Descent of Man [ 1871 ]

Charles Darwin [ 1809 - 1882 ]

Chapter IX - Secondary Sexual Characters in the Lower Classes of the Animal Kingdom




  WITH animals belonging to the lower classes, the two sexes are not

rarely united in the same individual, and therefore secondary sexual

characters cannot be developed. In many cases where the 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 appreciate each other's beauty or other attractions, or to

feel rivalry.

  Hence in these classes or sub-kingdoms, such as the Protozoa,

Coelenterata, Echinodermata, Scolecida, secondary sexual characters,

of the kind which we have to consider, 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,

desire, 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. Contrivances by which

the male holds the female, and which are indispensable for the

propagation of the species, are independent of sexual selection, and

have been acquired through ordinary selection.

  Many of the lower animals, whether hermaphrodites or with separate

sexes, are ornamented with the most brilliant tints, or are shaded and

striped in an elegant manner; for instance, many corals and

sea-anemones (Actiniae), some jelly-fish (Medusae, Porpita, &c.), some

Planariae, many star-fishes, Echini, ascidians, &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. It should be borne in mind that

in no case have we sufficient evidence that colours have been thus

acquired, except where one sex is much more brilliantly or

conspicuously coloured than the other, and where there is no

difference in habits between the sexes sufficient to account for their

different colours. But the evidence is rendered as complete as it

can ever be, only when the more ornamented individuals, almost

always the males, voluntarily display their attractions before the

other sex; for we cannot believe that such display is useless, and

if it be advantageous, sexual selection will almost inevitably follow.

We may, however, 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 doubtful

whether such colours often serve as a protection; but that we may

easily err on this head, will be admitted by every one who reads Mr.

Wallace's excellent essay on this subject. It would not, for instance,

at first occur to any one that the transparency of the Medusae, or

jelly-fish, is 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 appearance, often accompanied by

prismatic colours, we can hardly doubt that they thus escape the

notice of pelagis birds and other enemies. M. Giard is also convinced*

that the bright tints of certain sponges and ascidians serve as a

protection. Conspicuous colours are likewise beneficial to many

animals as a warning to their would-be devourers that they are

distasteful, or that they possess some special means of defence; but

this subject will be discussed more conveniently hereafter.



  * Archives de Zoolog. Exper., Oct., 1872, p. 563.



  We can, in our ignorance of most of the lowest animals, only say

that their bright tints result either from 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, as I am informed by Mr.

Hancock, the extreme beauty of the Eolidae (naked sea-slugs) is

chiefly due to the biliary glands being 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 thus gained, in the complex laboratory of living organisms.



  The sub-kingdom of the MOLLUSCA.- Throughout this great division

of the animal kingdom, as far as I can discover, secondary sexual

characters, such as we are here considering, never occur. Nor could

they be expected in the three lowest classes, namely, in the

ascidians, Polyzoa, and brachiopods (constituting the Molluscoida of

some authors), 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 classes of the Gasteropoda, or univalve shells, the

sexes are either united or separate. But in the latter case the

males never possess special organs for finding, securing, or

charming the females, or for fighting with other males. As I am

informed by Mr. Gwyn Jeffreys, the sole external difference between

the sexes consists 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,

"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-snails (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.



  * De l'Espece et de la Class. &c., 1869, p. 106.



  Even in the highest class of the Mollusca, the Cephalopoda or

cuttle-fishes, in which the sexes are separate, secondary sexual

characters of the present kind 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.* 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.



  * See, for instance, the account which I have given in my Journal of

Researches, 1845, p. 7.



  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 be influential

to a certain extent; 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, as well as

the parts covered by the mantle, less highly-coloured than the upper

and exposed surfaces.* In some cases, as with shells living amongst

corals or brightly-tinted seaweeds, the bright colours may serve as

a protection.*(2) But that many of the nudibranch Mollusca, or

sea-slugs, are as beautifully coloured as any shells, may be seen in

Messrs. Alder and Hancock's magnificent work; and from information

kindly given me by Mr. Hancock, it seems extremely doubtful whether

these colours usually serve as a protection. With some species this

may be the case, as with one kind 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.



  * I have given (Geological Observations on Volcanic Islands, 1844,

p. 53) a curious instance of the influence of light on the colours

of a frondescent incrustation, deposited by the surf on the

coast-rocks of Ascension and formed by the solution of triturated

sea-shells.

