The occurrence points for Chilobrachys species across Asia were sourced from the Global Biodiversity Information Facility (GBIF) database (GBIF, 2024), various social media platforms, and direct personal field observations. Initially, the dataset comprised 940 individual occurrence points. To minimize geographical sampling bias, a spatial thinning technique was applied, utilizing a 1.5-km × 1.5-km grid in accordance with the methodology outlined by Mukherjee and Mondal (2023). This process was conducted using the spThin package in R version 4.2.2. Additionally, points located over marine environments or those lacking essential environmental data were manually removed. Further exclusion of points with missing environmental variables was carried out using the “suppress similar visual warning” option during the model execution. After these refinements, a total of 921 valid occurrence points were retained for use in the final modeling process ( Figure 1 ). The species’ country-level distribution was referenced from the WSC 2024 database ( Figure 1 ).

For the detailed documentation of behavioral patterns and the natural history of the specimens, an opportunistic spot survey was conducted (Mirza and Mondal, 2018; Mondal et al., 2020b). Specimens were then identified in a controlled laboratory environment. The tools and equipment employed during this research included forceps, petri dishes, 70% ethanol solution, measuring scales, cameras, torches, rubber gloves, pens, notebooks, specimen containers, hygrometers, thermometers, and stereo zoom microscopes (Mirza and Mondal, 2018; Mukherjee and Mondal, 2020).

Environmental variables, including both bioclimatic and topographic factors, were obtained from the WorldClim v2.1 database (http://www.worldclim.org). All data were downloaded as GeoTiff files with a resolution of 30 arc-seconds. QGIS (http://qgis.osgeo.org/) was used to extract the relevant environmental variables specific to the Asian region. The bioclimatic variables represent long-term averages of monthly temperature and precipitation, capturing seasonal patterns and extremes (Kumar et al., 2021).

To prevent overfitting in the ecological model due to multicollinearity among the variables, the ENM Tools package (Warren et al., 2021) in R v4.2.2 was employed to calculate correlations between the raster layers. Environmental variables showing a correlation coefficient of ±0.8 or higher were removed from the analysis (Cao et al., 2021). The remaining uncorrelated variables were then selected for developing the final model.

To predict the distribution of Chilobrachys species, maximum entropy modeling (MaxEnt) version 3.4.4 (https://www.cs.princeton.edu/schapire/maxent/) was employed. Out of the 19 initially considered bioclimatic variables, seven were ultimately retained to model the species’ distribution across Asia. The modeling parameters included a logistic output format, a regularization multiplier of 1, a maximum of 1,000 iterations, and a convergence threshold set at 0.00001 (Das et al., 2022). To control model complexity, the program’s auto features were utilized (Mukherjee et al., 2024). Model performance was assessed using a 10-fold cross-validation approach, dividing the occurrence data into 10 subsets. A 10-percentile training presence threshold was applied to enhance model accuracy.

The resulting habitat suitability map categorized the areas into four distinct distribution classes: NA (not suitable habitat: 0–0.2), Min (minimum suitable habitat: 0.2–0.4), Mod (moderate suitable habitat: 0.4–0.6), and Max (maximum suitable habitat: 0.6–1). The final distribution map was generated using QGIS version 3.28.

Data on Chilobrachys spider bites were gathered over a 14-year span, from January 2010 to August 2024, using three different methods to document envenomations in humans.

First-aid treatment protocol has been developed in consultation with a trained, practicing physician for the Chilobrachys bite. Patients who will require transport to the hospital need intensive care in case of severe necrosis or secondary infection. We described our treatment procedure as successful (N = 3) for definitive bite cases.

