August 21, 1890] 



NATURE 



405 



SEXUAL SELECTION IN SPIDERS. 

 T7 VERY student of zoology who, of late years, has attempted 

 -^ to follow the drift of all that has been written on the 

 subject of natural selection cannot fail to have observed that the 

 less important, though not less interesting, hypothesis of sexual 

 selection has received relatively little attention. It must be 

 seen, in fact, that to all intents and purposes the hypothesis has 

 remained in the state in which it was left by Mr. Wallace's 

 criticisms — lately repeated and extended in "Darwinism" — of 

 Mr. Darwin's views. And further, it will probably be admitted 

 that this circumstance is to be attributed, not to the fact that 

 there exists amongst zoologists unanimity of opinion on the 

 point — far from it ; but to the fact that, owing to the great 

 practical difficulties in the way of making fresh observations on 

 the courtship of new groups of animals, there has been little or 

 nothing to add to what has been already said. Consequently 

 any contribution to the subject should be gladly welcomed ; and 

 no apology is needed for drawing attention to a paper on the 

 " Sexual Selection of Spiders," which was published last year 

 in the United States of America. 



Moreover, since the paper in question is one of the occasional 

 papers of the Natural History Society of Wisconsin, it is not 

 likely to obtain a wide circulation, at all events on this side of 

 the Atlantic, and to meet with that attention which all who read 

 it must admit that it deserves. The author, Mr. G. W, Peckham, 

 whose name has been for some years past well known to those 

 who have devoted their time to the study of spiders, has in the 

 present instance produced a work of much greater general 

 interest than any that he has published before. For the series 

 of observations which constitute a large part of its subject-matter 

 affords a means of testing in practically an unworked order of 

 animals — and one especially favourable for the purpose — the two 

 hypotheses respecting sexual ornamentation formulated by Mr. 

 Darwin and Mr. Wallace. And since the conclusions to be 

 derived from these observations are, in the opinion of the author, 

 all in favour of Mr. Darwin's views, it will be of interest to see 

 from a critical examination of the contents of the paper to what 

 extent this opinion is supported by the facts therein set forth. 



Mr. Darwin's theory of sexual selection, or in other words the 

 theory that the brilliant colours and ornaments of the male are 

 due to the constant preference by the female of the best-decked 

 males, is too well known to need further explanation here. Mr. 

 Wallace, rejecting this theory on the grounds that there is but 

 little evidence in its favour and much that is directly opposed to 

 it, has put forward an alternative hypothesis which may be 

 briefly epitomized as follows. The production of colour in organ- 

 isms is normal, and needs no special accounting for ; the more 

 brilliant colouring of the male, the development of plumes, &c., 

 is attributable to the greater vigour and activity of this sex, and 

 when this colouring becomes intensified at the breeding season 

 it is because vitality is then at a maximum ; the duller colouring 

 of the female, at all events in birds, is due, through the agency of 

 natural selection, to the toning down or elimination of the 

 normal tints on account of her special need for concealment. 



Before proceeding to test these two theories in the light thrown 

 upon the subject by a consideration of the secondary sexual 

 characters of spiders, Mr. Peckham, turning his attention toother 

 groups, urgesthe followingcases as inexplicable by Mr. Wallace's 

 views. If, it is asked, there is a causal relation between high 

 vitality and ornamentation, how are we to account for the 

 brilliant colours of some tropical pigeons which are remarkable 

 neither for pugnacity nor activity ? and how for the gaudy plumes 

 of the birds of Paradise, which are by no means noticeable for 

 fierceness of disposition ? But in the case of these two objections 

 the flaw seems to be the assumption that pugnacity and activity 

 are the only criteria of high vitality. 



It is clear, moreover, that at the time these were advanced the 

 author had never read Mr, Wallace's last work, " Darwinism " ; 

 for on p. 292 of this volume reference is made to the birds of 

 Paradise, and their gaudy adornment is spoken of in connection 

 with the wonderful activity they display. The following question 

 is also raised. Quoting verbatim — " Perhaps the most difficult 

 fact to reconcile with the [Mr. Wallace's] theory is the absence 

 of ornamentation and bright colour in the bats. They have a wide 

 expanse of integument, and great activity, the conditions specified 

 by Mr. Wallace for the development of gaudy pigment, and no- 

 thing, apparently, in their habits to keep it down ; but, except in 

 the frugivorous bats, we find little difference between the sexes, 

 nor is there any appreciable approach to bright colours " (p. 9). 



