Tools to evaluate the health of invertebrates, specifically in the Arachnida class, are extremely limited. These limitations are becoming more relevant as people seek care for their nontraditional pets. In the order Araneae, trauma, dysecdysis, hair loss, and prolonged anorexia appear to be the most common medical problems observed under human care.1 Recent methods on how to address trauma, amputation of limbs, hemolymph hemorrhage, and anesthetization of tarantulas and scorpions have been published, but healthy diagnostic indicators are relatively new to invertebrate medicine.1 Dysecdysis is a common reason for owners and keepers to seek out veterinary care, and unless closely monitored between periods of ecdysis, it can be difficult to determine where an animal is within their growth cycle. Hydration status is an important variable for an arthropod animal to successfully molt.2 Increased water intake beginning 2 months before ecdysis is the first indication that molting is underway.2 Some authors have reported treating dysecdysis with administration of intracardiac fluids when detected early; however, there is a low risk of laceration of the cuticle or causing fatal leakage of hemolymph.1 Keepers and owners have historically assessed hydration status based on body position, such as if the limbs are held retracted in toward the body in contrast to a somewhat extended position, and the animal’s ability to move normally as limb movement is dependent on hemolymph and hydraulics.3 Molting is a process that has many steps that can go awry, including asymmetrical molting, dysecdysis or partial ecdysis, deformed limbs, retained cuticle, and premature molting. Therefore, veterinary understanding and knowledge regarding where a tarantula is within their ecdysis cycle, or perhaps even if they are stuck in a molt, is invaluable. The arthrodial membranes lack an exocuticle and the ventral opisthosoma has a thinner exocuticle, making them potentially suitable locations for tonometry readings with less interference of chitin.4–6
In this study, we are utilizing the concept of tonometry to measure the tension of the cuticle or potential pressure inside the cuticle, in contrast to how the tool is traditionally used for its intended purpose with intraocular pressure (IOP). The objective of this study was to determine if tonometry can be used to assess tarantula estimated ventral opisthosoma pressures and if any detected change over time is consistent with the timing of an ecdysis cycle. If estimated ventral opisthosoma pressures prove to be consistent, tonometry may prove to be a valuable diagnostic indicator of health or disease in spiders and other arachnids, where antemortem diagnostics are currently limited.
Methods
This study protocol was not reviewed by an IACUC committee as the use of invertebrates in research does not require IACUC oversight.7
This study included 5 juvenile (yearling) female curlyhair tarantulas (Tliltocatl albopilosus) and 1 juvenile male curlyhair tarantula. These animals were sourced from a zoological collection and were captive born from the same clutch. Spiders were housed in individual 12 X 18 X 9-inch (30.5 cm X 46 cm X 23-cm) plastic cambros (Cambro Manufacturing; product ID: 12189CW) with holes drilled in the lids for ventilation.8 Within the containers were 2 to 3 inches (5 to 7.5 cm) of coco husk substrate, a black plastic hide, a small and shallow water dish, and a deeper dish filled with moistened paper towels. The containers were housed within a Darwin Environmental Chamber (Darwin Chambers; model #DB031-LT) measuring 33.5 X 29.5 X 80.75 inches (85 cm X 75 cm X 205 cm) at a constant temperature of 25.7 °C with humidity maintained via dishes of water placed within the chamber.9 Water dishes were filled with dechlorinated water daily. Animals were fed 1-half- to 3-quarter-inch prekilled banded crickets/Jamaican field crickets (Gryllus assimilis), increasing in amount accordingly as they grew. Additionally, when crickets were not available for feeding, giant mealworms (Zophobas morio) or Dubia roaches (Blaptica dubia) were substituted as prey items for that week. Pressures maintained consistency throughout any week when there was a change in diet.
