings, whilst the females are wingless; the sexes differ
in the form of the body and elytra; in the second joints of their
antennae and in their tarsi; but as the signification of these
differences is quite unknown, they may be here passed over. The females
are generally larger and more robust than the males. With British, and,
as far as Mr. Douglas knows, with exotic species, the sexes do not
commonly differ much in colour; but in about six British species the
male is considerably darker than the female, and in about four other
species the female is darker than the male. Both sexes of some species
are beautifully marked with vermilion and black. It is doubtful whether
these colours serve as a protection. If in any species the males had
differed from the females in an analogous manner, we might have been
justified in attributing such conspicuous colours to sexual selection
with transference to both sexes.
Some species of Reduvidae make a stridulating noise; and, in the case of
_Pirates stridulus_, this is said[447] to be effected by the movement of
the neck within the pro-thoracic cavity. According to Westring,
_Reduvius personatus_ also stridulates. But I have not been able to
learn any particulars about these insects; nor have I any reason to
suppose that they differ sexually in this respect.
Order, _Homoptera._--Every one who has wandered in a tropical forest
must have been astonished at the din made by the male Cicadae. The
females are mute; as the Grecian poet Xenarchus says, "Happy the Cicadas
live, since they all have voiceless wives." The noise thus made could be
plainly heard on board the "Beagle," when anchored at a quarter of a
mile from the shore of Brazil; and Captain Hancock says it can be heard
at the distance of a mile. The Greeks formerly kept, and the Chinese now
keep, these insects in cages for the sake of their song, so that it
must be pleasing to the ears of some men.[448] The Cicadidae usually sing
during the day; whilst the Fulgoridae appear to be night-songsters. The
sound, according to Landois,[449] who has recently studied the subject,
is produced by the vibration of the lips of the spiracles, which are set
into motion by a current of air emitted from the tracheae. It is
increased by a wonderfully complex resounding apparatus, consisting of
two cavities covered by scales. Hence the sound may truly be called a
voice. In the female the musical apparatus is present, but very much
less developed than in the male, and is never used for producing sound.
With respect to the object of the music, Dr. Hartman in speaking of the
_Cicada septemdecim_ of the United States, says,[450] "the drums are now
(June 6th and 7th, 1851) heard in all directions. This I believe to be
the marital summons from the males. Standing in thick chestnut sprouts
about as high as my head, where hundreds were around me, I observed the
females coming around the drumming males." He adds, "this season (Aug.
1868) a dwarf pear-tree in my garden produced about fifty larvae of _Cic.
pruinosa_; and I several times noticed the females to alight near a male
while he was uttering his clanging notes." Fritz Mueller writes to me
from S. Brazil that he has often listened to a musical contest between
two or three males of a Cicada, having a particularly loud voice, and
seated at a considerable distance from each other. As soon as the first
had finished his song, a second immediately began; and after he had
concluded, another began, and so on. As there is so much rivalry between
the males, it is probable that the females not only discover them by the
sounds emitted, but that, like female birds, they are excited or allured
by the male with the most attractive voice.
I have not found any well-marked cases of ornamental differences between
the sexes of the Homoptera. Mr. Douglas informs me that there are three
British species, in which the male is black or marked with black bands,
whilst the females are pale-coloured or obscure.
Order, _Orthoptera_.--The males in the three saltatorial families
belonging to this Order are remarkable for their musical powers, namely
the Achetidae or crickets, the Locustidae for which there is no exact
equivalent name in English, and the Acridiidae or grasshoppers. The
stridulation produced by some of the Locustidae is so loud that it can be
heard during the night at the distance of a mile;[451] and that made by
certain species is not unmusical even to the human ear, so that the
Indians on the Amazons keep them in wicker cages. All observers agree
that the sounds serve either to call or excite the mute females. But it
has been noticed[452] that the male migratory locust of Russia (one of
the Acridiidae) whilst coupled with the female, stridulates from anger or
jealousy when approached by another male. The house-cricket when
surprised at night uses its voice to warn its fellows.[453] In North
America the Katy-did (_Platyphyllum concavum_, one of the Locustidae) is
described[454] as mounting on the upper branches of a tree, and in the
evening beginning "his noisy babble, while rival notes issue from the
neighbouring trees, and the groves resound with the call of
_Katy-did-she-did_, the live-long night." Mr. Bates, in speaking of the
European field-cricket (one of the Achetidae), says, "the male has been
observed to place itself in the evening at the entrance of its burrow,
and stridulate until a female approaches, when the louder notes are
succeeded by a more subdued tone, whilst the successful musician
caresses with his antennae the mate he has won."[455] Dr. Scudder was
able to excite one of these insects to answer him, by rubbing on a file
with a quill.[456] In both sexes a remarkable auditory apparatus has
been discovered by Von Siebold, situated in the front legs.[457]
[Illustration: Fig. 10. Gryllus campestris (from Landois).
