the control of dragline spinning in the garden spider by r ... · the control of dragline spinning...

557

The control of dragline spinning in the garden spider

By R. S. WILSON

(From the Department of Zoology, University of Bristol)

With one plate (fig. 3)

SummaryA study of the tarsal claws, the spinning apparatus, and other factors affecting drag-line spinning has been made using certain orb-web spiders, principally Araneusdiadematus. The structure of the tarsal claws suggests that they are incapable ofsmoothly controlling the speed at which the silk thread is extracted, although they cangrip and release the thread quickly.

Evidence is presented that smooth control of spinning is achieved through the inter-action of the intra-abdominal pressure and the control valve which lies in the silkduct. The intra-abdominal pressure is shown to be equivalent to from 3 to 4 cm ofmercury, and to be responsible for moving the fluid silk out of the gland and up theduct. The experimental results also suggest that the control valve regulates theamount of fluid silk available for spinning as a thread and that it also acts as the func-tional spinning orifice, its aperture controlling the diameter of the thread spun.

IntroductionALTHOUGH much is now known concerning the secretion and use of spidersilk (see Millott, 1949; Savory, 1952; Bristowe, 1958) little attention has beenpaid to the way in which the spider controls its silk-spinning. It is widelyrecognized that the change from fluid silk to silk thread only occurs undertension, and as spiders are able to pull out the silk thread with their hind legs,and also by their body-weight, any internal control mechanism seems un-necessary.

New light has recently been shed on this aspect of silk-spinning, however,with the discovery that control valves are present in the ducts of those glandswhich secrete the dragline silk in Araneus diadematus and other orb-webspiders (Wilson, 1962). These valves are absent from spiders which spinother types of web, and it seems therefore that the orb-web spiders possessa specially great degree of control over their dragline spinning. Since the drag-line is used for the framework and radii of the orb-web, it seems reasonableto assume that this extra control is associated with the spinning of the orb-web itself.

In view of these findings, therefore, it was thought that a review of themechanism of dragline spinning in orb-web spiders might prove to be ofvalue, and studies were made both of the tarsal claws and their manipulationof the silk thread, and of the properties of the silk duct, the control valve, andthe spigot in relation to silk-thread production.[Quart. J. micr. Sci., Vol. 104, pt. 4, pp. 557-71, 1962.]

558 Wilson—Dragline spinning by the spider

The tarsal claws and their control of the silk threadThe tarsal claws oiAraneus, shown in fig. i, have been described previously

by Nielson (1932), and his observations and conclusions have been confirmedin this study. Of the 3 claws, the 2 outer ones have large teeth and the medianone very small teeth, and the whole set can be pivoted backwards and for-wards on a terminal sclerite. A set of barbed hairs is associated with the claws,

FIG. 1. Drawing of the tip of the left-hand fourth leg ofAraneus, showing the 3 principal tarsal claws in relation tothe accessory claws. Most of the surrounding hairs have been

removed.

attached to the last tarsal segment. The hairs are springy and have theirbarbs facing outwards.

The median claw is the one principally concerned with manipulating thesilk. When the claws are pivoted forwards on the terminal sclerite, the medianclaw is positioned so that it can be hooked over the silk thread, and as the silkengages in the median claw it is trapped simultaneously by the barbs of thebarbed hairs and is held very firmly. When the median claw is raised slightly,the trip on the silk is relaxed, but the thread can only be released completelyby pivoting the claws right back so as to allow it to escape from the barbedhairs. The method of gripping and releasing the silk is illustrated in fig. 2.

Thus the tarsal claws can pick up, grip or release the silk quickly, but theyappear unable to exercise a smooth control over the speed at which the silk

A

silk debris

spigot

coiling of brokenend of silk

Wilson—Dragline spinning by the spider 559

is being pulled out. It is likely, therefore, that fine regulation of the spinningprocess is achieved by means of the internal control mechanism.

Intra-abdominal pressure and its influence on silk spinningParry and Brown (1959) found the normal 'resting' pressure in the leg of

Tegenaria atrica, the common house spider, to be about 5 cm Hg, rising tran-siently to over half an atmosphere when the spider is stimulated. The authors

100>/

FIG. 2. Diagrammatic drawings of the tarsal claws of Araneus illus-trating A, the claws pivoted back so as to release the silk, and B, theclaws pivoted forwards showing how the silk is trapped between themedian claw and the accessory claws. For clarity, only one accessory

claw has been represented.

suggested, however, that these pressures might occur only in the cephalo-thorax, the abdomen being too elastic to resist such internal pressures withoutstretching.

