Trichobothria mechanically respond to such electric fields, as well as to air flow

When one thinks of airborne organisms, spiders do not usually come to mind. However, these wingless arthropods have been found 4 km up in the sky [], dispersing hundreds of kilometers []. To disperse, spiders “balloon,” whereby they climb to the top of a prominence, let out silk, and float away. The prevailing view is that drag forces from light wind allow spiders to become airborne [], yet ballooning mechanisms are not fully explained by current aerodynamic models []. The global atmospheric electric circuit and the resulting atmospheric potential gradient (APG) [] provide an additional force that has been proposed to explain ballooning []. Here, we test the hypothesis that electric fields (e-fields) commensurate with the APG can be detected by spiders and are sufficient to stimulate ballooning. We find that the presence of a vertical e-field elicits ballooning behavior and takeoff in spiders. We also investigate the mechanical response of putative sensory receivers in response to both e-field and air-flow stimuli, showing that spider mechanosensory hairs are mechanically activated by weak e-fields. Altogether, the evidence gathered reveals an electric driving force that is sufficient for ballooning. These results also suggest that the APG, as additional meteorological information, can reveal the auspicious time to engage in ballooning. We propose that atmospheric electricity adds key information to our understanding and predictive capability of the ecologically important mass migration patterns of arthropod fauna [].

A review of the possible causative factors and significance of ballooning in spiders.

Glick, P.A. (1939). The distribution of insects, spiders, and mites in the air. United States Department of Agriculture, technical bulletin no. 673. https://naldc.nal.usda.gov/download/CAT86200667/PDF .

Results and Discussion

9 Blackwall J. Mr Murray’s paper on the aerial spider. 10 −1) [ 4 Humphrey J.A.C. Fluid mechanic constraints on spider ballooning. 5 Reynolds A.M.

Bohan D.A.

Bell J.R. Ballooning dispersal in arthropod taxa: conditions at take-off. 11 Suter R.B. An aerial lottery: the physics of ballooning in a chaotic atmosphere. 12 Schneider J.M.

Roos J.

Lubin Y.

Henschel J.R. Dispersal of Stegodyphus dumicola (Araneae eresidae): they do balloon after all!. 13 Sparkes J.

Holland C. Analysis of the pressure requirements for silk spinning reveals a pultrusion dominated process. 10 14 Weyman G.S.

Sunderland K.D.

Jepson P.C. A review of the evolution and mechanisms of ballooning by spiders inhabiting arable farmland. 15 Greenstone M.H. Meteorological determinants of spider ballooning: the roles of thermals vs. the vertical windspeed gradient in becoming airborne. 16 Tolbert W.W. Aerial dispersal behavior of two orb weaving spiders. 17 Vugts H.F.

Van Wingerden W.K.R.E. Meteorological aspects of aeronautic behaviour of spiders. 18 Yeargan K.V. Factors influencing the aerial dispersal of spiders (Arachnida: Araneida). 14 Weyman G.S.

Sunderland K.D.

Jepson P.C. A review of the evolution and mechanisms of ballooning by spiders inhabiting arable farmland. 15 Greenstone M.H. Meteorological determinants of spider ballooning: the roles of thermals vs. the vertical windspeed gradient in becoming airborne. 19 Richter C.J.J. Aerial dispersal in relation to habitat in eight wolf spider species (Pardosa, Araneae, Lycosidae). 20 Bishop L. Meteorological aspects of spider ballooning. −1 [ 11 Suter R.B. An aerial lottery: the physics of ballooning in a chaotic atmosphere. 15 Greenstone M.H. Meteorological determinants of spider ballooning: the roles of thermals vs. the vertical windspeed gradient in becoming airborne. 17 Vugts H.F.

Van Wingerden W.K.R.E. Meteorological aspects of aeronautic behaviour of spiders. 19 Richter C.J.J. Aerial dispersal in relation to habitat in eight wolf spider species (Pardosa, Araneae, Lycosidae). 20 Bishop L. Meteorological aspects of spider ballooning. 21 Simonneau M.

Courtial C.

Pétillon J. Phenological and meteorological determinants of spider ballooning in an agricultural landscape. 12 Schneider J.M.

Roos J.

Lubin Y.