An iCare TONOVET TV01 tonometer (Tonovet) set to “canine” species was used to measure the estimated ventral opisthosoma pressures of spiders.10 A measurement sequence includes 6 measurements, discards the highest and lowest value, and averages the remaining 4 values. Readings were only accepted if they were within the range of acceptable SD recommended by the manufacturer (≤3.5 mm Hg); these were indicated by no bar, low bar, or middle bar on the instrument. Pressures were evaluated once per week for the first 2 months, then once every 2 weeks for 1 month, then back to once per week for the remainder of the study. Additional measurements were obtained 2 days following each observed episode of ecdysis to ensure that the exoskeleton was strong enough to withstand handling without damage. Pressures were measured from the center of the ventral opisthosoma, the thinnest area of exocuticle throughout the body and providing the most consistent readings throughout the body.4,5 Three pressures were recorded on each occasion, and the median of the 3 was used for statistical analyses to reduce measurement variability.
The tonometer was held in an upright position at a 4-to-8-mm distance from the probe to the surface. In order to increase safety for the handler and the spider and to minimize the potential impact of direct physical restraint on pressure readings, a small restraint device was created from Hdpe Hardware Cloth Rolled Fencing with Mesh Size 3/4 inch X 1 inch (Tenax) and plastic zip ties.11
The restraint device, referred to as the “spider pocket” (Figure 1), allowed for safe and consistent maneuvering and restraining while obtaining tonometry readings (Figure 2). An Escali 125-g scale was used to monitor individuals’ weights each month.
A piece of plastic hardware cloth is folded in half and held together with plastic zip ties on 2 of the 3 sides to create a pocket that can be utilized for tarantula restraint. The author is using a paintbrush to gently coax the tarantula to back out of the pouch after measurements were taken.
Citation: American Journal of Veterinary Research 85, 11; 10.2460/ajvr.24.02.0053
A TonoVet tonometer is held upright with the probe perpendicular to the surface of the spider’s opisthosoma while the animal is held in the author’s “spider pocket” to minimize falsely altering pressures with manual restraint.
Citation: American Journal of Veterinary Research 85, 11; 10.2460/ajvr.24.02.0053
Two months into data collection, all tarantulas were showing areas of “alopecia.” Although spiders do not have hair, they do have setae, which are hair-like structures that cover their bodies and appendages.12 Observation of traumatic loss of these setae (not truly alopecia but consistent with this term as applied to mammals) from the dorsal abdomen can be common in some species of tarantula that are in premolt as well as an indication of sustained or chronic stress. This is due to their defensive behavior of rubbing the dorsal abdomen with hind legs to release the hair-like setae into the air. The structure of these setae allows them to become lodged in the tissue of a would-be predator, causing itching, pain, and discomfort. In some cases, regular or sustained disturbance of these animals under human care via interactions, vibrations, or even increased air currents can result in moderate-to-severe flicking of setae and “alopecia” of the dorsal abdomen. Possible stressors were evaluated, and it was found that the average humidity of the chamber was maintained at 40% to 42%. Based on general care references, it was suspected that this humidity was below the preferred range for the species and may be contributing to the suspected stress behavior.1 Two shallow dishes of tap water were added in the chamber outside of the individual enclosures to increase and maintain the humidity between 60% and 70% for the remainder of the study. In addition to increasing humidity, the frequency of tonometry readings was decreased to every 2 weeks during this time to attempt to minimize potential stress of handling until the behavior ceased. One month later, all spiders had molted and displayed new setae. There were no new indications of stress with no new areas of “alopecia,” the humidity remained constant between 60% and 70%, and the frequency of tonometry readings was returned to every week for the remainder of the study.
At the end of the 9-month period of measurements, each animal was anesthetized for hemolymph sampling with a 0.5-mL insulin syringe and a 28-gauge needle. Based on previous studies,13–16 the target volume to be safely sampled from each spider was 2% of each tarantula’s bodyweight. Each tarantula was weighed prior to sample collection, and the amount to be drawn was calculated for each animal. Due to the nature of drawing hemolymph from arachnids, there is often a small amount of hemolymph that leaks out before “hemostasis” is achieved. Manual pressure with a cotton-tipped applicator was immediately applied following removal of hemolymph.