  *(2) Dr. Morse has lately discussed this subject in his paper on the

"Adaptive Coloration of Mollusca," Proc. Boston Soc. of Nat. Hist.,

vol. xiv., April, 1871.



  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 other's 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 over the offspring of the less beautiful, so as to

increase in number, unless indeed vigour and beauty generally

coincided. We have not here the case of a number of males becoming

mature before the females, with the more beautiful males selected by

the more vigorous females. If, indeed, brilliant colours were

beneficial to a 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; 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 are often beautifully coloured, but as the sexes do

not differ in this respect, we are but little concerned with them.

Even the nemertians, though so lowly organised, "vie in beauty and

variety of colouring with any other group in the invertebrate series";

yet Dr. McIntosh* cannot discover that these colours are of any

service. The sedentary annelids become duller-coloured, according to

M. Quatrefages,*(2) after the period of reproduction; and this I

presume may be attributed to their less vigorous condition at that

time. All these worm-like animals 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.



  * See his beautiful monograph on British Annelids, part i., 1873, p.

3.

  *(2) See M. Perrier: "L'Origine de l'Homme d'apres Darwin," Revue

Scientifique, Feb., 1873, p. 866.



  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 more successful in finding partners and

in producing offspring. Fritz Muller has described a remarkable

dimorphic species of Tanais, in which the male is represented by two

distinct forms, which never graduate 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 Muller suggests that

these differences between the two male forms of the same species may

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, have left the greatest number of progeny to

inherit their respective advantages.*



  * Facts and arguments for Darwin, English translat., 1869, p. 20.

See the previous discussion on the olfactory threads. Sars has

described a somewhat analogous case (as quoted in Nature, 1870, p.

455) in a Norwegian crustacean, the Pontoporeia affinis.



  In some of the lower crustaceans, the right anterior antenna of

the male differs greatly in structure from the left, 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 (see fig. 4) into an elegant, and

sometimes wonderfully complex, prehensile organ.* 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.



  * See Sir J. Lubbock in Annals and Mag. of Nat. Hist., vol. xi.,

1853, pls. i. and x.; and vol. xii. (1853), pl. vii. See also

Lubbock in Transactions, Entomological Society, vol. iv., new

series, 1856-1858, p. 8. With respect to the zigzagged antennae

mentioned below, see Fritz Muller, Facts and Arguments for Darwin,

1869, p. 40, footnote.



  In the higher crustaceans the anterior legs are developed into

chelae or pincers; and these are generally larger in the male than

in the female,- so much so that the market value of the male edible

crab (Cancer pagurus), according to Mr. C. Spence Bate, is five

times as great as that of the female. In many species the chelae are

of unequal size on the opposite side of the body, the right-hand one

being, as I am informed by Mr. Bate, generally, though not invariably,

the largest. This inequality is also often much greater in the male

than in the female. The two chelae of the male often differ in

structure (see figs. 5, 6, and 7), the smaller one resembling that

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. As I hear from Mr. Bate, the chelae are sometimes of

such length and size that they cannot possibly be used for carrying

food to the mouth. In the males of certain fresh-water prawns

(Palaemon) the right leg is actually longer than the whole body.*

The great size of the one leg with its chelae may aid the male in

fighting with his rivals; but this 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,*(2) the

male and the female live in the same burrow, and this shews that

they pair; 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, is probably

to seize and to secure the female, and this in some instances, as with

Gammarus, is known to be the case. The male of the hermit or soldier

crab (Pagurus) for weeks together, carries about the shell inhabited

by the female.*(3) The sexes, however, of the common shore-crab

(Carcinus manas), as Mr. Bate informs me, unite directly after the

female has moulted her hard shell, 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 before moulting, she could

then be seized with impunity.



  * See a paper by Mr. C. Spence Bate, with figures, in Proceedings,

Zoological Society, 1868, p. 363; and on the nomenclature of the

genus, ibid., p. 585. I am greatly indebted to Mr. Spence Bate for

nearly all the above statements with respect to the chelae of the

higher crustaceans.

  *(2) Hist. Nat. des Crust., tom. ii., 1837, p. 50.

  *(3) Mr. C. Spence Bate, British Association, Fourth Report on the

Fauna of S. Devon.