Henschel J.R. Dispersal of Stegodyphus dumicola (Araneae eresidae): they do balloon after all!. In the early 1800s, two competing hypotheses were proposed to explain how ballooning animals become airborne, invoking (1) the aerodynamic drag from wind acting on the silk or (2) atmospheric electrostatic forces []. Aware of the prevailing arguments, Charles Darwin mused over how thermals might provide the forces required for ballooning as he watched hundreds of spiders alight on the Beagle on a calm day out at sea []. Darwin’s observation, however, did not provide further evidence in support of either hypothesis. The physical force required for ballooning has since been attributed to aerodynamic drag at low wind speeds (<3 ms) [], yet the involvement of electrostatic forces in ballooning has never been tested. Several issues have emerged when models using aerodynamic drag alone are employed to explain ballooning dispersal. For example, many spiders balloon using multiple strands of silk that splay out in a fan-like shape. Instead of tangling and meandering in light air currents, each silk strand is kept separate, pointing to the action of a repelling electrostatic force []. Questions also arise as to how spiders are able to rapidly emit ballooning silk into the air with the low wind speeds observed in ballooning; the mechanics of silk production requires sufficient external forces to pull silk from spinnerets during spinning []. And, how do low wind speeds provide the high initial accelerations seen in ballooning takeoff []? Attempts to find weather patterns that predict the prevalence of ballooning have been made, but results remain inconsistent []. Mass ballooning events occur sporadically, and weather conditions on days with abundant aeronauts cannot be readily distinguished from days void of them. Although reports claim thermal air currents and temperature gradients on fair-weather days are the driving force [], ballooning can be observed when skies are overcast, as well as in rainy conditions ([] and E.L.M, unpublished data). Humidity is potentially an important predictor [], but causal and testable explanations are lacking. One consistent predictor of ballooning is wind speed; spiders only take flight when wind speed is below 3 ms], a very light breeze, but models show that these conditions should not allow large spiders to balloon, despite observation to the contrary [].

th century, atmospheric electricity was intensively studied, establishing the ubiquity of the atmospheric potential gradient (APG) [ 6 Wilson C.T.R. Atmospheric electricity. −1 ( 6 Wilson C.T.R. Atmospheric electricity. 22 Rycroft M.J.

Nicoll K.A.

Aplin K.L.

Harrison R.G. Recent advances in global electric circuit coupling between the space environment and the troposphere. 23 Bennett A. Measurement of atmospheric electricity during different meteorological conditions. PhD thesis. 24 Feynman R.P.

Leighton R.B.

Sands M.L. The Feynman Lectures on Physics: Mainly Electromagnetism and Matter. 25 Clarke D.

Morley E.

Robert D. The bee, the flower, and the electric field: electric ecology and aerial electroreception. 26 Borra J.-P. Étude de la formation des décharges électriques de pointe dans la nature: analyse des produits gazeux et particulaire en conditions controlées. PhD thesis. 25 Clarke D.

Morley E.

Robert D. The bee, the flower, and the electric field: electric ecology and aerial electroreception. 26 Borra J.-P. Étude de la formation des décharges électriques de pointe dans la nature: analyse des produits gazeux et particulaire en conditions controlées. PhD thesis. 27 Clarke D.

Whitney H.

Sutton G.

Robert D. Detection and learning of floral electric fields by bumblebees. −1), the electric field ∼10 m above the canopy of a 35-m-tall tree can exceed 2 kVm−1 ( 26 Borra J.-P. Étude de la formation des décharges électriques de pointe dans la nature: analyse des produits gazeux et particulaire en conditions controlées. PhD thesis. 28 Aubrecht L.

Stanek Z.

Koller J. Corona discharge on coniferous trees-spruce and pine. 29 Aubrecht L.

Koller J.

Stanek Z. Onset voltages of atmospheric corona discharges on plants. 30 Borra J.-P.

Roos R.

Renard D.

Lazar H.

Goldman A.

Goldman M. Electrical and chemical consequences of point discharges in a forest during a mist and a thunderstorm. 31 Bent R.B.

Collin H.L.

Hutchinson W.C.A.