Tarantulas were anesthetized in a mouse induction box (10 X 4.5 X 4.5 inches, volume of 202.5 cubic inches) with 5% isoflurane gas and oxygen flow at 2 L/min for 10 minutes.13–16 By 10 minutes, all tarantulas had lost righting reflex and response to stimuli. At 10 minutes, isoflurane was turned off and inflow was disconnected with active scavenging outflow maintained at 1.5 L/min for 45 seconds to evacuate the chamber with room air. After removing the tarantula from the chamber, the dorsal opisthosoma was prepped for hemolymph draw by gently wiping the exoskeleton surface with a sterile cotton-tipped applicator, causing a small area of setae to come off. Hemolymph was then drawn to obtain the precalculated amount of hemolymph. Pressure was applied with a cotton-tipped applicator to the injection site until “hemostasis” was achieved.17 The tarantulas were then weighed again, and the actual percentage taken was calculated from the difference in weight to determine accuracy (animal ID/%: A, 1.12%; B, 2.88%; C, 2.69%; D, 2.48%; E, 2.34%; F, 2.27%).
The hemolymph samples were transferred to a collection tube with no additives and immediately processed prior to clotting. The hemolymph centrifuged for 5 minutes before the supernatant was loaded into an avian/reptile rotor and analyzed by a VetScan VS2 (Zoetis) for chemical analysis.18,19 Sodium was over the upper limit of the VetScan for all samples. The remaining hemolymph sample was evaluated via iStat. All tarantulas began moving within 1 to 2 minutes of hemolymph sampling while in room air; gentle stimuli were provided with a paintbrush to encourage movement.1 Tarantulas were returned to their enclosures for recovery once they could fully right themselves when placed on their dorsum.
Estimated ventral opisthosoma pressures were taken before anesthesia, during anesthesia before hemolymph sampling, after hemolymph sampling, and following recovery from anesthesia. Estimated ventral opisthosoma pressures were additionally obtained 1 hour after recovery, 3 hours after recovery, and once per day for the following 5 days. This process was repeated twice after a 2-week washout period between each procedure but without drawing hemolymph to determine if anesthesia alone decreased pressures.
Statistical analysis
Statistical analyses were used to examine pressure changes during normal molt cycles as well as following post hemolymph draw and response to anesthesia. The estimated ventral opisthosoma pressure changes during molt cycles were examined using 2 different methods. The first method used a single predictor variable that classified each observation taken during the normal molt cycle as “before molt” if the measurement occurred prior to a molt and “after molt” if the measurement occurred after, or neither (describing the majority of observations). The outcome for this method was change in pressure from the previous observation. The purpose of this analysis was to determine whether changes from the previous time of measurement were different around the time of molt.
The second method used the numeric predictor variables “days before molt” and “days after molt” based on the 2 nearest molts to each observation. The outcome for this method was the concurrent tonometry pressure. The purpose of this analysis was to determine whether there are general trends in tonometry pressure as tarantulas move closer to and further away from their molts.
Both methods use marginal linear models with a continuous autoregressive structure; this allows for the measures closer together in time within the same tarantula to be more similar to one another, accounting for lack of independence of observations within the same tarantula.
The model for pressure changes following anesthesia used a predictor variable of time following anesthesia numerically. Trial (1, 2, or 3) was a second predictor variable since the tarantulas were anesthetized on 3 different occasions and the first involved a hemolymph draw, whereas the others did not. An interaction of trial and time was incorporated as a predictor as well to test whether the changes in pressure following the hemolymph draw were different from the anesthesia only trials. The outcome variable was concurrent tonometry pressure. The purpose of this model was to allow for identification of an overall increase or decrease in pressure as time passed following the anesthesia. This analysis used the same type of marginal linear model outlined in the previous analyses. The observations were again allowed to be correlated using a continuous autoregressive structure, where points that were closer together in time were assumed to be more correlated.
All statistical analyses were performed using the lme package in R, version 4.2.3.20
The second method used the variables “days before molt” and “days after molt” based on the 2 nearest molts. The outcomes for this method were the estimated ventral opisthosoma pressures themselves, which allows for determination of general trends in pressure as the tarantulas move closer to and further away from their molts.
Both methods use marginal linear models with a continuous autoregressive structure; this allows for the measures closer together in time within the same tarantula to be more similar to one another.