The abdomen in both Tegenaria and Araneus, however, is encircled imme-diately below the cuticle with relatively large bands of muscle (fig. 3, A), whichmust be able to limit stretching, or even maintain a positive hydrostatic pres-sure. Measurements were therefore made using both Araneus and Tegenaria,in order to discover whether any appreciable positive pressure indeed existsin their abdomens.

Measurement of the intra-abdominal pressure

Preliminary experiments using the helical glass pressure gauge developedby Harris and Crofton (1957) for direct measurement of internal pressures

FIG. 3 (plate), A, photograph of a dissection of the abdomen of Araneus, showing the sub-cuticular muscle bands.

B, photograph of part of the silk duct of Araneus showing the broken end of the silk lyingnear the second bend in the duct after the thread had broken during an extraction experiment.


in Ascaris, indicated that positive hydrostatic pressures were present in theabdomens of both Araneus and Tegenaria. Precise measurement proved im-possible, however, because the insertion of a hypodermic needle into theabdomen resulted in fluid leakage round the puncture, and therefore a dropin the pressure.

hinge

FIG. 4. Diagram of the apparatus used for measuring the intra-abdominalpressure in Araneus. For further details see text.

Indirect measurement of the hydrostatic pressure was therefore made,using the apparatus illustrated in fig. 4. The spider was strapped to a plasti-cine block so that the abdomen was held above the level of the rest of the bodywithout being in any way constricted. The block and spider were then placedon a rack-and-pinion device so that they could be raised and lowered, and werepositioned under a horizontally mounted strip of \ in. Perspex. The Perspexstrip was hinged at one end, and had the other end resting on a support. Thespider's abdomen was then painted with black printers' ink, and the blockracked up until the abdomen pressed against the perspex strip and just liftedthe free end from its support. The block was then lowered, the spider's abdo-men re-inked and racked up again so that the abdomen pressed againsta different point on the Perspex strip. A series of prints were thus obtained


showing the area of contact between the abdomen and the Perspex at differentpoints along the strip. The area of each print, and the force required to liftthe Perspex strip at each corresponding position, was measured, and fromthese figures the force per unit area was calculated.

This procedure was repeated with the same spider after it had been relaxedin ethyl acetate, and under these conditions the force per unit area corre-sponds to the intrinsic elasticity of the spider's body. For any given load,therefore, subtracting the values obtained for the active and relaxed conditions,will give a measure of the intra-abdominal pressure.

The measurements were successfully repeated on 4 adult female Araneus,and the results showed that there was a constant difference in each casebetween the active and relaxed conditions under loads of from 4 to 23 g.Table 1 gives the values for an applied load of 5 g.

TABLE I

The intra-abdominal pressures of 4 adult female Araneus

Spider

1234

Pressure (glcrrfi for applied loadofSg)

Active

90-882-0

I I I -Oioo-o

Relaxed

43-539'442-S55-6

Difference

47'342-658-S44'4

Equivalentcm of Hg

3'483-i44-3°3-27

The values of intra-abdominal pressure in Araneus obtained by this methodrange from 3-14 to 4-3 cm Hg, and are thus comparable to the resting pressureof 5 cm Hg in the leg of Tegenaria (Parry and Brown, 1959). There is alsosome indication from the preliminary experiments that such a pressure alsoexists in the abdomen of Tegenaria, but this has not been measured.

The influence of intra-abdominal pressure on silk-spinning

The next step was to determine whether intra-abdominal pressures of thisorder can effect silk-spinning in Araneus.

Accordingly, adult female spiders were etherized and strapped to a plasti-cine block, and their abdomens were subjected to pressures varying from zeroto 7-8 g. The pressures were applied to each spider's abdomen by means ofa balance fitted with a pressure plate (fig. 5), and the range chosen corre-sponded roughly to from o to 4 cm Hg. The pressure was altered in 7 equalsteps, starting and finishing at a maximum, with zero half-way. At each pres-sure, silk was wound out by hand on to a slide, and its diameter measured.