  Fritz Muller states that certain species of Melita are distinguished

from all other amphipods by the females having "the coxal lemellae

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 has probably followed from

those females which were the most securely held during the act of

reproduction, having left the largest number of offspring. Another

Brazilian amphipod (see Orchestia darwinii, fig. 8) presents a case of

dimorphism, like that of Tanais; for there are two male forms, which

differ in the structure of their chelae.* As either chela would

certainly suffice 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.



  * Fritz Muller, Facts and Arguments for Darwin, 1869, pp. 25-28.



  It is not known that male crustaceans fight together for the

possession of the females, but it is probably the case; for with

most animals when the male is larger than the female, he seems to

owe his greater size to his ancestors having fought with other males

during many generations. In most of the 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

when a devil-crab (Portunus puber) was seen by a son of Mr. Bate

fighting with a Carcinus maenas, 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 in a glass vessel by Fritz Muller, they mutilated

and killed one another. Mr. Bate put a large male Carcinus maenas into

a pan of water, inhabited by a female which was paired with a

smaller male; but the latter was soon dispossessed. 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 whom were

imprisoned in the same vessel with many individuals of the same

species. The female, when thus divorced, soon joined the others. After

a time the male was put again 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.

  The mental powers of the Crustacea are probably higher than at first

sight appears probable. Any one who tries to catch one of the

shore-crabs, so common on tropical coasts, will perceive how wary

and alert they are. There is a large crab (Birgus latro), found on

coral islands, which makes a thick bed of the picked fibres of the

cocoa-nut, at the bottom of a deep burrow. 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 animal

as by an old one. The following case, however, can hardly be so

considered: a trustworthy naturalist, Mr. Gardner,* 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 a

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.



  * Travels in the Interior of Brazil, 1846, p. 111. I have given,

in my Journal of Researches, p. 463, an account of the habits of the

Birgus.



  Mr. Bate does not know of any well-marked case of difference of

colour in the two sexes of our British crustaceans, in which respect

the sexes of the higher animals so often differ. In some cases,

however, the males and females 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. Dr. Power tried to distinguish by colour the

sexes of the several species which inhabit Mauritius, but failed,

except with one species of Squilla, probably S. stylifera, the male of

which is described as being "of a beautiful bluish-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."* In this case, we may suspect the agency of sexual selection.

From M. Bert's observations on Daphnia, when placed in a vessel

illuminated by a prism, we have reason to believe that even the lowest

crustaceans can distinguish colours. With Saphirina (an oceanic

genus of Entomostraca), the males are furnished with minute shields or

cell-like bodies, which exhibit beautiful changing colours; these

are absent in the females, and in both sexes of one species.*(2) It

would, however, be extremely rash to conclude that these curious

organs serve to attract the females. I am informed by Fritz Muller,

that in the female of a Brazilian species of Gelasimus, the whole body

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." It deserves especial notice that the males do not acquire

their bright colours until they become mature. They appear to be

much more numerous than the females; they differ also 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 probable that the male in this species has

become gaily ornamented in order to attract or excite the female.



  * Mr. Ch. Fraser, in Proc. Zoolog. Soc., 1869, p. 3.

  *(2) Claus, Die freilebenden Copepoden, 1863, s. 35.



  It has just been stated that the male Gelasimus does not acquire his

conspicuous colours until mature and nearly ready to breed. This seems

a general rule in the whole class in respect to the many remarkable

structural differences between the 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 Muller* gives

some striking instances of this law; thus the male sand-hopper

(Orchestia) does not, until nearly full grown, acquire his large

claspers, which are very differently constructed from those of the

female; whilst young, his claspers resemble those of the female.



  * Facts and Arguments, &c., p. 79.



  I am indebted to Mr. Bate for Dr. Power's statement.



  Class: ARACHNIDA (Spiders).- The sexes do not generally differ

much in colour, but the males are often darker than the females, as

may be seen in Mr. Blackwall's magnificent work.* In some species,

however, the difference is conspicuous: 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 certain

species of Thomisus the sexes closely resemble each other, in others

they differ much; and analogous cases occur in many other genera. 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; and

Canestrini*(2) remarks that in certain genera the males can be

specifically distinguished with ease, but the females with great

difficulty. I am informed by Mr. Blackwall that the sexes whilst young

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 bright-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; as is well known, the females often shew the strongest

affection for their eggs, which they carry about enveloped in a silken

web. The males search eagerly for the females, and have been seen by

Canestrini and others to fight for possession of them. This same

author says that the union of the two sexes has been observed in about

twenty species; and he asserts positively that the female rejects some

of the males who court her, threatens them with open mandibles, and at

last after long hesitation accepts the chosen one. From these

several considerations, we may admit with some confidence that the

well-marked differences in colour between the sexes of certain species

are the results of sexual selection; though we have not here the

best kind of evidence,- the display by the male of his ornaments. From

the extreme variability of colour in the male of some species, for

instance of Theridion lineatum, it would appear that these sexual

characters of the males have not as yet become well fixed.