Chalmers J.A. Space charges produced by point discharge from trees during a thunderstorm. Figure 1 Quantifying Electric Fields in Nature Show full caption (A) Atmospheric potential gradient (APG) measured for 30 min periods across 3 days using a field mill (Chillworth JCI131) at the University of Bristol School of Veterinary Sciences, Langford. Colors depict recordings from different days in June 2016. (B) Scale bar for (C) and (D). (C) Finite element analysis (FEA) model of electric field (e-field) enhancement around a geometrically domed oak tree in an APG strength of 4 kVm−1. (D) FEA model detailing the e-field around geometrically sharp tree branches in an APG strength of 4 kVm−1. (E) Two-dimensional plot of the e-field along cut lines (red; left inset) of (C) oak modeled as geometrically domed (solid) and (D) branches (dashed) in an APG of 4 kVm−1 (red) and 1 kVm−1 (black). Inset: detail of area indicated by the gray box. See also Figure S1 In the early 20century, atmospheric electricity was intensively studied, establishing the ubiquity of the atmospheric potential gradient (APG) []; from fair to stormy weather, an APG is always present, varying in strength and polarity with local meteorological conditions. Over a flat field on a day with clear skies, the APG is approximately 120 Vm Figure S1 ). In more unsettled meteorological conditions, charged clouds passing overhead modify the APG, with rainclouds, storm clouds, and mist or fog generating APGs of several kilovolts per meter [] ( Figure 1 A). Any electrically grounded, geometrically sharp structure protruding from this flat field will cause a substantial enhancement of local electric fields (e-fields) [] ( Figures 1 C and 1D). Fundamentally, this is why lightning rods work to channel a safe, predictable, path for lightning to reach ground. Because they are rooted in the earth and contain a high proportion of water and electrolytes, plants tend to equalize to ground potential [], and the electric field strength surrounding leaves and branches, due to their sharp geometry, can reach many kilovolts per meter [] ( Figures 1 B–1E). For example, in mildly unsettled weather (APG of 1 kVm), the electric field ∼10 m above the canopy of a 35-m-tall tree can exceed 2 kVm Figures 1 B–1E and S1 ). Closer to the tree, around sharp leaf, needle, and branch tips, e-fields easily reach tens of kilovolts per meter ( Figures 1 B–1E and S1 ). Local e-fields can become very high under observed atmospheric conditions; the potential difference between a grounded plant and the surrounding air is often high enough to initiate ion emission by corona discharge [].

27 Clarke D.

Whitney H.

Sutton G.

Robert D. Detection and learning of floral electric fields by bumblebees. 32 Greggers U.

Koch G.

Schmidt V.

Dürr A.

Floriou-Servou A.

Piepenbrock D.

Göpfert M.C.

Menzel R. Reception and learning of electric fields in bees. 33 Vollrath F.

Edmonds D. Consequences of electrical conductivity in an orb spider’s capture web. 7 Gorham, P.W. (2013). Ballooning spiders: the case for electrostatic flight. arXiv, arXiv:1309.4731v, http://arxiv.org/abs/1309.4731v. −1 control conditions, 1.25 kVm−1, or 6.25 kVm−1, encompassing APG values observed in overcast, misty, and stormy weather [ 23 Bennett A. Measurement of atmospheric electricity during different meteorological conditions. PhD thesis. 6 Wilson C.T.R. Atmospheric electricity. 25 Clarke D.

Morley E.

Robert D. The bee, the flower, and the electric field: electric ecology and aerial electroreception. 30 Borra J.-P.

Roos R.

Renard D.

Lazar H.

Goldman A.