If the abdominal pressure had any effect on silk-spinning, the variations inapplied pressure should be reflected in the thickness of the silk thread pro-duced, and the results shown in table 2 reveal that this is the case. To accountfor this result one must postulate that the pressure is capable of moving thefluid silk down the duct, and that the diameter of the thread depends to a large


extent on the amount of fluid silk available for spinning. The results werecomplicated by the spider recovering from the anaesthetic towards the end ofeach experiment, but this in itself shows that the spider is able to control thethickness of its thread, despite an artificial increase in its abdominal pressure.

FIG. 5. Diagram of the apparatus used for compressing the abdomens of Araneus.For further details see text.

TABLE 2

The diameter of dragline silk extracted from anaesthetized adultfemale Araneus whose abdomens were subjected to externally

applied pressures

Spider

i

2

3

Force applied toabdomen

(g)o

finger pressure

7-8

2-6o-o2-6S'2*7-8*

7-85-22-6o-o2-65-2

7-8*

Silk diameter

0*)87

1 2 2

1 3 9

1 2 9

u s1 2 81 2 01 2 6

1 3 41 3 2

1 3 11 2 01 2 01 2 61 3 0

* Spider recovering from anaesthetic.

Measurement of the diameter of the silk proved difficult because it variedwith the humidity of the air and with the tension under which the silk wasmounted on the slide. Relative measurements were obtained, however, bothfrom dry-mounted silk, and from silk stained in shirlastain A (I.C.I.) andmounted in de Faure's aqueous medium. This treatment ensures that each ofthe silk samples was in a comparable physical state, and allowed more reliable


relative measurements of the diameters to be made. The figures given forsilk diameters throughout this study refer in each case to treated silk.

Mechanical extraction of the draglineSince fluid silk is converted into silk thread by tension (Richards, 1953;

Savory, 1952), it was thought that there might be a relationship between the

FIG. 6. Diagram showing the layout and connexions between itemsof equipment used for measuring the force required to extract thedragline from Araneus and Zygiella at various speeds. For further

details see text.

forces required to extract the silk at different speeds, and that these mightcorrelate with the diameter of the silk produced. It was determined, there-fore, to extract the dragline mechanically from anaesthetized spiders, and tryto discover what limitations were imposed on silk-spinning by the structureand functioning of the spigot, the silk duct and the control valve in the duct.It was necessary to use anaesthetized spiders so that they could not activelycontrol the spinning process.

Materials and method

Fig. 6 shows the apparatus used in these experiments. After etherizationuntil movements had ceased, a spider, held in a small spring wire clip roundits pedicel, was hung on the arm of the balance so that the spinnerets on the


abdomen pointed upwards. This was achieved by counterweighting the spiderwith a small ball of plasticine hung from the other end of the clip.

The balance was constructed from a sensitive microammeter movement, themeter pointer being replaced by the balance arm, one end of which wasfashioned into a small hook from which the spider was hung, while the othercarried a small vane of black, opaque paper. An oil dashpot was fitted to pro-vide damping.

The operation of the balance was as follows: as the balance arm was de-pressed by the weight of the spider, the black paper vane uncovered the sensi-tive surface of a cadmium sulphide photoresistive cell to the light from a smalltorch bulb. The cadmium cell was connected in the base circuit of a singletransistor amplifier, and the change in current produced by the light fallingon the cell was amplified and fed back into the meter coil so as to oppose theoriginal displacement of the balance arm. In this way the arm was stabilizedwhile the weight of the spider could be measured in terms of the currentthrough the meter coil. In practice the voltage drop across the coil was dis-played on a specially calibrated voltmeter and on the screen of an oscilloscope(Nagard double beam, type DT 103).

After the spider had been placed on the balance, the dragline silk waspicked up with a needle, and given a turn round a brass drum which wasmounted on a spindle. The spindle was driven by a thyratron-controlled,variable-speed motor. The drum was fitted with a paper sleeve which couldbe renewed for each experiment, and served for individual storage of thesilks.

The motor was then switched on and its speed either gradually increaseduntil the silk broke, or held at certain constant speeds according to the experi-ment. The force required to extract the silk, equal to the apparent reductionin weight of the spider, was continuously recorded by a moving film camera(Langham-Thompson, type series 200), running at one inch per second,attached to the oscilloscope. The whole apparatus was carefully adjustedbefore each run so that a known displacement of the spot represented a knownforce applied to the balance. The second beam of the oscilloscope was usedto display the waveform from an 'on/off' device mounted on the spindle. Thiswas constructed from a potentiometer, and served as a revolution marker.

