The Zoologist/4th series, vol 2 (1898)/Issue 679/Stridulation in some African Spiders
STRIDULATION IN SOME AFRICAN SPIDERS.
By R.I. Pocock, of the British Museum.
To most readers of 'The Zoologist' the Spiders which form the subject-matter of the following pages are probably best known by the comprehensive title "Mygale," a term which was applied to the group of which they are members in the first decade of this century, and has been almost up to the present time universally adopted for them by the compilers of text-books, and the writers of articles on popular natural history. They are also sometimes called Crab-Spiders, presumably from the great size to which most of the species attain; sometimes Bird-eating Spiders, from their alleged propensity for capturing and devouring small birds, a propensity which suggested to Lamarck the generic term Avicularia, still in use for one of the South American genera. But during the last fifty years our knowledge of this group has increased by leaps and bounds; the genus has expanded into a family, represented by numbers of genera which are rapidly becoming more and more accurately defined and classified.
Apart from their large size and usually heavy build, these Spiders, referred to a family variously termed Mygalidæ, Theraphosidæ, and Aviculariidæ, may be recognized from the vast majority of other Spiders by possessing two pairs of lung-sacs, and by the circumstance that the mandibles or jaws project horizontally forwards; while the fang closes almost longitudinally backwards.
So far as habits are concerned, it may be added that none of the species spread nets for the capture of prey. Most of them live on the ground beneath stones, or in deep burrows which they excavate in the soil, and line with a layer of tough silk to prevent the infall of loose particles of earth or sand. At nightfall the Spiders may be seen watching at the entrance of their burrows for passing insects, and during the breeding season the females are to be found at its further extremity mounting guard over their egg-cocoon. Other species again live in trees, and spin a silken domicile either between forked branches or in the hollow trunk, or in large leaves rolled up for the purpose. There is no doubt that their food consists almost wholly of insects of various kinds. Nevertheless cases are on record of the destruction of small reptiles, mammals, and birds by these monstrous Spiders.
The discovery of stridulatory organs in the members of this family dates back to the year 1876, when Prof. Wood-Mason came across one in an Assamese species now known as Musagetes stridulans. Since that year organs like that which he described have been found, not merely in the solitary species as he and most of his successors appear to have thought would be the case, but in a great number of genera ranging from India to Queensland. For the proper comprehension, however, of the mechanism of this and the other organs of like nature described in this paper, it is necessary to add a few words in explanation of a spider's external anatomy. The fore part of the body, the part namely that lies in front of the waist, and is termed the cephalo-thorax, is furnished with six pairs of appendages arranged radially round its margin. The first appendage on each side, known as the mandible, consists of a short stout basal segment, covered above with hair and furnished below with a thick fringe of bristles, called, from its proximity to the mouth, the oral fringe (Fig. 1, c). Articulated to the tip of this basal segment is the second segment, modified to form a long stout pointed fang (Fig. 1, a). Behind the mandible on each side comes a short leg-like appendage called the palp, and consisting of six segments, of which the basal is usually termed the maxilla, from its function as a chewing organ. Its inner surface is furnished above with a suture, and below with an oral fringe (Fig. 2, B). Following the palp are the four walking legs, each of which is composed of seven segments, the basal being known as the coxa, and the second, like the second segment of the palp, as the trochanter.
Now these appendages are so arranged that their coxæ, and to a lesser extent their trochanters, are in contact with the corresponding segments of the appendange in front or behind; so that when a limb is raised upwards the adjacent surfaces of the segments in question slide over one another. These surfaces therefore are areas favourable for the development of stridulating organs; for in the great majority of cases—the Cicada, by the way, being a notable exception—stridulation in the articulated animals results from the friction of two mutually roughened adjacent chitinous areas.