Right-hand figure, under side of part of the wing-nervure, much
magnified, showing the teeth, _st_.
Left-hand figure, upper surface of wing-cover, with the projecting,
smooth nervure, _r._, across which the teeth (_st_) are scraped.]
In the three Families the sounds are differently produced. In the males
Of the Achetidae both wing-covers have the same structure; and this in
the field-cricket (_Gryllus campestris_, fig. 10) consists, as
described by Landois,[458] of from 131 to 138 sharp, transverse ridges
or teeth (_st_) on the under side of one of the nervures of the
wing-cover. This toothed nervure is rapidly scraped across a projecting,
smooth, hard nervure (_r_) on the upper surface of the opposite wing.
First one wing is rubbed over the other, and then the movement is
reversed. Both wings are raised a little at the same time, so as to
increase the resonance. In some species the wing-covers of the males are
furnished at the base with a talc-like plate.[459] I have here given a
drawing (fig. 11) of the teeth on the under side of the nervure of
another species of Gryllus, viz. _G. domesticus_.
[Illustration: Fig. 11. Teeth of Nervure of Gryllus domesticus (from
Landois).]
In the Locustidae the opposite wing-covers differ in structure (fig. 12),
and cannot, as in the last family, be indifferently used in a reversed
manner. The left wing, which acts as the bow of the fiddle, lies over
the right wing which serves as the fiddle itself. One of the nervures
(_a_) on the under surface of the former is finely serrated, and is
scraped across the prominent nervures on the upper surface of the
opposite or right wing. In our British _Phasgonura viridissima_ it
appeared to me that the serrated nervure is rubbed against the rounded
hind corner of the opposite wing, the edge of which is thickened,
coloured brown, and very sharp. In the right wing, but not in the left,
there is a little plate, as transparent as talc, surrounded by nervures,
and called the speculum. In _Ephippiger vitium_, a member of this same
family, we have a curious subordinate modification; for the wing-covers
are greatly reduced in size, but "the posterior part of the pro-thorax
is elevated into a kind of dome over the wing-covers, and which has
probably the effect of increasing the sound."[460]
[Illustration: Fig. 12. Chlorocoelus Tanana (from Bates). _a_, _b_.
Lobes of opposite wing-covers.]
We thus see that the musical apparatus is more differentiated or
specialised in the Locustidae, which includes I believe the most powerful
performers in the Order, than in the Achetidae, in which both wing-covers
have the same structure and the same function.[461] Landois, however,
detected in one of the Locustidae, namely in Decticus, a short and narrow
row of small teeth, mere rudiments, on the inferior surface of the
right wing-cover, which underlies the other and is never used as the
bow. I observed the same rudimentary structure on the under side of the
right wing-cover in _Phasgonura viridissima_. Hence we may with
confidence infer that the Locustidae are descended from a form, in which,
as in the existing Achetidae, both wing-covers had serrated nervures on
the under surface, and could be indifferently used as the bow; but that
in the Locustidae the two wing-covers gradually became differentiated and
perfected, on the principle of the division of labour, the one to act
exclusively as the bow and the other as the fiddle. By what steps the
more simple apparatus in the Achetidae originated, we do not know, but it
is probable that the basal portions of the wing-covers overlapped each
other formerly as at present, and that the friction of the nervures
produced a grating sound, as I find is now the case with the wing-covers
of the females.[462] A grating sound thus occasionally and accidentally
made by the males, if it served them ever so little as a love-call to
the females, might readily have been intensified through sexual
selection by fitting variations in the roughness of the nervures having
been continually preserved.
[Illustration: Fig. 13, Hind-leg of Stenobothrus pratorum: _r_, the
stridulating ridge; lower figure, the teeth, forming the ridge, much
magnified (from Landois).]
In the last and third Family, namely the Acridiidae or grasshoppers, the
stridulation is produced in a very different manner, and is not so
shrill, according to Dr. Scudder, as in the preceding Families. The
inner surface of the femur (fig. 13, _r_) is furnished with a
longitudinal row of minute, elegant, lancet-shaped, elastic teeth, from
85 to 93 in number;[463] and these are scraped across the sharp,
projecting nervures on the wing-covers, which, are thus made to vibrate
and resound. Harris[464] says that when one of the males begins to play,
he first "bends the shank of the hind-leg beneath, the thigh, where it
is lodged in a furrow designed to receive it, and then draws the leg
briskly up and down. He does not play both fiddles together, but
alternately first upon one and then on the other." In many species, the
base of the abdomen is hollowed out into a great cavity which is
believed to act as a resounding board. In Pneumora (fig. 14), a S.