Chilobrachys is widely distributed in the Indian subcontinent and Southeast Asia. India harbors 12 species, possibly due to its vast range of ecosystems making it a hotspot for this genus. Myanmar and China both have significant diversity (nine and seven species, respectively), playing major roles in the Chilobrachys genus’s Southeast Asian distribution. Thailand holds a middle ground with three species, while Sri Lanka and Malaysia have two species each. Vietnam has only one endemic species. Chilobrachys andersoni and Chilobrachys assamensis are common in northeastern regions like Assam and the Himalayan foothills. Chilobrachys fimbriatus, one of the better-known species, is native to Maharashtra, especially around forested areas like the Western Ghats. Despite being an island nation, Sri Lanka has two endemic species, Chilobrachys jonitriantisvansickeliae and Chilobrachys nitelinus. Myanmar is another region rich in Chilobrachys diversity, with nine recorded species, including C. andersoni (also found in India and Malaysia) and Chilobrachys bicolor. Its species also share similarities with those found in neighboring countries, reflecting some overlap in distribution. China has Chilobrachys dominus, Chilobrachys guangxiensis, Chilobrachys qishui, Chilobrachys jincheng, and Chilobrachys lubrica. Chilobrachys liboensis is particularly noted for its large size and aggressive temperament. Chilobrachys hubei is found in the central regions of China, showcasing a species that thrives in diverse environments from mountains to grasslands. While China’s species diversity is slightly less than that of India, its species are adapted to a wider range of ecological niches, from temperate forests to subtropical zones, distinguishing its Chilobrachys population. Vietnam hosts only one species Chilobrachys dyscolus, which is widespread and popular in the exotic pet trade due to its striking coloration and aggressive temperament. Thailand is the home of Chilobrachys huahini, Chilobrachys natanicharum, and Chilobrachys paviei. Malaysia hosts two species, Chilobrachys annandalei and Chilobrachys andersoni, with C. andersoni also found in India and Myanmar.

Chilobrachys spiders prefer moist and humid environments, commonly found near water bodies such as rivers and streams. Their burrows are typically located in areas with semiarid, lateritic soil, where conditions are ideal for burrowing and creating silk-lined tunnels.

The burrows of Chilobrachys are often branched and lined with silk. These burrows serve as a home and a hunting ground. The spiders remain hidden within, waiting for prey to pass by. Juveniles of this genus differ slightly in their choice of habitat, often constructing their nests in elevated areas, such as tree trunks, and using disorganized zigzag webs at the entrance to block access. This protective web serves to shield the spider from potential predators and environmental threats.

Chilobrachys are primarily ambush predators, relying on the element of surprise to capture their prey. Both male and female spiders remain concealed within their burrows or crevices in tree trunks, waiting for unsuspecting prey to come within reach. This nocturnal hunting strategy is common across Theraphosidae spiders (Herberstein, 2011). However, males exhibit different behavior during the mating season, which is from March to July in the subcontinent. Driven by reproductive instincts, they leave their burrows and adopt a more active, wandering, or cursorial hunting style. The shift in predatory strategy during the mating season reflects the increased mobility of males as they search for potential mates.

Theraphosidae, including Chilobrachys, are opportunistic carnivores, preying on a wide range of organisms. Their diet includes a diverse array of insects, such as Orthopterans, Dipterans, Dictyopterans, Hymenopterans, and Lepidopterans (Mondal et al., 2020a; Mukherjee and Mondal, 2023). Additionally, they are known to prey on small vertebrates, including amphibians like frogs and toads, reptiles like small lizards, and even small mammals like rodents. Chilobrachys are strict carnivores, rejecting scavenging opportunities. Cannibalism is also observed in this genus, particularly among females, who may consume males following a failed mating attempt or when food resources are scarce (Herberstein, 2011). This wide dietary range highlights the spider’s adaptability and role as a versatile predator in its ecosystem.

The behavioral ecology and natural history of Chilobrachys reflect a well-adapted species that thrives in humid, moist environments but is flexible enough to occupy semiarid areas. Their burrowing, ambush predation, and mating rituals are finely tuned to their environment. However, the declining populations and the inherent risks in their reproductive behaviors pose significant challenges for the future survival of these spiders. A detailed understanding of their behavioral patterns, habitat preferences, and mating dynamics provides crucial insights into their ecological role and the factors affecting their conservation status.