Strictly speaking, however, this is not the case with the stridulating organs that have been found in the Spiders now under discussion; for in all cases these organs consist of modified bristles. In the Oriental members of the family two such organs exist, namely, the one discovered by Wood-Mason, and another discovered by myself in several more genera.[1] In both cases the organ lies between the outer surface of the mandible and the inner surface of the maxilla—the basal segment of the palp; and each consists of a set of vibratile bristles, which are set a-twanging by a series of spines. But whereas in Wood-Mason's instrument the vibratile bristles or notes are placed on the maxilla, and the spines or scraper on the mandible, exactly the opposite obtains in the other instrument, the notes being on the mandible and the scraper on the maxilla.
In some of the African Theraphosidæ I have also had the good fortune to discover two stridulating organs, which are not only quite different from each other, but also quite different from those possessed by the genera inhabiting Tropical Asia. One of these organs occurs in the genus Harpactira, the common "Mygale" of Cape Colony. It occupies the same position as the analogous organs existing in the Oriental species, being situated between the mandible and the maxilla. A glance at Fig. 1 will show the construction of the organ. The outer surface of the mandible (A) is furnished with a large pad of feathery hairs (b), and on the area between this pad and the oral fringe (c) are two sets of bristles, both of which, judging from their colour and structure, originally formed part of the oral fringe, and have been derived from it. Those of the upper series are long, and have their free ends bent over and more or less interlacing with each other. Those of the lower series are less regularly arranged. In the species figured they are short and spiniform; but in some allied forms they are much less distinctly differentiated from the adjacent hairs of the oral fringe, being longer and more bristle-like, as, for example, in H. tigrina. These two rows of bristles are evidently designed to catch against and shake the tips of the long feathery bristles which rise up amongst the hairs clothing the area upon the maxilla between the suture (Fig. 1, B, d) and the oral fringe (B, e).
Fig. 1.—Stridulating organ of Harpactira chrysogaster.
A. Outer surface of mandible, showing a, fang; b, pad of feathery hairs; c, oral fringe and two rows of modified bristles between the pad and the fringe.
B. Inner surface of maxilla, showing the cluster of plumose bristles between the suture d and the oral fringe e.
Structurally, this organ, characteristic of Harpactira, calls to mind the organ possessed by the Oriental genera Citharognathus, Phormingochilus, &c. In these, too, the outer surface of the mandible is furnished with a pad of feathery hairs, and the notes or vibrating bristles are also plumose; they are not, however, situated on the maxilla, as in Harpactira, but upon the mandible, and result merely from the enlargement of a few of the hairs of the feathery pad.
The next organ to be described, though resembling the others in principle, differs entirely in position. Instead of being lodged between the mandible and maxilla, it is lodged between the palp and the first leg. It has been found in several genera (Hysterocrates, Phoneyusa, &c), ranging all over Central Africa, from Old Calabar and the Congo on the west, to Masailand on the east; and also in genera met with in Socotra and Madagascar. If a leg of the first pair in any of these genera be detached, it may be noticed that there is a fringe of hairs bordering the front edge of the upper surface of the first and second segments (coxa and trochanter). On the coxa immediately beneath this fringe, and partially buried in it, there are one or two long stout clavate spines, and some smaller ones as well (Fig. 2, A, a).
Fig. 2.—Stridulating organ of Phoneyusa sp.
A. Anterior surface of first and second segments of leg of first pair, with club-shaped bristles a on coxa and row of erect spines b on trochanter.
B. Posterior surface of first and second segments of palp, with rows of short spines c on maxilla and rigid brush-like bristles d on trochanter.
On the trochanter there is beneath the fringe a series of upstanding long curved spines (Fig. 2, A, b). When the limb is at rest in its normal position the front surface of these two segments are closely in contact with the posterior surface of the corresponding segments of the palp. It is here therefore that the remainder of the organ is found. It consists of a couple of irregular rows of spines on the basal segment (Fig. 2, B, c), and of a thick brush of very fine but stiff bristles upon the trochanter (Fig. 2, B, d). When the Spider is allowed to dry after removal from alcohol a distinct stridulation may be easily produced artificially by rubbing the leg and palp together, the long "notes" on the coxa of the first leg giving rise to a distinct "click, click" when scraped against the spines on the maxilla; while the spines on the trochanter of the first leg, when rubbed against the stiff brush of hairs on the trochanter of the palp, gives out a sound resembling the rustling of a silk dress.