Results
Throughout the 9 months, estimated ventral opisthosoma pressures were evaluated once per week for the first 2 months, then once every 2 weeks for 1 month, then back to once per week for the remainder of the study. What was considered to be a premolt pressure was determined retroactively as the pressures that were taken weekly were prior to an animal molting. Each individual spider appeared to have their own “normal” pressures, with an average of all premolt pressures of 26 mm Hg (SD, 3.5). Pressures were measured additionally 2 days postecdysis, when animals were able to be handled without concern of injury due to exoskeleton fragility; there was a drop postmolt to an average of 15 mm Hg (SD, 3.8), followed by a gradual increase back to premolt pressures over a 3-week period, with an average of 22 days (SD, 1.93) (Figure 3). Each individual spider had a different number of days in between episodes of ecdysis. To visualize the pressures over a “standardized” molt cycle, the days between ecdyses were divided by the total number of days and then multiplied by 100 to obtain a percentage of the molt cycle (Figure 4). Therefore, in the visualization, the first molt occurred at 0, and the second molt occurred at 100 on the x axis.
A graph demonstrating the mean tonometry value of 6 curlyhair tarantulas (Tliltocatl albopilosus) over the total time of the study (9-month period). Molts are indicated by red stars.
Citation: American Journal of Veterinary Research 85, 11; 10.2460/ajvr.24.02.0053
A graph of the mean tonometry pressures of 6 curlyhair tarantulas (T albopilosus) as a percentage of molt cycle. Colors of the trend line correspond to the individual tarantula. The first molt occurred at 0%, and the second molt occurred at 100% on the x axis.
Citation: American Journal of Veterinary Research 85, 11; 10.2460/ajvr.24.02.0053
Out of the 6 tarantulas in the study, 1 animal had slightly higher pressures overall than the others, with a baseline average of 28 to 33 mm Hg and average postecdysis pressure of 17 to 18 mm Hg. Because pressures were only measured once per week, it is presumed that pressures steadily increase between readings. Pressures did not appear to correlate with increases in weight or body condition score, though this is subjective in arachnids. Animals that increased steadily in weight and/or Body Condition Score (BCS) due to growth did not have an increase in baseline pressures.
While anesthetized, pressures decreased from each tarantula’s baseline. When hemolymph was collected from anesthetized spiders, pressures decreased at induction and then decreased further following the removal of hemolymph. These pressures gradually returned to baseline within 24 hours postprocedure. When this process was repeated with just anesthesia, pressures appeared to go up following induction and then return to baseline.
Normal molt cycles
The results of the model examining whether the type of measure (before molt, after molt, or between molts) are given in Table 1 (n = 203).
Results of pressure change model by type of measure.
| Variable | Coefficient | SE | t | DF | P values |
|---|---|---|---|---|---|
| Intercept | 0.812 | 0.325 | 2.50 | 200 | .013 |
| Premolt (between molts) | 0.370 | 1.359 | 0.27 | 200 | .786 |
| Postmolt (between molts) | −13.176 | 1.359 | −9.70 | 200 | < .001 |
DF = Degree of freedom.
To interpret the results of Table 1, note that “between molts” is the reference level for the type of measure. The intercept coefficient therefore represents the average change in pressure for a measure that is neither before nor after molt. This average pressure change is positive and is 0.8 (P = .013). Although the coefficient is positive, a measure before molt is not statistically significantly different from “between molts” (P = .786), meaning that there is no evidence that a measure prior to a molt is any different from any other measure at any other time. However, for a measure that is after a molt, the coefficient of −13.2 means that this change in pressure, on average, is less than one that is neither before nor after molt. Subtracting the intercept, the average change after molt from the previous observation is −12.4. This drop in estimated ventral opisthosoma pressure is significantly different (P < .001) from the slight increase in pressure experienced during a between molt observation. Therefore, this model provides evidence that the drop in pressure following the molt is a significantly different occurrence from the remainder of the molt cycle.
