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Ballooning spiders.

Ballooning Spiders

Fossil records of spiders date back 300 million years, and over the past 25 million years these creatures have changed very little. One would expect that they are well adapted for survival, and close inspection of their bodies and way of life confirms this. Among the most remarkable animals in the animal kingdom, spiders are equipped with varied and exquisitely refined sensory organs that allow them to disperse, feed, and reproduce effectively.

In Part I of this study (see ME, Sept. 1989, pp. 50-53) we briefly reviewed various aspects of organism dispersal in the wave-swept environment. In Part II we consider the land-air environment by reference to the fluid mechanics aspects of spider ballooning and prey capture. Ballooning is a form of aerial dispersal that critically affects the feeding and reproduction of spiders. Improved understanding of it should have important consequences in the control of insect pests in agriculture.

Bodies of land-based animals, such as insects and spiders, are approximately 1000 times as dense as air. As a result, when passively transported by air, the only forces that matter in determining their individual speeds and trajectories are gravity and drag.

An interesting example concerns the phenomenon of spider ballooning. In this case, air drafts stimulate a response in certain species to adopt favorable orientations with respect to the wind, which can result in aerodynamic lift. A spider's adoption of a preferred attitude with respect to the wind is an "active" interaction with the flow field environment in the sense discussed in Part I of this work. Once levitated, however, the organism is entirely passive as far as its aerodynamic transport is concerned. This transport is briefly examined here.

Spiders display yet another form of active body-flow field interaction. This relates to their response to characteristic signals in the air oscillations generated by, for example, insects flapping their wings, which, if detected by a spider, may elicit behavioral patterns appropriate to the pursuit of prey or the avoidance of predators.

Ballooning

Although a misnomer, ballooning is the word commonly used by arachnologists to describe the aerial dispersion of spiders resulting from the action of drag on their bodies and, more importantly, on one or more long silk filaments attached to their spinnerets. The ballooning phenomenon is of considerable ecological interest. While this mode of spider dispersal is not well understood, it potentially affects where in the landscape spiders will be found, and thereby the amount of biomass, mostly in the form of insect prey, that a ballooning species will consume. Although the amount of biomass eaten by spiders on a yearly basis is known to be very large, the benefits to mankind in general, and to the control of crop pests in particular, remain unknown.

There is a strong correlation between increases in environment temperature and ballooning activity. A distinction must be made between the two roles of temperature as a factor stimulating and sustaining the ballooning activity. Richter's experiments on Pardosa purbeckensis support the role of temperature as a stimulant for the "tip-toe" behavior of spiders prior to ballooning [1]. During this stage, shown in Figure 1, a ballooning spider orients itself facing the wind while raising its abdomen and straightening its back legs. Then, assisted by wind drag, it extrudes one or more silk filaments from the exposed spinnerets.

In contrast, the work by Vugts and Van Wingerden points to the role of temperature for sustaining the ballooning activity [2]. These authors show that variations of the gradient Richardson number, Ri, defined as Ri = (g/T)(dT/dz)[(dV.sub.x/dz.sup.)2] correlate strongly with variations in the ballooning activity of Erigone arctica. (In the definition of Ri, g is the acceleration of gravity, T is the absolute temperature, [V.sub.x] is the mean horizontal wind velocity, and z denotes the vertical coordinate direction.) Their observation is significant because the gradient Richardson number is a relative measure of buoyancy-induced to shear-induced turbulence in the flow or, equivalently, of the ratio between vertical and horizontal velocity fluctuations. Vugts and Van Wingerden show that large negative values of Ri, characteristic of unstably stratified wind flow conditions on calm, sunny days, correlate strongly with pronounced ballooning activity.

Irrespective of important biotic factors, which can affect the initiation of spider ballooning, there are important mechanical constraints that limit the parameter ranges over which the activity can be sustained on a purely physical basis. The "rules" involved have been investigated by Humphrey using a simple "lollipop" configuration to represent the spider (approximated as an equivalent sphere) and the silk filament (approximated as a rigid rod) to which it is attached [3]. The analysis predicts that the minimum vertical wind speed, [V.sub.z], required to levitate a spider of mass m (or, equivalently, of diameter D), attached to a silk filament of length l and diameter d is given by: [Mathematical Expression Omitted] where [mu] is the kinematic viscosity of air. A plot of [V.sub.z] as a function of mass m for different values of l is shown in Figure 2. The abscissa provides scales for the modulus of the wind velocity in units of m/s or mph and equivalent temperature differences, [Delta] T([degree]C), for pure free convection. The values of [Delta] T were estimated from: [V.sub.t] = (2gL [Delta] T/[T.sub.o])[.sup.1/2] where [T.sub.o] = 300 K and L = 5 m are the assumed surrounding temperature and characteristic thermal length scales, respectively. Even though the assumptions underpinning the analysis render the predictions qualitative, it is clear that winds in excess of about 2.5 m/s correspond to unrealistically large temperature differences. This allows a distinction to be made between low-speed air currents induced primarily by heating of the ground (through solar irradiation) and high-speed air currents due to large-scale climatological conditions.