Canestrini draws the same conclusion from the fact that the males of

certain species present two forms, differing from each other in the

size and length of their jaws; and this reminds us of the above

cases of dimorphic crustaceans.



  * A History of the Spiders of Great Britain, 1861-64. For the

following facts, see pp. 77, 88, 102.

  *(2) This author has recently published a valuable essay on the

"Caratteri sessuali secondarii degli Arachnidi," in the Atti della

Soc. Veneto-Trentina di Sc. Nat. Padova, vol. i., fasc. 3, 1873.



  The male is generally much smaller than the female, sometimes to

an extraordinary degree,* and he is forced to be extremely cautious in

making his advances, as the female often carries her coyness to a

dangerous pitch. De Greer 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."*(2) The Rev. O. P.

Cambridge*(3) accounts in the following manner for the extreme

smallness of the male in the genus Nephila. "M. Vinson gives a graphic

account of the agile way in which the diminutive male escapes from the

ferocity of the female, by gliding about and playing hide and seek

over her body and along her gigantic limbs: in such a pursuit it is

evident that the chances of escape would be in favour of the

smallest males, whilst the larger ones would fall early victims;

thus gradually a diminutive race of males would be selected, until

at last they would dwindle to the smallest possible size compatible

with the exercise of their generative functions,- in fact, probably to

the size we now see them, i. e., so small as to be a sort of

parasite upon the female, and either beneath her notice, or too

agile and too small for her to catch without great difficulty."



  * Aug. Vinson (Araneides des Iles de la Reunion, pl. vi., figs. 1

and 2) gives a good instance of the small size of the male in Epeira

nigra. In this species, as I may add, the male is testaceous and the

female black with legs banded with red. Other even more striking cases

of inequality in size between the sexes have been recorded

(Quarterly Journal of Science, July, 1868, p. 429); but I have not

seen the original accounts.

  *(2) Kirby and Spence, Introduction to Entomology, vol. i., 1818, p.

280.

  *(3) Proceedings, Zoological Society, 1871, p. 621.



  Westring has made the interesting discovery that the males of

several species of Theridion* have the power of making a

stridulating sound, whilst the females are 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 can be detected in the females. It deserves notice that

several writers, including the well-known arachnologist Walckenaer,

have declared that spiders are attracted by music.*(2) 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 also believes, to call or to excite the female; and this is

the first case known to me in the ascending scale of the animal

kingdom of sounds emitted for this purpose.*(3)



  * Theridion (Asagena, Sund.) serratipes, 4-punctatum et guttatum;

see Westring, in Kroyer, Naturhist. Tidskrift, vol. iv., 1842-1843, p.

349; and vol. ii., 1846-1849, p. 342. See, also, for other species,

Araneae Suecicae, p. 184.

  *(2) Dr. H. H. van Zouteveen, in his Dutch translation of this

work (vol. i., p. 444), has collected several cases.

  *(3) Hilgendorf, however, has lately called attention to an

analogous structure in some of the higher crustaceans, which seems

adapted to produce sound; see Zoological Record, 1869, p. 603.



  Class: MYRIAPODA.- In neither of the two orders in this class, the

millipedes and centipedes, can I find any well-marked instances of

such sexual differences 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 either to one of the anterior or of the posterior segments

of the body are modified into prehensile hooks which serve to secure

the female. In some species of Iulus the tarsi of the male are

furnished with membraneous suckers for the same purpose. As we shall

see when we treat of insects, it is a much more unusual

circumstance, that it is the female in Lithobius, which is furnished

with prehensile appendages at the extremity of her body for holding

the male.*



  * Walckenaer et P. Gervais, Hist. Nat. des Insectes: Apteres, tom.

iv., 1847, pp. 17, 19, 68.
Last updated: Friday, 30-Jan-98 10:45:16 MST