African genus belonging to this same family, we meet with a new and
remarkable modification: in the males a small notched ridge projects
obliquely from each side of the abdomen, against which the hind femora
are rubbed.[465] As the male is furnished with wings, the female being
wingless, it is remarkable that the thighs are not rubbed in the usual
manner against the wing-covers; but this may perhaps be accounted for by
the unusually small size of the hind-legs. I have not been able to
examine the inner surface of the thighs, which, judging from analogy,
would be finely serrated. The species of Pneumora have been more
profoundly modified for the sake of stridulation than any other
orthopterous insect; for in the male the whole body has been converted
into a musical instrument, being distended with air, like a great
pellucid bladder, so as to increase the resonance. Mr. Trimen informs me
that at the Cape of Good Hope these insects make a wonderful noise
during the night There is one exception to the rule that the females in
these three Families are destitute of an efficient musical apparatus;
for both sexes of Ephippiger (Locustidae) are said[466] to be thus
provided. This case may be compared with that of the reindeer, in which
species alone both sexes possess horns. Although the female orthoptera
are thus almost invariably mute, yet Landois[467] found rudiments of the
stridulating organs on the femora of the female Acridiidae, and similar
rudiments on the under surface of the wing-covers of the female
Achetidae; but he failed to find any rudiments in the females of
Decticus, one of the Locustidae. In the Homoptera the mute females of
Cicada, have the proper musical apparatus in an undeveloped state; and
we shall hereafter meet in other divisions of the animal kingdom with
innumerable instances of structures proper to the male being present in
a rudimentary condition in the female. Such cases appear at first sight
to indicate that both sexes were primordially constructed in the same
manner, but that certain organs were subsequently lost by the females.
It is, however, a more probable view, as previously explained, that the
organs in question were acquired by the males and partially transferred
to the females.
[Illustration: Fig. 14. Pneumora (from specimens in the British Museum).
Upper figure, male; lower figure, female.]
Landois has observed another interesting fact, namely that in the
females of the Acridiidae, the stridulating teeth on the femora remain
throughout life in the same condition in which they first appear in both
sexes during the larval state. In the males, on the other hand, they
become fully developed and acquire their perfect structure at the last
moult, when the insect is mature and ready to breed.
From the facts now given, we see that the means by which the males
produce their sounds are extremely diversified in the Orthoptera, and
are altogether different from those employed by the Homoptera. But
throughout the animal kingdom we incessantly find the same object
gained by the most diversified means; this being due to the whole
organisation undergoing in the course of ages multifarious changes; and
as part after part varies, different variations are taken advantage of
for the same general purpose. The diversification of the means for
producing sound in the three families of the Orthoptera and in the
Homoptera, impresses the mind with the high importance of these
structures to the males, for the sake of calling or alluring the
females. We need feel no surprise at the amount of modification which
the Orthoptera have undergone in this respect, as we now know, from Dr.
Scudder's remarkable discovery,[468] that there has been more than ample
time. This naturalist has lately found a fossil insect in the Devonian
formation of New Brunswick, which is furnished with "the well-known
tympanum or stridulating apparatus of the male Locustidae." This insect,
though in most respects related to the Neuroptera, appears to connect,
as is so often the case with very ancient forms, the two Orders of the
Neuroptera and Orthoptera which are now generally ranked as quite
distinct.
I have but little more to say on the Orthoptera. Some of the species are
very pugnacious: when two male field-crickets (_Gryllus campestris_) are
confined together, they fight till one kills the other; and the species
of Mantis are described as manoeuvring with their sword-like
front-limbs, like hussars with their sabres. The Chinese keep these
insects in little bamboo cages and match them like game-cocks.[469] With
respect to colour, some exotic locusts are beautifully ornamented; the
posterior wings being marked with red, blue, and black; but as
throughout the Order the two sexes rarely differ much in colour, it is
doubtful whether they owe these bright tints to sexual selection.
Conspicuous colours may be of use to these insects as a protection, on
the principle to be explained in the next chapter, by giving notice to
their enemies that they are unpalatable. Thus it has been observed[470]
that an Indian brightly-coloured locust was invariably rejected when
offered to birds and lizards. Some cases, however, of sexual differences
in colour in this Order are known. The male of an American cricket[471]
is described as being as white as ivory, whilst the female varies from
almost white to greenish-yellow or dusky. Mr. Walsh informs me that the
adult male of _Spectrum femoratum_ (one of the Phasmidae) "is of a
shining brownish-yellow colour; the adult female being of a dull,
opaque, cinereous-brown; the young of both sexes being green." Lastly, I
may mention that the male of one curious kind of cricket[472] is
furnished with "a long membranous appendage, which falls over the face
like a veil;" but whether this serves as an ornament is not known.
