Answer Key for Scorpions as Seismologists Laboratory for Introduction to Marine Science 2007 Prof. Laura Wetzel — Eckerd College Background

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Answer Key for Scorpions as Seismologists
Laboratory for Introduction to Marine Science 2007

Prof. Laura Wetzel — Eckerd College
All movement produces waves. Rain drops falling on a puddle form ripples. Earthquakes moving the seafloor create tsunamis. Plucking a guitar string makes music. Spiders crawling across the desert shake the sand. Waves from all of these sources—rain drops, earthquakes, guitars, and spiders—can be detected. Our ears hear music. Seismographs detect earthquakes. Scorpions feel the footfalls of spiders. In this lab you will investigate how scorpions use waves to locate sources of movement.


  • To show how scorpions and seismologists use similar methods to locate wave sources.

  • To create graphs relating distance and time (travel-time curves).

  • To use the constructed time-travel graphs to locate seismic sources.

  • To develop the concepts of forward and inverse problems.

Desert-dwelling scorpions hunt at night and have poor eyesight, so how do they catch prey? When Paruroctonus mesaensis, a nocturnal sand scorpion living in the Mojave Desert, detects a disturbance, it abruptly turns and moves forward quickly with its pincers at the ready (Fig. 1). It accurately judges both distances and angles—in front, behind, or to the side. The scorpion does not see, hear, or smell distant prey; it feels the tiny waves produced by walking and burrowing animals.

Figure 1. Desert scorpion. (Illustration from

The sand scorpion hunts by sitting and waiting until it detects movement. Fine hairs on its feet feel compressional primary (P) waves and a special sensor on its ankle called the slit sensillum detects surface Rayleigh waves. The P wave is faster and vibrates sand back and forth. The Rayleigh wave is slower and moves the sand up and down in an elliptical pattern similar to an ocean wave. In this exercise, you will think like a scorpion and use information about the P and Rayleigh waves to determine the location of a spider.

Forward Problem
First, consider waves produced by walking across the desert. The eight legs of a scorpion form a nearly circular array of sensors four to six centimeters across. They can detect movement as far as 30 cm away and accurately locate prey that makes waves within 15 cm. Follow these steps to show how the waves from the spider fan out in a circular pattern like ripples from a stone dropped in a pond:

  1. Place the point of the drawing compass at the center of the spider in Figure 2 and extend the pencil on the compass to the center of one of the scorpion legs. As shown for the closest leg, draw an arc to represent a wave produced by a walking spider. Repeat this process for all of the legs to create eight arcs centered on the spider.

  2. Each of the scorpion’s legs detects the spider at a slightly different time. Number the legs in order, (1) for the first and (8) for the last leg that detects the spider.

Figure 2. An overhead view of a spider and the legs of a scorpion shown at actual size. As shown for the first leg, use a compass to draw arcs, like ripples on a pond, through each of the scorpion’s legs. Place the metal tip of the compass on the spider and extend the pencil to the center of each of the scorpion’s legs. Number the legs in order, (1) for the leg closest to the spider and (8) for the leg furthest away.

  1. Use a ruler to measure the distances in cm from the center of the spider to the centers of each of the scorpion’s legs; record your results in Table 1. Measure distances to the closest tenth of a centimeter (e.g., 7.2 cm). The correct answers for some legs are provided for you to check your results.

Table 1. Scorpion to spider distances and wave travel times. Leg 1 is closest to the spider and Leg 8 is furthest away.

Leg Number

Leg 1

Leg 2

Leg 3

Leg 4

Leg 5

Leg 6

Leg 7

Leg 8

Distance (cm)









Slow Wave Time (ms)

Vslow = 5 cm/ms









Fast Wave Time (ms)

Vfast = 12 cm/ms









  1. When the spider moves, how long does it take for the scorpion to feel it? Loose sand conducts two wave types particularly well, P waves that travel at 95 to 120 m/s and Rayleigh waves that travel at 40 to 50 m/s. Use the maximum velocities of 120 m/s and 50 m/s to calculate the fast and slow wave times in milliseconds and record your results in Table 1. Record all calculations to three significant figures.

Slow Wave Velocity: 50 meters/second = 5,000 cm/s = 5 cm/millisecond (cm/ms)

Fast Wave Velocity: 120 meters/second = 12,000 cm/s = 12 cm/millisecond (cm/ms)

  1. On the graph shown on the next page (Fig. 4), plot the slow travel times using a colored pencil. To save time, plot the data for the odd legs only (1, 3, 5, and 7).

  2. Fill in the appropriate box in the legend.

  3. Use a ruler to draw a straight line through the data starting from the origin. (These data are perfect, so all of your points should be on the line. If not, check your measurements and calculations.)