At first sight this objection seems to be valid ; for, if we admit 



NO. 1086, VOL. 42] 



the truth of the author's premises, it seems that his conclusion 

 is a just one. But can this be done? Is it the case that there 

 is nothing in the habits of bats to keep down the development 

 of gaudy pigment? Surely not. These animals are nocturnal 

 or crepuscular, spending the day suspended from the roof of 

 some cave or other dark secluded spot, and only issuing forth at 

 nightfall to exercise their activity in hunting for prey. Thus 

 that great expanse of integument above referred to — namely, the 

 wings — is never exposed to the rays of the sun, and few things, 

 if any, are more antagonistic to the development of gaudy pigment 

 than absence of light. 



These cases, then, do not seem upon examination to be 

 seriously opposed to Mr. Wallace's views. But turning to the 

 spiders, we find that the objections are of a somewhat different 

 order from those hitherto considered. For after passing in re- 

 view all the principal families of this group, and studying their 

 moults, it is concluded (i) that there is no evidence to show that 

 there is a causal relation between high vitality and adornment, 

 since, in the first place, as a rule, the savage and powerful 

 female is less, sometimes very much less, brightly coloured than 

 the male, who is comparatively weak and unaggressive ; and, in 

 the second place, many of the sluggish and sedentary spiders, 

 such as, e.g. , the Epeirida:, are brilliantly tinted, whilst other 

 active, restless forms, such as most Lycosidce, are relatively dull- 

 coloured ; (2) that when the male differs from the female he 

 departs in proportion from the normal colouring of the group^ 

 and that when the female, as well as the male, is showily attired, 

 the resemblance between them is due to the partial or complete 

 transmission of the male colouring to the female, the complete- 

 ness of the resemblance depending upon the age of the male at 

 the time of the acquisition of his adornments ; and (3) that there 

 is no reason to think that the females have had their colours 

 toned down for the sake of concealment at the time of nesting, 

 for in the Attida, where sexual differences of colour are best 

 marked, the females have covered nests. 



Mr. Wallace's theory, then, however satisfactory it may be in 

 the case of butterflies and birds, fails apparently in ever)' 

 essential respect when applied to spiders. It is necessary, 

 therefore, to look elsewhere for an explanation. 



Now, when we consider the secondary sexual characters of the 

 Aitidce, the first fact that strikes the attention is that these cha- 

 racters exist in the males as modifications in the form of the 

 falces, palpi, first pair of legs, or clypeus — that is, of those 

 portions of the anterior part of the body which are in full view 

 when the male is approaching the female — and, moreover, that 

 these modifications in form result often in an increase of surface 

 for the development of gaudy, often strongly iridescent, hues. 



Thus, in Salliais forftiicarius the ? has short vertical reddish- 

 black falces, while those of the S are horizontal, much en- 

 larged, and copper-green in colour ; and in Icius palmarum " the 

 falces in the <5 are compressed, horizontal, and three times as 

 long as the face, the fang equalling the falx in length ; the front 

 sur^ce of the falces is dark bronzy rufus, and on each outer 

 edge is a wide band of snowy-white hairs. In the ? the falces 

 are vertical, and only as long as the face, and the snowy-white 

 hairs are absent. The 6 is rendered still more striking by the 

 long snowy-white hairs which cover his clypeus, while the fore- 

 head, and a space just below the first row of eyes, is covered 

 with bright red hairs. All this ornamentation is lacking in 

 the ?." 



The clypeus, too, is liable to a considerable amount of varia- 

 tion with sex. ThxAy'va. Dendryphantes capitatus this part in 

 the $ is conspicuously marked by several white bands, which 

 contrast strikingly with the dark colour of the rest of the face ; 

 in the ? the whole clypeus is whitish, and in no way conspicuous. 

 Again, in Mopsus mormon there is a high vertical ridge of hairs 

 extending over the forehead in the S ; in the 9 , on the other 

 hand, these hairs are wholly absent. So, too, in the Theridiida, 

 the heads of the males are frequently higher, in many species 

 very much higher, than in the females. 



With regard to the palpi and first pair of legs, it will be 

 sufficient to say that in Keyserling's " Arachniden Australiens " 

 thirty-four males are described (in the family Attidie), having 

 well -developed fringes or tufts of hair on the palpi, while there 

 are only five females so ornamented, and several of these to only 

 a moderate extent ; and that in the 6 of Synageles picata the 

 tibia of the first pair of legs is enlarged and flattened, and the 

 anterior face of the enlargement is of a brilliant steel-blue 

 colour ; in the 6 of PhilcEus vietallesccns the anterior legs are 

 elongate, of a brilliant steel-blue colour, and ornamented with 

 rings, spots, and fringes of hairs, whilst in the females of these 