Order, _Neuroptera_.--Little need here be said, except in regard to
colour. In the Ephemeridae the sexes often differ slightly in their
obscure tints;[473] but it is not probable that the males are thus
rendered attractive to the females. The Libellulidae or dragon-flies are
ornamented with splendid green, blue, yellow, and vermilion metallic
tints; and the sexes often differ. Thus, the males of some of the
Agrionidae, as Prof. Westwood remarks[474] "are of a rich blue with black
wings, whilst the females are fine green with colourless wings." But in
_Agrion Ramburii_ these colours are exactly reversed in the two
sexes.[475] In the extensive N. American genus of Hetaerina, the males
alone have a beautiful carmine spot at the base of each wing. In _Anax
junius_ the basal part of the abdomen in the male is a vivid
ultra-marine blue, and in the female grass-green. In the allied genus
Gomphus, on the other hand, and in some other genera, the sexes differ
but little in colour. Throughout the animal kingdom, similar cases of
the sexes of closely-allied forms either differing greatly, or very
little, or not at all, are of frequent occurrence. Although with many
Libellulidae there is so wide a difference in colour between the sexes,
it is often difficult to say which is the most brilliant; and the
ordinary coloration of the two sexes is exactly reversed, as we have
just seen, in one species of Agrion. It is not probable that their
colours in any case have been gained as a protection. As Mr. MacLachlan,
who has closely attended to this family, writes to me, dragon-flies--the
tyrants of the insect-world--are the least liable of any insect to be
attacked by birds or other enemies. He believes that their bright
colours serve as a sexual attraction. It deserves notice, as bearing on
this subject, that certain dragon-flies appear to be attracted by
particular colours: Mr. Patterson observed[476] that the species of
Agrionidae, of which the males are blue, settled in numbers on the blue
float of a fishing line; whilst two other species were attracted by
shining white colours.
It is an interesting fact, first observed by Schelver, that the males,
in several genera belonging to two sub-families, when they first emerge
from the pupal state are coloured exactly like the females; but that
their bodies in a short time assume a conspicuous milky-blue tint, owing
to the exudation of a kind of oil, soluble in ether and alcohol. Mr.
MacLachlan believes that in the male of _Libellula depressa_ this change
of colour does not occur until nearly a fortnight after the
metamorphosis, when the sexes are ready to pair.
Certain species of Neurothemis present, according to Brauer[477] a
curious case of dimorphism, some of the females having their wings
netted in the usual manner; whilst other females have them "very richly
netted as in the males of the same species." Brauer "explains the
phenomenon on Darwinian principles by the supposition that the close
netting of the veins is a secondary sexual character in the males." This
latter character is generally developed in the males alone, but being,
like every other masculine character, latent in the female, is
occasionally developed in them. We have here an illustration of the
manner in which the two sexes of many animals have probably come to
resemble each other, namely by variations first appearing in the males,
being preserved in them, and then transmitted to and developed in the
females; but in this particular genus a complete transference is
occasionally and abruptly effected. Mr. MacLachlan informs me of another
case of dimorphism occurring in several species of Agrion in which a
certain number of individuals are found of an orange colour, and these
are invariably females. This is probably a case of reversion, for in
the true Libellulae, when the sexes differ in colour, the females are
always orange or yellow, so that supposing Agrion to be descended from
some primordial form having the characteristic sexual colours of the
typical Libellulae, it would not be surprising that a tendency to vary in
this manner should occur in the females alone.