But what is to be said respecting the function of these organs, and what evidence, it may be asked, can be adduced in support of the view that they subserve stridulation? To this question the answer must be that so far as the African species are concerned there is no direct evidence based upon observation of the living animal to show what part they play in the Spider's economy. But that their true and probably sole function is the emission of sound, as has been claimed in the preceding pages, is so strongly supported as to reach practical certainty from what is known of the function of the analogous organ detected by Wood-Mason in the Assamese genus Musagetes.
Mr. Peal, it appears, was the first to notice the phenomenon. His gardener, while engaged in digging up a field, unearthed one of these great Spiders, and, not being a collector, naturally enough proceeded to strike at it with his hoe, with the object of ridding the world of such vermin. Thereupon the Spider raised itself upon its two pairs of hind legs, brandished the two remaining pairs in the air, opened its jaws, and waved its palpi up and down, scraping the basal segment to and fro against the outer surface of the mandible, and emitting a sound subsequently described by Wood-Mason as resembling that produced by rapidly dropping shot on a china plate. Fortunately Mr. Peal rescued this historic Spider from the gardener, and afterwards had the satisfaction of seeing it repeat the performance when attacked by a cat. In confirmation of this story, it may be added that Mr. E.W. Pickard-Cambridge told me recently, in course of conversation, that one day, when leaving his bungalow at Coremia in Assam, he met one of these Spiders coming up the steps, and on his approach the beast reared itself up, waved its legs, and hissed at him. And lastly, Prof. Baldwin-Spencer has made similar observations upon an allied genus Phlogius, observed by him in Australia, his account being accompanied by a beautifully executed illustration of the organ by which the sound is produced.[2]
From the knowledge thus supplied touching the function of the instrument in the Spiders just mentioned, one is perfectly justified in concluding that organs constructed upon the same principle, and occupying the same or similar positions, will in all probability be found to perform the same office; and no further basis need be sought for the belief that the African Spiders, Harpactira and Phoneyusa, and their allies, can stridulate as well as their Oriental relations.
Two other little points connected with the organs may here be mentioned. These are the fringes of hair surmounting the "notes" or vibrating bristles on the leg in Phoneyusa, and the pad of hair above the two series of bristles on the mandible of Harpactira. From the position of these hair-tufts it may be inferred that they serve to keep the bristles below them free from dirt, which would of course seriously interfere with the performance of their function.
What now is the use to the Spider of the sounds that these organs give forth? It has been suggested that, like the call of the Cicada and the chirrup of the Cricket, they have a sexual significance, and serve to inform one sex of the whereabouts of the other. This belief, however, has no foundation in fact; for, in the first place, there is not a particle of evidence that these Spiders possess an auditory sense; and, in the second place, these stridulatory organs are equally well developed in the males and females, and are not, like the sexual stridulating organs known in other groups, confined to the male, or at all events better developed in that sex than in the female. Moreover, they appear in the young at an early age, and become functionally perfected long before the attainment of sexual maturity. So the supposition that they act as a sexual signal may be regarded as unsupported by evidence.
As a matter of fact, the true key to their function is supplied by the behaviour of the living Spiders. From the accounts above quoted from Mr. Peal and Mr. Cambridge, it is evident that the Spiders emit the sound when on their defence and acting under the stimulus of fear or anger, in exactly the same way as the Rattlesnake makes use of its rattle. So far as I am aware, the only explanation that has been suggested touching the function of the snake's rattle is that it serves as an advertisement of the whereabouts of the poisonous reptile, so that it may be avoided by enemies which might otherwise inadvertently injure it. Similarly poisonous and noxious insects are decked with warning colours, so that they may be readily recognized and not slain in mistake for harmless or edible species. If this be the true explanation of the so-called warning coloration of the insects in question, and of the whirring noise made by the Rattlesnake, there seems to be no reason to doubt that the same significance is to be attached to the stridulation emitted by the peculiar organs recently discovered in the great African Spiders and described in the preceding pages.
This work is in the public domain in the United States because it was published before January 1, 1929.
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