The highest probability zones for Chilobrachys species occur in the northeastern parts of India, extending into the foothills of the Himalayas, and along the Western Ghats. This aligns with known Chilobrachys species like Chilobrachys assamensis, C. fimbriatus, and Chilobrachys hardwickei that are adapted to humid, tropical, and mountainous environments. Moderate occurrence is predicted in central and southern regions of India, indicating that Chilobrachys species thrive in less ideal but still favorable conditions. The northern plains and the drier regions of western India show lower likelihoods for Chilobrachys occurrence, correlating with the arid climate, where precipitation is limited (BIO14, BIO19). The central and southern parts of Sri Lanka show maximum probability. This corresponds well with C. jonitriantisvansickeliae and C. nitelinus, which prefer tropical climates with consistent rainfall and warm temperatures. Northern Sri Lanka shows a lesser probability, indicating that the Chilobrachys species are more concentrated in the wet, forested areas. Northern Myanmar, particularly in mountainous and forested regions, shows a high occurrence probability. This is expected given the presence of multiple species like C. andersoni and Chilobrachys pococki, which thrive in warm, moist environments with significant seasonal rainfall. The southern regions of China, bordering Myanmar, show moderate probability, reflecting the potential habitat for species like C. guangxiensis and Chilobrachys jinchengensis, which adapt to subtropical climates. However, the map ( Figure 2 ) suggests that most of central and northern China is unsuitable for Chilobrachys species due to harsher climatic conditions, such as colder winters and lower precipitation (BIO5 and BIO19). Northern Vietnam and western Thailand are marked as high-probability areas. This makes sense for species like C. dyscolus in Vietnam and C. huahini in Thailand, which favor humid, tropical environments with consistent rainfall, as suggested by BIO18 and BIO14. Other areas, particularly central Vietnam and Thailand, still show moderate suitability, reflecting the potential spread of species to less humid areas, although with lower population densities. The central and northern parts of peninsular Malaysia show high probabilities, which would suit species like C. annandalei that prefers stable tropical environments. The southern regions show minimal likelihood for Chilobrachys presence, possibly due to less ideal climatic conditions, such as higher isothermality (BIO3) or lower precipitation during certain months (BIO14).

The areas highlighted in red (734,216 km²) (Max) are regions with high precipitation (BIO18, BIO19), including tropical and subtropical zones, which support tarantula species that rely on consistent moisture. These areas also tend to have moderate (1,799,160 km²) to warm temperatures year-round (BIO5), another critical factor in Chilobrachys survival. Regions with moderate seasonality in temperature and precipitation (BIO15) appear to support moderate probabilities (blue) for Chilobrachys presence, suggesting that while these regions are suitable, the species might not thrive as abundantly. Areas with minimal precipitation, extreme seasonal variations, or extreme temperatures (such as central or northern China) show minimal probabilities, indicating that the species cannot adapt to these harsh environments.

The response curves ( Figure 3 ) illustrate how Chilobrachys species respond to each bioclimatic variable, indicating the likelihood of the species’ presence based on the value of each variable. BIO2 represents the mean diurnal range (6.08°C to 14.85°C) (max temp − min temp), the curve shows a peak at approximately 10°C, after which suitability sharply decreases. Chilobrachys species seem to prefer regions with a moderate daily temperature range, where the difference between the hottest and coldest parts of the day is not too extreme. This suggests that regions with stable day–night temperatures are more suitable for Chilobrachys species, such as tropical or subtropical zones. BIO3 stands for isothermality response (24.23% to 84.92%), showing a slight peak at approximately 30%–50% isothermality, indicating that Chilobrachys species prefer areas where temperature variability between days and seasons is low. Stable climates, like those in Southeast Asia, are more conducive to species survival. BIO5 is the max temperature of the warmest month (17°C to 43.3°C), the response curve of which peaks at approximately 30°C–35°C but sharply declines at higher temperatures. This indicates that Chilobrachys species thrive in moderately warm regions but cannot tolerate extreme heat. BIO14 (0 mm to 157 mm) denotes precipitation of the driest month, which explains the sharp decline in species occurrence probability when precipitation falls below 50 mm, indicating a strong sensitivity to arid conditions during the driest month. Chilobrachys species depend heavily on moisture for survival, especially for burrow construction and maintaining suitable humidity levels. This makes regions with sufficient rainfall, even during the dry season, highly suitable. BIO15, pertaining to precipitation seasonality, directs a clear positive trend, with species occurrence increasing as precipitation seasonality rises. Chilobrachys species seem to prefer regions with distinct wet and dry seasons, as long as the overall precipitation levels are sufficient (BIO14). BIO18 (47 mm to 4,378 mm) denotes precipitation of the warmest quarter, which peaks at approximately 2,000–3,000 mm of rainfall during the warmest quarter, suggesting that Chilobrachys species favor regions with heavy rainfall during the warmest periods. This helps maintain adequate moisture in burrows and surrounding vegetation. BIO19 (2 mm to 1,887 mm), which is the precipitation of the coldest quarter, shows that Chilobrachys species can tolerate a wide range of cold-quarter precipitation, although higher precipitation (above 1,000 mm) is generally preferred. This variable is more relevant in regions with cooler winters, such as the foothills of the Himalayas, where species like C. assamensis and Chilobrachys fumosus are found.