Various constraints of an entirely mechanical nature limit the ballooning activity to the window shown in Figure 2. They are, respectively: * Minimum size constraint: Even the smallest ballooning spider is finite in size. The data available suggest m [tilda or minus] 2 x [10.sup.-4] grams. * Minimum velocity constraint: Spiders are sensitive to the slightest air movements through their tactile hairs and trichobothria. Richter's experiments show that isothermal velocity fluctuations stimulate spider ballooning activity and that this takes place for values of a "spider" Reynolds number, [Re.sub.D] = [DV.sub.zp]/[mu], of order 10 [1]. This condition fixes the minimum velocity that will both stimulate and sustain the ballooning activity. * Maximum velocity constraint: The work of Richter [1] and Vugts and Van Wingerden [2] shows that spider ballooning activity decreases with increasing wind speed. This allows the definition of a range in [Re.sub.D] beyond which ballooning cannot be accomplished safely. The data suggest 250[is less than or equal to][Re.sub.D][is less than or equal to] 400, approximately. However, at high wind speeds ballooning is unlikely because the substratum (leaf, twig, etc.) upon which the spider rests starts oscillating. Assuming that the characteristic dimension of the substratum, L, lies between 0.2 and 2 cm gives values of (Re.sub.L) (Reynolds based on L) ranging from 400 to 4000 when [V.sub.z] = 3.5 m/s. The data in Richter and Vugts and Van Wingerden suggest that 3.5 m/s is an appropriate maximum wind speed beyond which ballooning activity ceases entirely. * Surrounding environment constraint: If, upon being emitted, the silk filament attaches to the surroundings, ballooning cannot take place. Thus, there is a practical limit to L that here has been set at l[is less than or equal to] 4 m.

The conclusion of this analysis is that purely mechanical arguments define a range of conditions, or a window, outside of which the ballooning activity cannot be physically sustained irrespective of the biotic factors involved. Although qualitative, the rules established through improved understanding and modeling help to render the ballooning phenomenon predictable in a more general context.

Sensing Air Flow

While it is not yet entirely clear what the main environmental factors are that stimulate ballooning, an important stimulus appears to be the detection of favorable wind conditions by spiders. Both insects and spiders have thread hairs sensitive to extremely weak currents of air. The deflections of these hairs help distinguish between the presence of prey, predator, or mate from relatively high-frequency substrate and air-borne vibrations. Therefore, it seems reasonable to suppose that these sensory organs also play a role in transmitting information concerning the low-frequency signal content of background wind favorable to ballooning activity. Hair Sensillae. Hair sensillae are the most widespread type of sensillum on the arthropod exoskeleton. Barth and Blickhan have shown that they have mechanoreceptive, chemoreceptive, and thermoreceptive functions [4]. Two types of mechanoreceptive hair, sensilla in spiders are shown in Figure 3. In a mechanoreceptive hair, the dimensions and structure of the shaft, as well as the stiffness and structure of its articulation, affect the hair's sensitivity to the intensity and direction of air motion. Of interest here are the very fine hairs on insects (filiform or thread hair) and arachnids (trichobothria) that respond to the slightest movement of air.

The sensory hair apparatus consists of the hair shaft, its articulating membrane, and one or more sensory cells attached to the hair base. Typically, the thread hairs on insects have diameters less than 10 [mu]m and range between 400 and 2000 [mu] m in length. Corresponding dimensions for spider trichobothria are of order 1 [mu]m for the hair diameter, for hairs ranging between 100 and 1500 [mu] m in length. However, much longer trichobothria, up to 2500 [mu] m, have been observed in bird spiders.

Small deflections of the sensory hairs trigger action potentials that determine behavioral responses to air motion. Of special interest is the fluid mechanics of the stimulation process that, in the case of spiders, is known to elicit prey capture behavior or defensive behavior. Prey Capture. The dynamic properties of the trichobothria's stimulus-transmitting structures have been investigated by Reibland and Gorner [5] for the case of orb-weaving spiders (Agelenidae, Araneae) and by Hergenroder and Barth [6] in the case of a wandering spider (Cupiennius Salei). Reibland and Gorner show that the air motion generated by the wings of a fly hovering about 1 cm from an orb-weaving spider, such as a Tegenaria or Agelena, deflect the trichobothria on a spider's extremities to new positions around which they oscillate. (It is important to note that the trichobothria are not pressure detectors; they react solely to the movement of air and can be modeled as displacement receivers.) Experiments in steady flows showed that trichobothria of medium length (400 [mu] m) on a house spider were deflected 5 degrees in air speeds of 10 mm/s. Since action potentials are known to occur in response to deflection angles even smaller than 1 degree, it is clear that these hairs are quite sensitive, even to the weakest of flows. The stiffness of the hairs prevents them from bending significantly during deflection.