The results of the model comparing time before and after molt to pressure readings are given in Table 2. Here there are n = 133 observations since days before molt and days after molt could only be calculated for instances where both the previous molt and following molt occurred on known days. Additionally, 2 observations that occurred on the day of molt were removed due to the uncertainty of how to define “days before molt” on these days.
Results of pressure model by days before and after molt.
| Variable | Coefficient | SE | t | DF | P values |
|---|---|---|---|---|---|
| Intercept | 20.862 | 1.682 | 12.40 | 130 | < .001 |
| Days before molt | 0.014 | 0.013 | 1.03 | 130 | .304 |
| Days after molt | 0.052 | 0.013 | 3.89 | 130 | < .001 |
From Table 2, days before molt is not statistically significant; days after molt is statistically significant with P < .001. The coefficient for days after molt is positive, meaning that on average, pressure increases each day following a molt.
While both models are likely a simplification of the true nature of the relationship of the molt cycle to pressure readings, both models indicate that the estimated ventral opisthosoma pressure decreases by a relatively large amount following a molt and then increases on some basis in the time following each molt.
Estimated ventral opisthosoma pressure changes following anesthesia
The overall results of the model demonstrated that while trial was highly statistically significant (χ2[2] = 31.76, P < .001), there was not a statistically significant interaction of time and trial (χ2[2] = 4.68, P = .096), and time did not have a statistically significant effect at the 0.05 level (χ2[1] = 2.97, P = .085), but there is not sufficient evidence that pressure changed over time following the anesthesia with or without the removal of hemolymph.
Note, a second version of this model that treated the timepoints categorically was also analyzed, in case the change following anesthesia was not linear, and this model also resulted in no statistically significant effect of time.
Discussion
Foelix provides a comprehensive overview of spider anatomy and physiology.12 In spiders, the molt into the adult stage is the final molt in males, whereas females molt approximately once per year following sexual maturation.21 Juvenile developmental life stages between molts are separated into instars.22 Young tarantulas grow rapidly; they may molt once every 2 to 3 months under typical captive care.23 The periods of time between molts are influenced by many external factors, including temperature, humidity, hormones, and nutrition.24 A new cuticle is synthesized in a folded state under the old cuticle, and exuvial liquid increases within the intercuticular space, lubricating the new cuticle from the old. The increase in pressure eventually results in splitting the old cuticle, resulting in a full molt of the exoskeleton. After the old exoskeleton is split and discarded, the tarantula is soft and vulnerable for a period of several hours to weeks. The new cuticle only reaches 50% of its strength by 24 hours after ecdysis and full strength at 16 to 20 days.2
Rebound tonometry was developed to be a routine diagnostic test for measuring the IOP of mammals during an ophthalmological examination.25,26 The only other use of tonometry that has been reported in the literature for nonocular tissue is digital tonometry, being utilized in vivo in humans during brain surgery to determine brain stiffness to identify the boundaries of dysplastic tissue.27 With rebound tonometry, a lightweight magnetized steel wire shaft covered with a round plastic probe is launched toward the cornea and bounces back. The moving magnet induces voltage into the solenoid, and the motion of the probe is analyzed.28 Deceleration and the contact time of the probe change as a function of IOP.
This data demonstrates that estimated ventral opisthosoma pressures of tarantulas change throughout an ecdysis cycle with predictability. Pressures of all animals in the study decreased postmolt and gradually increased back to the pressure that the animal was at before molting within 3 weeks. One animal had slightly higher pressures overall than the others, with a baseline average of 28 to 33 mm Hg and average postecdysis pressure of 17 to 18 mm Hg.
When anesthetized the first time, pressures decreased after induction of anesthesia and decreased further post hemolymph draw. Pressures gradually increased over 3 days back to the animals’ normal, and 1 tarantula even molted 1 day postprocedure. When anesthetized the second time to see if anesthesia truly decreased pressures, 2 out of 6 spiders increased pressures, 2 spiders decreased pressure, and the other 2 spiders showed no significant increase or decrease in pressures. When anesthetized for the third time to confirm results, all 6 spiders increased pressures, though these increases were not statistically significant.