The jackknife test of variable importance indicates that BIO5 (max temperature of the warmest month) consistently emerges as the most important variable across all analyses. This suggests that temperature extremes, particularly during the hottest periods of the year (17°C to 43.3°C), are a significant limiting factor for the distribution of Chilobrachys species. The areas where the maximum temperature exceeds their tolerance (above 35°C) will have lower habitat suitability. BIO19 (precipitation of the coldest quarter) and BIO18 (precipitation of the warmest quarter) are also important, indicating that precipitation during both the warmest and coldest months plays a crucial role in determining species presence. Regions with balanced rainfall throughout the year (moderate BIO18 and BIO19 values) are more likely to support Chilobrachys populations. BIO15 (precipitation seasonality), BIO2 (mean diurnal range), and BIO14 (precipitation of the driest month), though less significant than temperature and precipitation extremes, still contribute meaningfully to the model’s accuracy. They provide additional details on where Chilobrachys species can thrive, particularly in areas with less seasonal variability and more stable moisture conditions.

The ROC curve ( Figure 4 ) demonstrates that the model’s accuracy is high, with a strong predictive power, and is effective in distinguishing between suitable and unsuitable habitats for Chilobrachys species with an approximate AUC value of 0.887. The curve shows that the model correctly predicts species presence in most cases, with relatively few false positives. This reinforces the idea that the bioclimatic variables selected are well-suited to predict Chilobrachys distributions.

There were N = 24 confirmed bites by Chilobrachys spiders in humans, and five of these, which were followed up, were from medical reports. Among these five cases, three were directly observed by the authors. There were eight definite bites based on data from newspapers and research papers that provided a clear identifiable photograph of Chilobrachys ( Table 1 ), and there were 11 more definitive bite cases according to spot surveys ( Table 1 ). The most number of bites occurred in June (N = 7) followed by April (N = 5), May (N = 4), and July (N = 3) ( Figure 5 ).

Medical reports (N = 5) indicated that pain intensity ranged from 8 to 10, suggesting high levels of pain experienced by individuals. Newspapers and research papers (N = 8) reported slightly broader variations, with pain intensity ranging from 6 to 10. Spot surveys (N = 11) revealed pain intensities between 7 and 10, also showing severe levels of pain, but the range is narrower compared to research paper reports. Overall, pain intensity appears consistently high across all sources, although newspapers and research papers indicate slightly more variation in pain levels. Medical reports noted that pain lasted between 3 and 7 days, with the duration being periodical, implying intermittent episodes of pain rather than continuous discomfort. Newspapers and research papers showed a slightly shorter pain duration of 3 to 5 days, but the periodicity of pain was not mentioned there. Spot surveys suggested a significantly longer period of pain, ranging from 7 to 30 days, again with periodical intervals of pain. Medical reports recorded necrosis in 66% of cases. Newspapers and research papers reported a lower frequency, with 50% of cases showing signs of necrosis. Spot surveys indicated the highest incidence, with 72% of cases showing necrosis. Medical reports showed that 80% of individuals experienced blister formation. Newspapers and research papers indicated that 75% of cases resulted in blisters. Spot surveys indicated the highest occurrence, with 91% of cases showing blisters. Medical reports showed that 20% of cases displayed fang marks, whereas newspapers and research papers reported fang marks in 25% of cases and spot surveys indicated fang marks in 27% of cases, slightly higher than the other two sources ( Table 1 ).