Although many dragon-flies are such large, powerful, and fierce insects,
the males have not been observed by Mr. MacLachlan to fight together,
except, as he believes, in the case of some of the smaller species of
Agrion. In another very distinct group in this Order, namely in the
Termites or white ants, both sexes at the time of swarming may be seen
running about, "the male after the female, sometimes two chasing one
female, and contending with great eagerness who shall win the
prize."[478]
Order, _Hymenoptera_.--That inimitable observer, M. Fabre,[479] in
describing the habits of Cerceris, a wasp-like insect, remarks that
"fights frequently ensue between the males for the possession of some
particular female, who sits an apparently unconcerned beholder of the
struggle for supremacy, and when the victory is decided, quietly flies
away in company with the conqueror." Westwood[480] says that the males
of one of the saw-flies (Tenthredinae) "have been found fighting
together, with their mandibles locked." As M. Fabre speaks of the males
of Cerceris striving to obtain a particular female, it may be well to
bear in mind that insects belonging to this Order have the power of
recognising each other after long intervals of time, and are deeply
attached. For instance, Pierre Huber, whose accuracy no one doubts,
separated some ants, and when after an interval of four months they met
others which had formerly belonged to the same community, they mutually
recognised and caressed each other with their antennae. Had they been
strangers they would have fought together. Again, when two communities
engage in a battle, the ants on the same side in the general confusion
sometimes attack each other, but they soon perceive their mistake, and
the one ant soothes the other.[481]
In this Order slight differences in colour, according to sex, are
common, but conspicuous differences are rare except in the family of
Bees; yet both sexes of certain groups are so brilliantly coloured--for
instance in Chrysis, in which vermilion and metallic greens
prevail--that we are tempted to attribute the result to sexual
selection. In the Ichneumonidae, according to Mr. Walsh,[482] the males
are almost universally lighter coloured than the females. On the other
hand, in the Tenthredinidae the males are generally darker than the
females. In the Siricidae the sexes frequently differ; thus the male of
_Sirex juvencus_ is banded with orange, whilst the female is dark
purple; but it is difficult to say which sex is the most ornamented. In
_Tremex columbae_ the female is much brighter coloured than the male.
With ants, as I am informed by Mr. F. Smith, the males of several
species are black, the females being testaceous. In the family of Bees,
especially in the solitary species, as I hear from the same
distinguished entomologist, the sexes often differ in colour. The males
are generally the brightest, and in Bombus as well as in Apathus, much
more variable in colour than the females. In _Anthophora retusa_ the
male is of a rich fulvous-brown, whilst the female is quite black: so
are the females of several species of Xylocopa, the males being bright
yellow. In an Australian bee (_Lestis bombylans_), the female is of an
extremely brilliant steel-blue, sometimes tinted with vivid green; the
male being of a bright brassy colour clothed with rich fulvous
pubescence. As in this group the females are provided with excellent
defensive weapons in their stings, it is not probable that they have
come to differ in colour from the males for the sake of protection.
_Mutilla Europaea_ emits a stridulating noise; and according to
Goureau[483] both sexes have this power. He attributes the sound to the
friction of the third and preceding abdominal segments; and I find that
these surfaces are marked with very fine concentric ridges, but so is
the projecting thoracic collar, on which the head articulates; and this
collar, when scratched with the point of a needle, emits the proper
sound. It is rather surprising that both sexes should have the power of
stridulating, as the male is winged and the female wingless. It is
notorious that Bees express certain emotions, as of anger, by the tone
of their humming, as do some dipterous insects; but I have not referred
to these sounds, as they are not known to be in any way connected with
the act of courtship.
Order, _Coleoptera_ (Beetles).--Many beetles are coloured so as to
resemble the surfaces which they habitually frequent. Other species are
ornamented with gorgeous metallic tints,--for instance, many Carabidae,
which live on the ground and have the power of defending themselves by
an intensely acrid secretion,--the splendid diamond-beetles which are
protected by an extremely hard covering,--many species of Chrysomela,
such as _C. cerealis_, a large species beautifully striped with various
colours, and in Britain confined to the bare summit of Snowdon,--and a
host of other species. These splendid colours, which are often arranged
in stripes, spots, crosses and other elegant patterns, can hardly be
beneficial, as a protection, except in the case of some flower-feeding
species; and we cannot believe that they are purposeless. Hence the
suspicion arises, that they serve as a sexual attraction; but we have no
evidence on this head, for the sexes rarely differ in colour. Blind
beetles, which cannot of course behold each other's beauty, never
exhibit, as I hear from Mr. Waterhouse, jun., bright colours, though
they often have polished coats: but the explanation of their obscurity
may be that blind insects inhabit caves and other obscure stations.
Some Longicorns, however, especially certain Prionidae, offer an
exception to the common rule that the sexes of beetles do not differ in
colour. Most of these insects are large and splendidly coloured. The
males in the genus Pyrodes,[484] as I saw in Mr. Bates' collection, are
generally redder but rather duller than the females, the latter being
coloured of a more or less splendid golden green. On the other hand, in
one species the male is golden-green, the female being richly tinted
with red and purple. In the genus Esmeralda the sexes differ so greatly
in colour that they have been ranked as distinct species: in one species
both are of a beautiful shining green, but the male has a red thorax. On
the whole, as far as I could judge, the females of those Prionidae, in
which the sexes differ, are coloured more richly than the males; and
this does not accord with the common rule in regard to colour when
acquired through sexual selection.
[Illustration: Fig. 15. Chalcosoma atlas. Upper figure, male (reduced);
lower figure, female (nat. size).]