The decrease in estimated ventral opisthosoma pressures that occur after ecdysis could be explained by the exoskeleton rigidity. It is known that the exoskeleton is soft and prone to injury following a molt, and it is commonly recommended that freshly molted spiders are not handled, disturbed, or fed for 2 to 3 weeks to allow the exoskeleton to harden first. This would also explain the time of 20 to 25 days it takes for tonometry pressures to gradually increase back to what they were before the molt. Another possible explanation for pressures decreasing postmolt would be sequestration of hemolymph to separate the old exoskeleton from the new exoskeleton. Sequestration of hemolymph to facilitate the molt process results in systemic dehydration. The spider then slowly rehydrates, and pressures increase back to normal. It was hypothesized that pressures would decrease following anesthetic induction. There was a decrease in all estimated ventral opisthosoma pressures after first anesthetic induction and then a further decrease post hemolymph draw. This prompted an additional procedure of solely anesthetizing the tarantulas and monitoring pressures throughout the induction and recovery phases without taking any hemolymph. When evacuating the chamber during the second procedure, there was a slight difference in protocol: oxygen flow was maintained at 2 L/min instead of disconnecting inflow and allowing evacuation on room air. The increase in pressures following induction could be attributed to oxygen flow decreasing the concentration of isoflurane in the animal faster than when left on room air. This could mean that they began waking up faster than when on room air, leading to more rigidity of the exoskeleton and redirecting of hemolymph to the abdomen to aid in recovery. Another possibility is that theraphosids respond to anoxia differently than what is observed in mammalian species. Each tarantula responded slightly differently throughout anesthetic induction, losing righting reflexes at different time points throughout the 10 minutes. All tarantulas appeared to go through an “excitatory phase” characterized by pulling all their legs in tightly toward the prosoma before relaxing completely and losing righting reflexes. Similar findings of an excitatory phase in tarantulas undergoing gas anesthesia have been observed by the authors previously. It is possible that the excitatory phase resulted in hemolymph sequestration that affected the readings. When the procedure was performed a third time, an increase in pressures postinduction was also observed. The increases in estimated ventral opisthosoma pressures may be an indicator that the circulatory system is compensating to the anesthesia and that the decrease in “pressure” readings is a response to reduced hemolymph volume, similar to reduced blood pressure when blood volume is reduced. Overall, these changes were not statistically significant with their unpredictable nature. It is not clear why there was such variability in results when assessing response to anesthesia, and this requires further investigation.
It was hypothesized that if the pressure being measured is analogous to blood pressure in mammalian species, there would be variation in the pressures when comparing before and after hemolymph removal. This decrease was observed in all individuals post hemolymph draw. Historically, spiders have been reported to have an “open circulatory system”; however, there is increasingly more literature that demonstrates features that are suggestive of a closed or semiclosed circulatory system with a distinct heart, branching “arteries,” and vasculature, with evidence histologically and through MRI contrast imaging.29,30 This information, in combination with the decrease observed in estimated ventral opisthosoma pressures post hemolymph draw and both increases and decreases in pressure in response to anesthesia, raises the question of whether or not arachnid circulatory systems behave more like vertebrate species than originally thought.