When the film had been developed, it provided a complete record of theforce applied to the silk, together with the speed of rotation of the drum,throughout each experiment. From these records the speeds of draglineextraction corresponding to the force needed to extract the silk were measuredand calculated.

Throughout each run the spider's spinnerets were kept under constantobservation through a stereoscopic microscope so that the silk could bewatched coming from the spigots. Such detailed observation was only pos-sible because of the stability of the balance arm. Afterwards the spider waspreserved in Pampel's fluid, and subsequently surface-dried, weighed, andmeasured. Each experiment therefore utilized a fresh spider, taken from


among those which had been collected from their webs in the field theprevious day.

Measurements were made of the diameter of the silk obtained in eachexperiment, after mounting it in de Faure's medium as previously described.

broke/

A Zygielto

E 50

" 40

10 20 30 40 50 60 70 80 90 100 1speed cm/sec 5Mk-*--

broke \

G Zygiella

•r

B Zygiella

,roke J

(

0 10 20 30 40

D Ar

50 60>ed cm/si

70 80

V

10 20 30 40 50 60 70 80 90 100 110speed cm/sec

FIG. 7. Graphs showing the relationship between force applied to the dragline silkand Zygiella and the speed of its extraction. Graph D is for Araneus and A, B,

Zygiella.

of Araneusand c for

Two sets of experiments were performed. In the first, the dragline wasextracted from adult female Zygiella at a gradually increasing speed. The runwas terminated by the silk breaking. In each case graphs were then drawn oftension in the silk against speed of extraction. Thirty-four runs gave reliableresults; many more were actually attempted, but were rejected either becauseof faults developing in the apparatus or through premature breakage of thedragline. Eight runs were also made with Araneus to check that the resultswere applicable to both species.

In the second set of experiments the dragline was extracted in stages fromlightly etherized adult female Araneus. Lengths of silk were wound out ata constant speed (18 cm/sec) at intervals of 2 min. Each length was keptseparate by moving the paper sleeve up the brass drum between each run.The tension in the silk was read directly from the voltmeter, instead of beingrecorded photographically from the oscilloscope, and was plotted against thediameter of the silk for each length. It was thus hoped to follow any changesthat might occur as the spider recovered from the anaesthetic.


Results and conclusions

Fig. 7 gives four graphs typical of those plotted from the first set of experi-ments. Although the rates of increase of tension with speed varied betweenthe different graphs, each could be seen to have the same overall pattern: aninitial sharp rise, followed by a flattening out of the curve, followed in turn bya sudden steep rise culminating with breakage of the silk.

Detailed assessment of the results, however, failed to reveal any clearrelationship between the diameter of silk, the size of the spider, and any of thevarious parameters which could be measured from the graphs of the individualruns. The only constant feature was the general shape of the curve as out-lined above.

TABLE 3

Values for dragline diameter and force required for its extraction at a constantspeed of 18 cm/sec. The dragline was extracted from lightly etherized adultfemale Araneus at intervals until the spider had recovered from the anaesthetic

or the silk broke

Spider

1

2

3

4

5

No. ofrun

1

2

345t

1

2

34t

1

2 *

3 *

1

2

3t

1

2

345*

Force required toextract silk at iS cm/sec

(mgs)5

2 2

456 0

55

3°42658 0

150-195

45-435

343636

41 0

16

5°0-110

Diameter ofsilk

0*)8-5879-8

io-69-8

8-6n-815-013-8

11-47-6-10-37-7-9-9

4-45-76-8

8-88-58-9

n-95-3-H-7

* Spider struggling. t Silk gave out.

In the second set of experiments the results given in table 3 indicate that,with each spider, tension in the silk rose with each successive run, and thatthis rise was correlated with an increase in the silk diameter. The last runs in


2 of the experiments, however, gave very variable results, due to the spiderregaining control of its muscles, but overall the results indicate clearly thatsome aspects of silk-spinning are governed by the internal state of the spider,probably by the body pressure which increases in step with the increase of thethread diameter.