[Illustration: Fig. 16. Copris isidis. (Left-hand figures, males.)]
[Illustration: Fig. 17. Phanaeus faunus.]
[Illustration: Fig. 18. Dipelicus cantori.]
[Illustration: Fig. 19. Onthophagus rangifer, enlarged.]
A most remarkable distinction between the sexes of many beetles is
presented by the great horns which rise from the head, thorax, or
clypeus of the males; and in some few cases from the under surface of
the body. These horns, in the great family of the Lamellicorns, resemble
those of various quadrupeds, such as stags, rhinoceroses, &c., and are
wonderful both from their size and diversified shapes. Instead of
describing them, I have given figures of the males and females of some
of the more remarkable forms. (Figs. 15 to 19.) The females generally
exhibit rudiments of the horns in the form of small knobs or ridges; but
some are destitute of even a rudiment. On the other hand, the horns are
nearly as well developed in the female as in the male of _Phanaeus
lancifer_; and only a little less well developed in the females of some
other species of the same genus and of Copris. In the several
subdivisions of the family, the differences in structure of the horns do
not run parallel, as I am informed by Mr. Bates, with their more
important and characteristic differences; thus within the same natural
section of the genus Onthophagus, there are species which have either a
single cephalic horn, or two distinct horns.
In almost all cases, the horns are remarkable from their excessive
variability; so that a graduated series can be formed, from the most
highly developed males to others so degenerate that they can barely be
distinguished from the females. Mr. Walsh[485] found that in _Phanaeus
carnifex_ the horns were thrice as long in some males as in others. Mr.
Bates, after examining above a hundred males of _Onthophagus rangifer_
(fig. 19), thought that he had at last discovered a species in which
the horns did not vary; but further research proved the contrary.
The extraordinary size of the horns, and their widely different
structure in closely-allied forms, indicate that they have been formed
for some important purpose; but their excessive variability in the males
of the same species leads to the inference that this purpose cannot be
of a definite nature. The horns do not show marks of friction, as if
used for any ordinary work. Some authors suppose[486] that as the males
wander much more than the females, they require horns as a defence
against their enemies; but in many cases the horns do not seem well
adapted for defence, as they are not sharp. The most obvious conjecture
is that they are used by the males for fighting together; but they have
never been observed to fight; nor could Mr. Bates, after a careful
examination of numerous species, find any sufficient evidence in their
mutilated or broken condition of their having been thus used. If the
males had been habitual fighters, their size would probably have been
increased through sexual selection, so as to have exceeded that of the
female; but Mr. Bates, after comparing the two sexes in above a hundred
species of the Copridae, does not find in well-developed individuals any
marked difference in this respect. There is, moreover, one beetle,
belonging to the same great division of the Lamellicorns, namely
Lethrus, the males of which are known to fight, but they are not
provided with horns, though their mandibles are much larger than those
of the female.
The conclusion, which best agrees with the fact of the horns having been
so immensely yet not fixedly developed,--as shewn by their extreme
variability in the same species and by their extreme diversity in
closely-allied species--is that they have been acquired as ornaments.
This view will at first appear extremely improbable; but we shall
hereafter find with many animals, standing much higher in the scale,
namely fishes, amphibians, reptiles and birds, that various kinds of
crests, knobs, horns and combs have been developed apparently for this
sole purpose.
[Illustration: Fig. 20. Onitis furcifer, male, viewed from beneath.]
[Illustration: Fig. 21. Left-hand figure, male of Onitis furcifer,
viewed laterally. Right-hand figure, female. _a._ Rudiment of cephalic
horn. _b._ Trace of thoracic horn or crest.]
The males of _Onitis furcifer_ (fig. 20) are furnished with singular
projections on their anterior femora, and with a great fork or pair of
horns on the lower surface of the thorax. This situation seems extremely
ill adapted for the display of these projections, and they may be of
some real service; but no use can at present be assigned to them. It is
a highly remarkable fact, that although the males do not exhibit even a
trace of horns on the upper surface of the body, yet in the females a
rudiment of a single horn on the head (fig. 21, _a_), and of a crest
(_b_) on the thorax, are plainly visible. That the slight thoracic crest
in the female is a rudiment of a projection proper to the male, though
entirely absent in the male of this particular species, is clear: for
the female of _Bubas bison_ (a form which comes next to _Onitis_) has a
similar slight crest on the thorax, and the male has in the same
situation a great projection. So again there can be no doubt that the
little point (_a_) on the head of the female _Onitis furcifer_, as well
of the females of two or three allied species, is a rudimentary
representative of the cephalic horn, which is common to the males of so
many lamellicorn beetles, as in Phanaeus, fig. 17. The males indeed of
some unnamed beetles in the British Museum, which are believed actually
to belong to the genus Onitis, are furnished with a similar horn. The
remarkable nature of this case will be best perceived by an
illustration: the Ruminant quadrupeds run parallel with the lamellicorn
beetles, in some females possessing horns as large as those of the male,
in others having them much smaller, or existing as mere rudiments
(though this is as rare with ruminants as it is common with
Lamellicorns), or in having none at all. Now if a new species of deer or
sheep were discovered with the female bearing distinct rudiments of
horns, whilst the head of the male was absolutely smooth, we should have
a case like that of _Onitis furcifer_.