Goldman M. Electrical and chemical consequences of point discharges in a forest during a mist and a thunderstorm. Figure 2 Spider Ballooning Behavior Show full caption (A) A spider showing a typical tiptoe stance. (B) Finite element model of the electric potential (left) and e-field (right) in the behavioral arena. The electric potential is the potential energy required to move a charge from one place to another without producing any acceleration: the amount of work per unit charge. It is a scalar quantity. The electric field is a vector quantity and a force that surrounds an electric charge. It exerts either an attractive or repelling force on other charges. The base is modeled as ground with 5,000 V applied to the top plate. A water moat surrounds the takeoff site to prevent spiders escaping over ground. The water was electrically floating, not connected to ground or a voltage. The scale bar shows electric potential (left) and e-field (right). Aside from small areas around the base of the arena, the e-field is fairly uniform with a strength of 6.25 kVm−1 (blue color indicated on the scale bar). (C and D) Boxplots showing the (C) number of dragline drops in response to 1.25 kVm−1, 6.25 kVm−1, and zero-voltage control and (D) the number of tiptoes in response to 1.25 kVm−1, 6.25 kVm−1 and zero-voltage control (D). Significance levels: ∗∗∗p < 0.001, ∗∗p < 0.01. See also Video S1 and Table S1 APGs and the e-fields surrounding all matter are relevant to biological systems; for example, bumblebees can detect e-fields arising between themselves and flowers [], and honeybees can use their charge to communicate within the hive []. But beyond bees, how widespread is the ability to detect and use electrostatic forces in terrestrial organisms? Spider silk has long been known as an effective electrical insulator; indeed, it was used in the first quantitative measurements of electrostatic charge by Michael Faraday and is positioned at the bottom of the triboelectric series, where it accumulates a net negative charge []. Previous theoretical considerations have proposed that when silk is charged, the APG can provide sufficient coulomb force to enable ballooning and aerial suspension using electrostatic forces alone []. Quite surprisingly, APG is rarely invoked, let alone quantified, in conventional weather descriptors and parameters collected by weather stations. As the APG plays a role in defining e-fields surrounding vegetation, it is reasonable to surmise that if e-fields are ecologically relevant, spiders should be able to detect and respond to an e-field by changing their behavior to engage in ballooning. Here, we presented adult Linyphiid spiders (Erigone) with e-fields quantitatively commensurate with atmospheric conditions. Spiders were placed on a vertical strip of cardboard in the center of a polycarbonate box, limiting air movement. This box also served as an APG simulator in the form of a parallel-plate capacitor. This entire setup was situated within an acoustic isolation and Faraday cage room (3 m × 2.8 m × 2.25 m). In their natural environment, ballooning spiders take off from protruding branches, leaves, or fences. We used a non-conductive, glue-free cardboard to construct a triboelectrically neutral takeoff site. This takeoff site generates a spatially uniform and moderate e-field within the arena ( Figure 2 A). Vertical e-field strengths across the arena were either 0 Vmcontrol conditions, 1.25 kVm, or 6.25 kVm, encompassing APG values observed in overcast, misty, and stormy weather [], as well as e-fields around grounded trees, grasses, and flowers [] ( Figure 1 ).

3 Weyman G.S. A review of the possible causative factors and significance of ballooning in spiders. 2 Bell J.R.

Bohan D.A.

Shaw E.M.

Weyman G.S. Ballooning dispersal using silk: world fauna, phylogenies, genetics and models. There are two behavioral proxies for ballooning in spiders: the upward extension of the opisthosoma and silk extrusion, referred to as tiptoeing ( Figure 2 A), and dropping on a silk dragline followed by extrusion of ballooning silk []. Although both behaviors allow spiders to become airborne, tiptoeing exclusively precedes ballooning and is an established predictor of ballooning propensity []. The occurrence of these behaviors was video recorded under the different experimental treatments and subsequent analysis scored blind.

−6; dragline drops ΔAIC between full and null model 28.1, AIC 310.5 versus 282.4, d.f. = 2, p < 10−6; −1 (Z = 2.95; p = 0.003) and 6.25 kVm−1 (Z = 4.87; p < 10−6), and there is a significant increase in the number of tiptoes at 6.25 kVm−1 (Z = 4.03; p < 10−6) ( Spiders show a significant increase in ballooning in the presence of e-fields (tiptoes ΔAIC [Akaike information criterion] between full and null model 42.1, AIC 153.1 versus 195.2, d.f. = 2, p < 10; dragline drops ΔAIC between full and null model 28.1, AIC 310.5 versus 282.4, d.f. = 2, p < 10 Figures 2 C and 2D). Significantly more dragline drops are elicited at 1.25 kVm(Z = 2.95; p = 0.003) and 6.25 kVm(Z = 4.87; p < 10), and there is a significant increase in the number of tiptoes at 6.25 kVm(Z = 4.03; p < 10) ( Table S1 ). The observed change in spider behavior establishes that they can detect APG-like e-fields. Moreover, the spider’s unlearned response to e-fields is to engage in ballooning, and, on becoming airborne, switching the e-field on and off results in the spider moving upward (on) or downward (off) ( Video S1 ).

eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiJlZWNlNWExN2I1NmQyNWVjNTA4NzI4YjEwNDRkMTQ4ZCIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjAxMTcxODQ3fQ.NPyECbntVpZBzVrCfaV51jJet2cxGplIYaitUwNZJqL6T3J4MzIdffZr8wA1wHJZ-O81echTG7KtjVGX95KlXxOnJ5hPtC6pe8_fR9eF4qfhtdngeQgcDOtZmau-bbtvqhBAu-RnzTiQqjajUEnwFAYkivJVTdlsbhbpH5UKaC9ES7fa0HxetXZmYliNPnjTv3TyYK-QQ2OcqEz8T-sECREud15OCg-1bnBDp7mr2vkFV9GdZrsYfTZqH-JXF0KvfaNin0aJdLVHt4nIjhAZ_HB3xJY4jrrOlTrTGrx75rz2YTP99b0QYRxW_sXC5qdUiejD7TQS0xqVP_F09fP_3w

34 Sutton G.P.

Clarke D.

Morley E.L.

Robert D. Mechanosensory hairs in bumblebees (Bombus terrestris) detect weak electric fields. 35 Barth F.G. Trichobothria - the measurement of air movement. 36 Reißland A.

Görner P. Trichobothria. 37 Humphrey J.A.C.

Barth F.G.

Reed M.

Spak A. The physics of arthropod medium-flow sensitive hairs: biological models for artificial sensors. 38 Shamble P.S.

Menda G.

Golden J.R.

Nitzany E.I.

Walden K.

Beatus T.

Elias D.O.

Cohen I.

Miles R.N.

Hoy R.R. Airborne acoustic perception by a jumping spider. 39 Reißland A.

Görner P. Mechanics of trichobothria in orb-weaving spiders (Agelenidae, Araneae). 39 Reißland A.

Görner P. Mechanics of trichobothria in orb-weaving spiders (Agelenidae, Araneae). 40 Hoffmann C. Bau und Funktion der Trichobothrien von Euscorpius carpathicus L. 41 Görner P. Mehrfach innervierte Mechanorezeptoren (Trichobothrien) bei Spinnen. Figure 3 Mechanical Displacement of Spider Trichobothria Show full caption Trichobothria in Erigone. (A) Diagram of a spider illustrating locations of metatarsal trichobothria and locations for non-contact laser Doppler vibrometry measurement (stars). (B) Scanning electron microscopy image of adult male Erigone metatarsi and trichobothria, with a close-up view of trichobothrium (inset). Arrows point to the base of trichobothrium. MT, metatarsus; T, tarsus. (C–H) Displacement of trichobothria in response to 0.5 ms−1 air flow (C and D), pseudo-DC efield (E and F), and 1 Hz sine e-field (G and H) measured using laser Doppler vibrometry (LDV). (C), (E), and (G) show single traces, and (D), (F), and (H) show the mean (black) and SD (gray). n = 6 (D), n = 5 (F), and n = 4 (H). Gray dashed lines indicate the stimulus. The behavioral experiments demonstrate that spiders can detect e-fields, but what is the sensory basis of spider e-field detection? In bumblebees, mechanosensory hairs are the putative electroreceptors sensitive to e-fields []. Arachnids have mechanosensory hairs known as trichobothria ( Figures 3 A and 3B ). Much is known about their mechanical and neural response to medium flows (air and water) []; they are exquisitely sensitive, detecting air motion close to thermal noise [], they detect sound [], and they are omnidirectional []. Early studies using electrostatic actuation as a tool to investigate trichobothria mechanics indicate that they may also be sensitive to e-fields [].

