Phalangium opilio’s ability to overcome obstacles without changing from it’s tripod gait

Betty J. RossieDepartment of Biology, McDaniel College, Westminster, Maryland, 21157

Introduction Objectives

Methods

Test Screens

Results

Conclusion

Literature cited

Acknowledgements

• Based on observations and video review the speed of locomotion versus the angulation of the screens, shows that the preferred direction for escape is in the direct vertical at 90º from the countertop. The 45º angled screen was not the preferred angulation based on the speed of locomotion up the screen and therefore is not the perferred angle for escape.• This is explained as Phalangium opilio would normally encounter surfaces of 0º or 90º in the wild habitats containing mostly trees, shrubs and fairly level substrate.

• The tripod stance is not forfeit when manuvering around obstacles, as there was only a slight decrease in speed. Phalangium opilio did not show a difference between the speed of locomotion across the uncut screen and the screen with a single series of cuts and the screen with multiple series of cuts.•Phalangium opilio continued to utilize the second set of limbs for sensing and not for walking over all surfaces. Only at rest are those limbs used as part of stabilizing and weight balance.

•There were some issues with the methods of my trials that could have been more precise with better equipment, but the best try was performed.

Terrestrial LocomotionSimilar to insects with 6 legs, Phalangium opilio, as with all species of Opliones, utilizes an alternating sets of tripods, legs L1, L4, R3 and L3, R4, R1. The second pair of limbs are most commonly used as tactile organs and so are not included in the tripodal stance, however, if needed for extra support these limbs area used (Sensing, 2006).

Figure 7. Multi-cut screen at 45º angle from flat countertop

Figure 6. Screen at 90º angle from flat countertop

Figure 5. Multi-cut screen 44.45cm x 22.86 cm

1. Observe the locomotion of the harvestmen (Phalangium opilio) when maneuvering over unnatural surfaces premade screens. The comparison of the speed with which the specimen moves across the area will determine which is the preferred angulation and if there is a change when holes are cut in the screen making more obstacles for the specimen to overcome.

2. Determine, by physical trials, the ability of the harvestmen to maneuver over obstacles without changes to normal hexapodal locomotion with tripod gait when encountering holes in the unnatural surfaces.

•It was determined that the ideal direction would be for the specimen to travel the quickest up the screen placed at a 90º from the countertop. Thereby, making the screen placed at a 45º angle from the countertop more difficult to overcome. All of the specimens were approximately the same size and weight

•In order to determine if there is a change in the locomotion when encountering obstacles, such as different surface angles and uneven surfaces, I began with screens made from balsa wood and 18 square per inch window screening.

•In order to perform laboratory trials, I had to first locate and collect specimens. After spending many evenings in the woods of Hashawa Nature Center, I encountered and collected a variety of species. •All were local native species such as Leiobunum vittatum (1), Leiobunum politum (2), Leiobunum serratipalpe (3), Leiobunum verrucosum (4), Hadrobunus grandis (5). The easiest to catch and maintain in the laboratory were Phalangium opilio (6) collected in the yard of my home in Finksburg, MD. Because of that, I decided to use Phalangium opilio for my trials.

I would not have been able to perform this research if not for the following: Dr. Brett McMillan – McDaniel College – my advisor

Dr. Jeffery Shultz – University of Maryland College ParkHashawa Nature Center – Westminster, MDAdam Hudson – McDaniel College – Exel guru and lab mateNick Minkowski – my laboratory assistant and sonJohn Rossie – my understanding husband and arachnophobic

•Opiliones are a seldom studied group of terrestrial Arachnids that occur in all areas of the planet. They are a varied in appearance, but their main features are recognizable to most cultures. There are over 3,200 species (and counting), 200 of which are native to the United States. Unlike a spider, a harvestman has long, stiltlike legs and a segmented abdomen, and lacks silk glands.

• The legs each have seven joints, making them highly flexible and allowing the animal to run rapidly over leaves and grass. Harvestmen feed on small insects, decayed animal matter, and the juices of fruits and vegetables.

• A pair of glands in the carapace secrete an unpleasant-smelling substance that deters predators. The female has an ovipositor, a specialized tubular organ that is extended from the abdomen to deposit eggs in damp soil. Harvestmen do not bite, so they are harmless to people. Some can, however, give off a bad odor if they are disturbed (Pinto-da-Rocha, 2007).

Normal harvestman behavior is cryptic and often their most of their actions occur during the night are not always easily observed. When escaping in nature the ideal movement was observed to be horizontal running over substrate and around tree trunks or as simple as moving from the surface of the leaf to the underside of the leaf (personal field observation). In the lab however, the escape maneuver was flat out running to escape, regardless of direction.

One of the most common opilionid encountered in human areas are Phalangium opilio.

Thought to be introduced in Maryland they are not currently considered invasive, but have been observed acting cannibalistic to their own species and other species.

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•Visual observations were made at the time of the trials as well as the review of the videos recorded using a Nikon CoolPix L18 digital camera with the help of a tripod. The videos were reviewed using Windows Media Player, frame by frame to observe any change of gait as well as timed at full speed. The timing was done on the continuous movement of the specimen with no further prompting other than the initial.

•All specimens were housed in prepared terrariums and small critter cages. The terrariums were prepared by starting with a base of small rocks, a layer of sand, and a layer of dirt. The critter cages are made of clear acrylic with vented lid.

Figure 8. Speed of specimens walking over screen at a 90º and 45º angles from the countertop. n=15; 9 males, 6 females.

What Looks Like a Spider but Isnt? You may know them as harvestmen, daddy longlegs, or by their proper name Opiliones. A member of the order Arachnida, they resemble members of the family Araneae – Spiders.

Figure 1. Phylogenetic tree for order Arachnida (Shultz, 1990)

Figure 2. Phalangium opilio adult male, tripod L1, L4, R3 stance indicated Figure 3. Phalangium opilio adult female,

tripod R1, R4, L3 stance indicated

Figure 4. Hand collected specimens

The specimens traveled over the screens an average of 4.2 cm/sec up the screen set at a 90 angle. The average speed of the specimens traveling over the screen set at a 45 angle was almost the same for the screen with no holes and the screen with holes at 5.3 cm/sec and 5.2 cm/sec respectively.

•All specimens were hand collected using a small fish net and a plastic 20 oz. soda bottle with the top cut off at the neck.

Further Research: It would be interesting to determine if there is a difference between species and whether or not there is a difference between the length of the limbs and total body weight. Also a closer look at the differences between the age of the specimens.

Pinto-da-Rocha, R., Machado, G., & Giribet, G. eds.,2007. The Biology of Opiliones. Harvard University Press, Cambridge, Massachusetts:chpts 1,7.

Sensing, A.T. and Shultz, J.W., 2006 (2007). Mechanical energy oscillations during locomotion in harvestmen, Leiobunum vittatum (Opiliones). Jrl of Arachnology 34:627-633.

Shultz, J.W. 1990. Evolutionary morphology and phylogeny of Arachnida. Cladistitcs 6: 1-38.