In this case the old belief of rudiments having been created to complete
the scheme of nature is so far from holding good, that all ordinary
rules are completely broken through. The view which seems the most
probable is that some early progenitor of Onitis acquired, like other
Lamellicorns, horns on the head and thorax, and then transferred them,
in a rudimentary condition, as with so many existing species, to the
female, by whom they have ever since been retained. The subsequent loss
of the horns by the male may have resulted through the principle of
compensation from the development of the projections on the lower
surface, whilst the female has not been thus affected, as she is not
furnished with these projections, and consequently has retained the
rudiments of the horns on the upper surface. Although this view is
supported by the case of Bledius immediately to be given, yet the
projections on the lower surface differ greatly in structure and
development in the males of the several species of Onitis, and are even
rudimentary in some; nevertheless the upper surface in all these species
is quite destitute of horns. As secondary sexual characters are so
eminently variable, it is possible that the projections on the lower
surface may have been first acquired by some progenitor of Onitis and
produced their effect through compensation, and then have been in
certain cases almost completely lost.
[Illustration: Fig. 22. Bledius taurus, magnified. Left-hand figure,
male; right-hand figure, female.]
All the cases hitherto given refer to the Lamellicorns, but the males of
some few other beetles, belonging to two widely distinct groups, namely,
the Curculionidae and Staphylinidae, are furnished with horns,--in the
former on the lower surface of the body,[487] in the latter on the upper
surface of the head and thorax. In the Staphylinidae the horns of the
males in the same species are extraordinarily variable, just as we have
seen with the Lamellicorns. In Siagonium we have a case of dimorphism,
for the males can be divided into two sets, differing greatly in the
size of their bodies, and in the development of their horns, without any
intermediate gradations. In a species of Bledius (fig. 22), also
belonging to the Staphylinidae, male specimens can be found in the same
locality, as Professor Westwood states, "in which the central horn of
the thorax is very large, but the horns of the head quite rudimental;
and others, in which the thoracic horn is much shorter, whilst the
protuberances on the head are long."[488] Here, then, we apparently have
an instance of compensation of growth, which throws light on the curious
case just given of the loss of the upper horns by the males of _Onitis
furcifer_.
_Law of Battle._--Some male beetles, which seem ill fitted for fighting,
nevertheless engage in conflicts for the possession of the females. Mr.
Wallace[489] saw two males of _Leptorhynchus angustatus_, a linear
beetle with a much elongated rostrum, "fighting for a female, who stood
close by busy at her boring. They pushed at each other with their
rostra, and clawed and thumped, apparently in the greatest rage." The
smaller male, however, "soon ran away, acknowledging himself
vanquished." In some few cases the males are well adapted for fighting,
by possessing great toothed mandibles, much larger than those of the
females. This is the case with the common stag-beetle (_Lucanus
cervus_), the males of which emerge from the pupal state about a week
before the other sex, so that several may often be seen pursuing the
same female. At this period they engage in fierce conflicts. When Mr. A.
H. Davis[490] enclosed two males with one female in a box, the larger
male severely pinched the smaller one, until he resigned his
pretensions. A friend informs me that when a boy he often put the males
together to see them fight, and he noticed that they were much bolder
and fiercer than the females, as is well known to be the case with the
higher animals. The males would seize hold of his finger, if held in
front, but not so the females. With many of the Lucanidae, as well as
with the above-mentioned Leptorhynchus, the males are larger and more
powerful insects than the females. The two sexes of _Lethrus cephalotes_
(one of the Lamellicorns) inhabit the same burrow; and the male has
larger mandibles than the female. If, during the breeding-season, a
strange male attempts to enter the burrow, he is attacked; the female
does not remain passive, but closes the mouth of the burrow, and
encourages her mate by continually pushing him on from behind. The
action does not cease until the aggressor is killed or runs away.[491]
The two sexes of another lamellicorn beetle, the _Ateuchus cicatricosus_
live in pairs, and seem much attached to each other; the male excites
the female to roll the balls of dung in which the ova are deposited; and
if she is removed, he becomes much agitated. If the male is removed, the
female ceases all work, and as M. Brulerie[492] believes, would remain
on the spot until she died.