  4. Repeat steps (5) through (7) with another color for the fast travel times from Table 1.

Figure 3. Scorpion. (Illustration from

Figure 4. On this travel-time graph, plot your data for slow and fast waves traveling through sand to the odd numbered legs of the scorpion (Table 1).

  1. Use your data from the odd legs in Table 1 to calculate the slopes. Recall that the slope is Y/X (rise/run). The slopes of the Slow and Fast lines should equal the inverse of the velocities. Record your answers using three significant figures. Show your work below:

Slow Line Slope:

To check your answer, calculate the velocity (VSlow). Does it match the slow velocity of 5 cm/ms used in Table 1? If not, check your work.


Fast Line Slope:

To check your answer, calculate the velocity(VFast). Does it match the fast velocity of 12 cm/ms used in Table 1? If not, check your work.


Inverse Problem
In the first case, you knew the spider’s location relative to the scorpion. Now consider the case where the scorpion is trying to find the spider. The scorpion feels the fast and slow waves with its feet. It does not know when or where the spider moved. All it knows is that each foot felt two waves arrive. In other words, the scorpion knows the Slow minus Fast time for each foot (Table 2). To find the spider, complete Table 2 by using the method outlined on the next page.
Table 2. Scorpion to spider distances and wave travel times. Note that the scorpion’s legs are designated from head (A) to tail (D), not in order of when they felt the waves.


Left Leg A

Left Leg B

Left Leg C

Left Leg D

Right Leg AA

Right Leg BB

Right Leg CC

Right Leg DD

Slow – Fast Time (ms)









Actual Distance (cm)









  1. Mark the Slow-Fast time for Leg A from Table 2 on the Y-axis in Figure 4.

  2. Place a piece of paper or an index card along the Y-axis. Mark the length from the origin to your average time mark on the Y-axis.

  3. Find the location where the vertical distance between the curves is equal to the distance you marked on the card. The easiest way to do this is to drag the card along the curves starting at the origin. Keep the vertical edge of the card parallel to the Y-axis.





Slow - Fast


Mark this length on a card.

This is the distance from the scorpion’s leg to the spider.



  1. When you match the length on the card with the gap between curves, draw a vertical line down to the X-axis. The point on the X-axis represents the distance from the scorpion’s leg to the spider. To ensure you have done this properly, compare your value to the following list:

5.3 6.0 6.5 7.4 8.2 9.3 10.4 11.5 12.7 13.1

If your value is NOT in this list, then you have made a mistake and you must reconsider your results and repeat steps (1) through (4). When you are confident of your value, record it as the “Actual Distance” in Table 2 for Leg A.

  1. Repeat steps (1) through (4) for Leg C, Leg AA, and Leg CC.

Somehow the scorpion analyzes all of this information to find its prey!

Follow these instructions to plot the spider’s location using triangulation.

  1. Where is the spider? Use the compass to measure the four distances you found for A, C, AA, and CC in Table 2 and draw them on Figure 5. Place the point of the drawing compass at a scorpion’s leg and draw a circle with the corresponding distance. For example, place the metal point of the compass at Left Leg A and draw a circle using the distance from Table 2 as the radius. If the radius is too big for the paper, draw as much of the circle as you can on the paper. Repeat this process to create four circles centered on four legs of the scorpion. These circles should intersect at one point—the spider! If the circles do not intersect at one point, then you have made a mistake in one or more of your calculations or you have not drawn the circles accurately with the compass. Erase and redo as necessary.

  2. Draw a square or your best sketch of a spider at the spider’s location.

  3. Name your spider and scorpion and place these labels next to their locations on Figure 5. Be sure to put your own name and the date on the front page of this exercise before turning it in.

Figure 5. An overhead view of the legs of a scorpion drawn at actual size. Use the Slow minus Fast Times in Table 2 to find the spider. Place the metal point of the compass on the appropriate leg and then draw semi-circles to find the spider. Although the scorpion will use information from all eight legs, you may draw circles from just four, legs A, C, AA, and CC, to find the spider.

Closing Remarks
Earthquakes and spiders create waves that travel through air, sand, and Earth. Perhaps this lab will motivate you to learn more about the relationships between physics, biology, and geology that allow seismologists and scorpions to locate their prey.

The information about scorpions is primarily from Prey Detection by the Sand Scorpion by P. H. Brownell, Scientific American (1984) vol. 251, issue 6, pages 86-97 and Compressional and Surface Waves in Sand: Used by Desert Scorpions to Locate Prey by P. H. Brownell, Science (1977) vol. 197, pages 479-482.

The arrays shown in Figures 1 and 5 are modified from How the Sand Scorpion Locates Its Prey by P. H. Brownell and J. L. van Hemmen (2000) from which was accessed 22 July 2004.

Last modified by LRW on June 7, 2007 at

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