Discussion

The results of this study show that the intra-abdominal pressure plays a veryimportant part in silk-spinning in Araneus, namely, that of moving the fluidsilk up the duct and keeping it available for spinning into a thread. Additionalevidence for this theory comes from a study of the effects of anaesthetizationon spinning. Normally the dragline can be picked up by touching the spigotwith a needle, but when the spider is fully relaxed, the silk can no longer bepicked up in this manner. Whereas in an active spider fluid silk must fill theduct right up to the tip of the spigot, and can be picked up, when the spider isrelaxed the muscle tension and hence the internal pressure falls, the fluid silkretracts from the spigot tip, and cannot be picked up. The structure of thegland and duct suggest an analogy with a pipette, the soft-walled gland beingequivalent to the bulb, and the duct, lined throughout with cuticle, acting asthe tube. There are no muscles round the gland which could directly forcethe fluid silk up the duct, and it may therefore be concluded that this functionis performed by the internal hydrostatic pressure. It was established by dis-section that the control valve remains open in fully relaxed spiders, and doesnot affect the above argument.

During spinning the change from fluid silk to silk thread must occur withinthe duct, and the point at which this occurs can be demonstrated as follows.The dragline is pulled from an anaesthetized spider until a certain tensiondevelops in the thread, and the spider is fixed in hot water while the silk ten-sion is maintained. The duct is then dissected out and examined under themicroscope. The point of thread formation can be seen to occupy a very shortlength of the duct, its position varying with the tension in the silk. At lowtensions it is at the spigot tip, and at high tensions it is found on the far sideof the control valve where the duct loops back on itself. This demonstrationis possible only with anaesthetized spiders where the fluid silk is not moved upby the body pressure, so that pulling the silk out converts more and more ofthe fluid silk already in the duct into thread, with a consequent increase oftension as the fluid silk is used up.

For the continuous spinning of a thread of uniform thickness, the silk drawnout as thread must be balanced by movement of fluid silk down the duct, andunder these conditions the point of thread formation will remain in one regionof the duct. If the silk thread is extracted faster than the fluid silk can replaceit, then either the diameter of the thread will decrease, or the fluid silk in theduct will be used up. This results in the thread running through an increasedlength of duct, consequently the friction will rise, and so will the force re-quired to maintain the given rate of extraction. Ultimately the point of thread

2421.4 Q q

5 68 Wilson—Dragline spinning by the spider

formation will move right back into the looped region of the duct and thethread will seize up and break. Fig. 8 illustrates these effects diagrammatically,while fig. 3, B is a photograph of the silk duct of Araneus dissected out afterthe silk had broken during an extraction experiment, showing the point of silk

FIG. 8. Diagram (not to scale) of the dragline duct of Araneus, showingthe position of the zone where fluid silk changes to silk thread undervarious conditions of spinning. A, transition zone at the spigot tipwhen excess fluid silk is available; B, transition zone at the controlvalve when the amount of fluid silk available is balanced by threadextraction; c, transition zone in the looped region of the duct wheninsufficient fluid silk is available to balance thread extraction; and D,

position of breakage of the silk under extreme tension.

breakage near the second duct bend. The coiling visible in the broken endof the thread suggests that the silk was under tension and recoiled as thethread snapped. On the other hand, if the silk is pulled out very slowly, or notat all, then the duct will fill up with fluid silk. Body pressure alone, however,does not seem capable of forcing the silk out of the spigot, possibly becausethe spigot shuts under its own elasticity (fig. 9). Surface tension effects willalso help to prevent the fluid silk being squeezed out of the spigot.

Apart from the characteristic S-shape of the force/speed graphs (fig. 7),the results obtained from the first set of dragline extraction experiments werenotable for their lack of correlation. This variability is probably due to differ-ent levels of anaesthetization being achieved in each spider, resulting in in-dividual values of body pressures and individual settings of the control valve.This is evidence that, under normal conditions, the spider can regulate its


dragline spinning through the interaction of these two factors. Moreover,variations in dragline thickness could be achieved if the control valve apertureitself were the functional spinning orifice, where the fluid silk is transformedinto the thread.

pin form glandspoo/s

flexor muscle

FIG. g. Drawings of the dragline spigot; A, showing wrinkling due totension in the spigot tip while accommodating a silk thread, and B,the spigot tip closed in the absence of silk, A also shows the cuticular

structures associated with the spigot.