The great mandibles of the male Lucanidae are extremely variable both in
size and structure, and in this respect resemble the horns on the head
and thorax of many male Lamellicorns and Staphylinidae. A perfect series
can be formed from the best-provided to the worst-provided or degenerate
males. Although the mandibles of the common stag-beetle, and probably of
many other species, are used as efficient weapons for fighting, it is
doubtful whether their great size can thus be accounted for. We have
seen that with the _Lucanus elaphus_ of N. America they are used for
seizing the female. As they are so conspicuous and so elegantly
branched, the suspicion has sometimes crossed my mind that they may be
serviceable to the males as an ornament, in the same manner as the horns
on the head and thorax of the various above described species. The male
_Chiasognathus grantii_ of S. Chile--a splendid beetle belonging to the
same family--has enormously-developed mandibles (fig. 23); he is bold
and pugnacious; when threatened on any side he faces round, opening his
great jaws, and at the same time stridulating loudly; but the mandibles
were not strong enough to pinch my finger so as to cause actual pain.
[Illustration: Fig. 23. Chiasognathus grantii, reduced. Upper figure,
male; lower figure, female.]
Sexual selection, which implies the possession of considerable
perceptive powers and of strong passions, seems to have been more
effective with the Lamellicorns than with any other family of the
Coleoptera or beetles. With some species the males are provided with
weapons for fighting; some live in pairs and show mutual affection;
many have the power of stridulating when excited; many are furnished
with the most extraordinary horns, apparently for the sake of ornament;
some which are diurnal in their habits are gorgeously coloured; and,
lastly, several of the largest beetles in the world belong to this
family, which was placed by Linnaeus and Fabricius at the head of the
Order of the Coleoptera.[493]
_Stridulating organs._--Beetles belonging to many and widely distinct
families possess these organs. The sound can sometimes be heard at the
distance of several feet or even yards,[494] but is not comparable with
that produced by the Orthoptera. The part which may be called the rasp
generally consists of a narrow slightly-raised surface, crossed by very
fine, parallel ribs, sometimes so fine as to cause iridescent colours,
and having a very elegant appearance under the microscope. In some
cases, for instance, with Typhaeus, it could be plainly seen that
extremely minute, bristly, scale-like prominences, which cover the whole
surrounding surface in approximately parallel lines, give rise to the
ribs of the rasp by becoming confluent and straight, and at the same
time more prominent and smooth. A hard ridge on any adjoining part of
the body, which in some cases is specially modified for the purpose,
serves as the scraper for the rasp. The scraper is rapidly moved across
the rasp, or conversely the rasp across the scraper.
[Illustration: Fig. 24. Necrophorus (from Landois). _r._ The two rasps.
Left-hand figure, part of the rasp highly magnified.]
These organs are situated in widely different positions. In the
carrion-beetles (Necrophorus) two parallel rasps (_r_, fig. 24) stand on
the dorsal surface of the fifth abdominal segment, each rasp being
crossed, as described by Landois,[495] by from 126 to 140 fine ribs.
These ribs are scraped by the posterior margins of the elytra, a small
portion of which projects beyond the general outline. In many
Crioceridae, and in _Clythra 4-punctata_ (one of the Chrysomelidae), and
in some Tenebrionidae, &c.,[496] the rasp is seated on the dorsal apex of
the abdomen, on the pygidium or pro-pygidium, and is scraped as above by
the elytra. In Heterocerus, which belongs to another family, the rasps
are placed on the sides of the first abdominal segment, and are scraped
by ridges on the femora.[497] In certain Curculionidae and Carabidae,[498]
the parts are completely reversed in position, for the rasps are seated
on the inferior surface of the elytra, near their apices, or along their
outer margins, and the edges of the abdominal segments serve as the
scrapers. In _Pelobius hermanni_ (one of Dytiscidae or water-beetles) a
strong ridge runs parallel and near to the sutural margin of the elytra,
and is crossed by ribs, coarse in the middle part, but becoming
gradually finer at both ends, especially at the upper end; when this
insect is held under water or in the air, a stridulating noise is
produced by scraping the extreme horny margin of the abdomen against the
rasp. In a great number of long-horned beetles (Longicornia) the organs
are altogether differently situated, the rasp being on the meso-thorax,
which is rubbed against the pro-thorax; Landois counted 238 very fine
ribs on the rasp of _Cerambyx heros_.
[Illustration: Fig. 25. Hind-leg of Geotrupes stercorarius (from
Landois). _r._ Rasp. _c._ Coxa. _f._ Femur. _t._ Tibia. _tr._ Tarsi.]