Because flies are common prey for the spiders investigated by Reibland and Gorner, these authors measured the energy spectra of Musca and Drosophilla, respectively. The fundamental oscillation of Musca was detected at 163 Hz, while that for Drosophilla was found at 185 Hz. Both spectra revealed higher harmonics but with considerably less energy. Trichobothria deflections in Tegenaria and Agelena spiders were measured as a function of hair length in sound fields ranging from 10 to 2500 Hz. The observations concerning mechanical directional sensitivity revealed essentially isotropic hairs. At low frequencies (less than 40 Hz) long hairs were deflected most, while at high frequencies (larger than 1000 Hz) the opposite was observed. Amplitude-frequency response measurements for Tegenaria showed that to induce a deflection of 1.64 degrees in a hair 255 [mu] m long, an effective sound particle velocity of 13 mm/s is required at 100 Hz but that this is only about 3 mm/s at 10 Hz. (This suggests that low-speed, low-frequency oscillations typical of the background wind favoring ballooning conditions are well within the range of detection of the spider trichobothria.)

Smoke flow visualization reveals a highly unsteady three-dimensional flow in the near wake of a fly. The mean speed of this flow is about 20 to 40 cm/s. The flow is well within the amplitude-frequency range of the trichobothria. Reibland and Gorner attempted to answer the question of whether or not the frequency content of the stimulus flow field generated by prey is recognized by spiders through their trichobothria. Frequency response diagrams showed that single hairs did not display resonances over the range of frequencies generated by prey. Although the maximum relative amplitude of hairs of different lengths was found to shift from longer to shorter hairs with increasing frequency, this was a minor effect and the authors considered that there was no frequency discrimination by the mechanical apparatus.

In contrast to this are the findings for insects, in particular those for the thoracal hairs of caterpillars of the cabbage moth Barathra brassicae, and on the cercal filiform sensilla of the cricket Gryllus bimaculatus. The mechanical aspects of these receptor hairs have been modeled as damped driven one-dimensional harmonic oscillators. In conjunction with experiments, such theoretical models allow one to identify the preferred frequency ranges within which short and long hairs deflect significantly. The analyses consider the boundary layer nature of the flows and the relation between boundary layer thickness and hair length for low- and high-frequency discrimination. In particular, it can be shown that long hairs are sensitive to velocity magnitudes while short hairs are sensitive to velocity changes, i.e., air accelerations.

In the case of Barathra brassicae, it is known that the vibration-sensitive thoracal hairs are most sensitive mechanically if stimulated by air motions with frequencies between 100 and 150 Hz. In this range, the resonance frequency of the hairs coincides exactly with the principal frequency component in the near field motion of the predator wasp, Dolichovespula media. The biological significance of this is quite obvious, since these caterpillars will deliberately drop from plants when their hair sensillae signal the presence of such a predator.

The fact that the trichobothria of spiders do not show a clear-cut mechanical resonance between 5 and 2500 Hz suggests that more advanced models are necessary for predicting the characteristics of such complex systems. It also remains to be seen how spiders can extract from a complex pattern of air flow the specific signals indicating the presence of prey or the appropriate conditions for ballooning. In this regard, the interaction between the trichobothria and the highly developed substrate vibration slit sense organs in spiders is a matter of intense research.

Nature abounds in animals whose biotic functions have been honed through natural selection over very long periods of time. In this sense, the bodies of these animals are matched (often extremely well) to their natural habitats for dispersing, feeding, and reproducing in the most effective way. If only for this reason, such animals are of considerable engineering interest.

A spider is the nearest thing one can imagine to a living microsensor. Most of its body and extremities are covered with hairlike receptors that are extremely sensitive to chemical, thermal, and mechanical stimuli. They inform the spider about its surroundings. An improved understanding of sensory systems perfected over millions of years, in spiders or other animals, can lead to better models of comparable engineering systems. In the case of spiders, an understanding of the function and mechanical properties of the trichobothria in relation to ballooning and prey capture behaviors could be put to practical use in, for example, the natural control of insect pests in crop fields.

PHOTO : Figure 1. On tip toes. A spider of the family Thomisidae (a crab spider) exhibits the tip-toe behavior that precedes ballooning. Here the spider is on a twig 1 m above the ground in an air updraft of approximately 1 m/s. The spider's body is about 3 mm long and the silk filament is 1 to 5 [mu] m in diameter and about 1 m long at takeoff. (Photo by C.E. Morgan courtesy of M.H. Greenstone, both of the USDA, Columbia, Mo.)

PHOTO : Figure 2. Limits within which spider ballooning can be physically maintained for a spider dragged by a silk filament of length I. Spider mass and equivalent diameter are plotted against wind speed shown in m/s, mph, and equivalent thermal plume-environment temperature difference.

PHOTO : Figure 3. Mechanosensitive hair sensilla from a leg of the wolf spider Lycosa gulosa. The slender trichobothrium (Tr) arises vertically from a prominent socket; the stronger tactile hair (T) emerges obliquely from a less-developed socket. Magnification is 1250x. (Photo from Chu-Wang and Foelix, Biology of Spiders, Harvard University Press, 1982).
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Title Annotation:Fluid Mechanics of Biotic Functions, part 2; aerial dispersion of spiders
Author:Humphrey, Joseph; Denny, Mark
Publication:Mechanical Engineering-CIME
Date:Nov 1, 1989
Words:2806
Previous Article:Electrohydraulic motion control.
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