Additional evidence for this interpretation is shown by the results of thesecond set of extraction experiments, where silk was extracted in stages atconstant speed from lightly anaesthetized spiders. It was found that the forcerequired for extraction increased at each successive stage in step with anincrease in silk diameter. As the spider recovered gradually from the anaes-thetic, its body pressure rose and forced more fluid silk down the duct, soaccounting for the increase in thread thickness. The increase in force requiredfor its extraction, however, must be due to friction as the thread passedthrough the spigot. It is significant that, as the spider recovered fully, therelationship between the force for extraction and the diameter of the threadwas destroyed, implying that the control valve is able to regulate the flow offluid silk under conditions of normal body pressure and control the thread

57° Wilson—Dragline spinning by the spider

thickness. It is likely, therefore, that spigot friction has little effect on normalspinning when the diameter of the thread is small, and that the control valveis the point at which the spinning process takes place.

The characteristic S-shape of the force/speed curves for dragline extractionmust now be considered. When rubber, wool, artificial silk, nylon, silkwormsilk, and also spider silk are subjected to progressively increasing loads, andthe values of the loads are plotted against the extension produced in the materialunder test, graphs of a similar S-shape are produced (Astbury, 1933; deWilde, 1943; Alfrey, 1945; Vincent, i960). It is well known in these casesthat the initial rise in the curve corresponds to elastic extension of the thread,the flattening out which follows indicates the region of molecular orientation,where movement of the molecular aggregates allows relatively easy extensionwith small increase in the applied load, and the final rise occurs when orienta-tion is complete and the material becomes relatively inextensible. Althoughthese load/extension curves refer to static conditions, and the force/speedcurves obtained in the dragline extraction experiments relate to dynamicconditions, it seems, nevertheless, that they reveal closely similar events, andmay be explained in the following way.

At the beginning of each extraction experiment, the fluid silk fills the ductup to the spigot tip, enabling it to be picked up on a needle and pulled outas a thread for winding onto the take-up drum. At the initial very slow ratesof extraction, the force required to extract the silk is related to the inherentelasticity in the thread. As the speed of extraction is increased, so that theamount of silk pulled out comes to be in excess of that supplied as fluid, thesilk is more and more stretched. By analogy with the static conditions, a pointwill be reached where molecular orientation becomes predominant, and theforce/speed graphs show a flattening out of the curve. With very high speedsof extraction, the stretching of the silk increases until all the available mole-cules are orientated, whereupon the force required for its extraction suddenlybuilds up and the thread snaps.

The results of these experiments reveal the importance of the control valvein dragline spinning in orb-web spiders. It can regulate the amount of fluidsilk moved up under body pressure, and acts as the functional spinning orifice.By altering the valve opening, the spider can control the thread thickness, andthe force required for its extraction. This internal control mechanism is there-fore designed for delicate regulation of the dragline spinning, whereas, onceit is spun, the thread may be manipulated by the tarsal claws. It is appro-priate that the orb-web should be the outcome of the delicately adjustedrelationship between two such mechanisms.

I should like to thank Professor J. E. Harris for the loan of the helical glasspressure gauge used in these experiments, and also all other members of theZoology and Physics Departments, especially Mr. D. F. Gibbs and Dr. J. C.Hartley, for their constant willingness to help with the problems arising inthe course of this work.


ReferencesALFREY, T., 1945. Mechanical behaviour of high polymers. New York (Interscience).ASTBURY, W. T., 1933. Fundamentals of fibre structure. London (Oxford University Press).BRISTOWE, W. S., 1958. The world of spiders. London (Collins).HARRIS, J. E., and CROFTON, H. D., 1957. J. exp. Biol., 34, 116.MILLOT, J,, 1949. 'Ordre des Araneides', a chapter in Traite de zoologie, tome vi, edited by

P.-P. Grasse. Paris (Masson).NIELSON, E., 1932. The biology of spiders. Copenhagen (Levin and Munksgaard).PARRY, D. S., and BROWN, R. H. J., 1959. J. exp. Biol., 36, 423-33.RICHARDS, A. C, 1953. 'Chemical and physical properties of cuticle', a chapter in hisect

physiology, edited by K. D. Roeder. New York (Wiley).SAVORY, T. H., 1952. The spider's web. London (Warne & Co. Ltd.).VINCENT, P. I., i960. Polymer, 1, 7.WILDE, J. DE, 1943. Arch, neerl. physiol. (ser. Ill), 27, 119-32.WILSON, R. S., 1962. Quart. J. micr. Sci., 104, 549.

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