−1) and e-fields using laser Doppler vibrometry (LDV). Pseudo-direct current (DC) electrical stimuli with 0.1 Hz and 0.01 Hz square waves were used to simulate a static deflection and rapid change in e-field, as happens when charged clouds pass overhead (−1). No response above instrumentation noise (typically 2–10 pm) was elicited from spines ( Figure 4 Velocity of Trichobothria Motion in Response to E-Fields Show full caption (A) Transient changes in velocity of a trichobothrium (black, solid line) in response to a 2 kVm−11 e-field oscillating at 0.1 Hz (gray, dashed line). (B) Transient changes in velocity of a metatarsal spine (black, solid line) in response to a 3.6 kVm−11 e-field oscillating at 0.1 Hz (gray, dashed line). (C) Spike rate (as seen in A and B) of trichobothria (black; n = 8; ±SD) and metatarsal spine (gray; n = 4; ±SD) across a range of e-field strengths. Spike rate was measured as the ratio between the total number of zero crossing of the e-field stimulus to the number of spikes coincident (within 25 ms) of stimulus zero crossings. (D) Histogram (binned every 25 ms) of the number of velocity spikes of the trichobothria (black; n = 8) and metatarsal spines (white; n = 4) in response to a 0.1 Hz square wave. The dashed gray line shows stimulus recording. (E) Velocity of a trichobothrium (black, solid line) in response to an e-field oscillating at 1 Hz (gray, dashed line). (F) Frequency response (FFT) of trichobothria (black; n = 6; ±SD) in response to a 1 Hz sine wave e-field. We tested the mechanical response of trichobothria on the front metatarsus to both air flow (0.5 ms) and e-fields using laser Doppler vibrometry (LDV). Pseudo-direct current (DC) electrical stimuli with 0.1 Hz and 0.01 Hz square waves were used to simulate a static deflection and rapid change in e-field, as happens when charged clouds pass overhead ( Figure 1 ). Also, a 1 Hz sine wave was used to investigate the response to slowly changing e-fields. The response to air flow, a stimulus long established to deflect trichobothria, was also measured for comparison. Trichobothria are displaced in different ways by DC air flow and DC e-fields ( Figures 3 C–3F). In response to air flow, trichobothria are statically displaced for the duration of stimulus presentation, a tonic response. In contrast, displacement to e-fields is maximal at the transient switch in voltage, decreasing back to the baseline over a period of around 30 s, a phasi-tonic response. Here, the direction of trichobothria displacement is independent of stimulus polarity; both positive-to-negative and negative-to-positive stimulus transitions produce displacement in the same direction, a response indicative of induction charging where forces are always attractive regardless of stimulus polarity. Notably, the different types of mechanical response generated by air movement and e-fields suggest that wind and electric field detection can be differentiated despite sharing a common peripheral receptor. The trichobothrium is also displaced in response to a 1 Hz sine wave ( Figures 3 G and 3H), showing that they mechanically respond to slowly varying e-fields, as well as to rapid changes in potential. Here, the frequency response of the trichobothrium is twice that of the stimulus ( Figures 4 E and 4F ); each zero crossing of the stimulus generates a change in the direction of displacement of the trichobothrium, providing additional evidence of electrostatic induction. The response of trichobothria, measured as the number of times the velocity spikes ( Figure 4 A), scales linearly with e-field strength within the range measured (3.6–0.4 kVm). No response above instrumentation noise (typically 2–10 pm) was elicited from spines ( Figures 4 B and 4C). The measurement of tibial spines is a useful control allowing the exclusion of non-stimulus specific air motion, electrical crosstalk, or the motion of the entire animal as potential drivers of the responses measured from trichobothria. Hence, the trichobothria’s mechanical response can be considered to result from forces applied to them by the electric field. Such sensitivity to ambient e-field strength is compatible with the notion that spider trichobothria can work as electromechanical receptors. The neuroethology of trichobothria in response to e-fields needs further characterization, to add to the detailed knowledge of their response to medium flows.

This is the first demonstration of aerial electroreception in spiders and in arthropods beyond Apidae. The phylogenetic distance between spiders and bees indicates that aerial electroreception could be widespread among the Arthropoda. Consequently, the electromechanical sensitivity of hair structures present in bumblebees and spiders indicates a possible dual function, as medium flow sensors and electroreceptors. The hypothesis thus emerges that the mechanosensory hairs of many arthropod species may exhibit the additional function of aerial electroreception.

The present evidence shows that the APG and resulting electrostatic forces are sufficient to elicit ballooning, yet they may not always be necessary. Aerodynamic drag associated with light wind and electrostatic forces can work in synergy to facilitate ballooning. As a result of this work, we propose that the APG serves at least three functions: an indicator of meteorological conditions, an informational trigger, and a physical driving force enabling ballooning. Several mechanistic questions now emerge, pertaining to the dielectric characteristics of ballooning silk and whether altitude control and navigation take place. Future work needs to disentangle the complex interplay between animal behavior and variations in the APG. Inclusion of the APG as a meteorological parameter has the potential to provide better predictions of dispersal events and the distribution of spider populations.