Major limitations of this study include the age of the animals, location of the reading, the use of tonometry on nonocular tissue, the absence of manometry, and utilizing “bar middle” values reported from the tonometer. Subadult/juvenile animals were selected rather than adults so that multiple ecdysis cycles would be anticipated over the 9-month period of the study. Due to tarantulas’ long lifespans, it is more likely that this technique would be applied to mature adult spiders in a clinical setting. This is a limitation that this data does not represent adult spiders at maturity over time. Another factor to be considered is the cuticle thickness and potential variability between individuals and species. Though both the ventral opisthosoma has thinner exocuticle and the arthrodial membranes lack an exocuticle, it was decided to take the readings on the ventral opisthosoma due to the inconsistent readings and error messages obtained when attempting to take readings of the arthrodial membranes. This is a limitation since arthrodial membranes would be the most ideal place to obtain a pressure without chitin interference but is impractical unless utilizing a larger species with greater arthrodial membrane surface area. The biggest limitation is the fact that tonometers are designed to account for the type of tissue it is measuring pressure inside of, and the iCare TONOVET being used is meant to be used on canine and feline globes. The tissue of the exoskeleton directly affects the readings that are reported, meaning the pressures are currently not representative of hemolymph pressure alone, hence why throughout the study it is referred to as the “estimated ventral opisthosoma pressure.” This is in addition to rebound tonometry being known to be significantly influenced by corneal thickness and other biomechanical factors.31 In human ophthalmology, meta-analysis confirms that low central corneal thickness values can result in low tonometry readings, and high central corneal thickness values can result in elevated tonometry readings.32 The thickness of the exoskeleton could vary from individual to individual as well as across species. To evaluate this aspect while taking the exoskeleton cuticle into account, manometry would need to be performed on multiple deceased tarantulas to develop a series of pressures to be able to calculate a regression equation and a conversion table to correlate the estimated pressure with the actual pressure. Additionally, there is likely more advanced physiology occurring that is not yet understood. Perhaps pressures increase as hemolymph is shifted to more vital organs while at the same time the volume in the abdomen was reduced, thereby decreasing estimated ventral opisthosoma pressures. When a tonometry gives “bar middle” on the display, it indicates that deviation is greater than normal, but the effect on the result is usually not relevant (deviation of > 3.5, > 2.5 mm Hg).33 New measurements are only recommended if the IOP if higher than a normal IOP, which is irrelevant in this study due to the novel use. If the study were to be repeated, “no bar” (< 1.8 mm Hg deviation) or “bar down” (< 2.5, > 1.8 mm Hg deviation) readings would be ideal to obtain the most accurate readings. However, throughout this study, “bar middle” readings were deemed acceptable due to difficulty obtaining “no bar” and “bar down” readings.
Though the sample size in this study of tarantulas was limited to 6 individuals, pressures across the group were consistent with similar values before and after ecdysis. Over 9 months, as the spiders progressed toward adult maturity, ecdysis happened less frequently, and authors observed a consistent plateau in pressures, with only minor fluctuations up and down (3 to 5 mm Hg). There was no increase in pressure pre-ecdysis; this was originally hypothesized due to the necessity of fluid buildup between the old exoskeleton and the new exoskeleton to successfully molt. The most consistent pattern observed was a decrease in pressure 2 days postecdysis that slowly returned to normal over the period of 3 weeks. Though, this could simply be due to the exocuticle hardening over this period of time, decreasing the elasticity of the exoskeleton. Pressures also returned to normal baseline for all tarantulas within 1 to 2 days following anesthesia and hemolymph draw. This could possibly indicate that tarantulas have control over sequestering hemolymph to where it is needed most. The total volume of hemolymph drawn from each spider in this study was based on previously published data from other studies.13–16 In these other published studies, authors cite that they calculated safe volumes for hemolymph removal from extrapolations based on standards used in other veterinary species. However, with a quick recovery following hemolymph collection, perhaps more than this amount could be taken without detriment to the spider.
The goal of the current study was to investigate if tonometry could be used to gauge normal estimated ventral opisthosoma pressures in tarantulas or as a diagnostic indicator, which are often limited in arachnids to drawing hemolymph and necropsy. This study demonstrates that tonometry can be used to generally assess the estimated ventral opisthosoma pressure, which could correlate with where a spider is in an ecdysis cycle. Further research needs to be performed, including attempting manometry on deceased arachnids to determine the true value of hemolymph pressure without exoskeleton interference. Additional future projects include assessing pressure values when a female is gravid, seeing if values change based on the type of prey fed or quantity of prey fed, and attempt assessing pressures among other arachnid species.
Acknowledgments
The authors thank Larry S. Christian for assistance with animal husbandry, organization, and logistical planning that greatly improved the study.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
Material support was obtained from the North Carolina Museum of Natural Sciences, Veterinary Science Department.
The authors thank the Robert J. Koller Endowment for financial support.
ORCID
M. V. Chung https://orcid.org/0009-0009-1319-7088
G. A. Lewbart https://orcid.org/0000-0003-0716-1387
H. D. Westermeyer https://orcid.org/0000-0003-4463-1245
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