Bark scorpions are common household pests in parts of the southwestern United States, prompting many residents to seek accurate information about their size and growth. Understanding the physical dimensions of these arachnids is important for identifying them and for implementing appropriate pest management strategies. This article provides definitive measurements and explains the biological processes that determine the ultimate size an individual bark scorpion reaches.
Which Species Are We Measuring?
The common name “bark scorpion” applies to a number of species globally, but the one of primary concern in North America is the Arizona Bark Scorpion, scientifically known as Centruroides sculpturatus. This species is the most frequently encountered scorpion in urban areas of the Sonoran Desert, including Arizona, southern Nevada, and parts of California. The genus Centruroides includes various other species, such as the Striped Bark Scorpion (Centruroides vittatus), which has a wider distribution. The measurements discussed in the following sections focus specifically on the Arizona Bark Scorpion, as this species represents the dimensions of the animal most people are trying to identify.
Maximum and Average Measurements
The Arizona Bark Scorpion is not a large arachnid, especially when compared to other scorpion species found globally or even locally. Adult length varies between 2.5 to 3.1 inches (approximately 6.5 to 8 centimeters) from the tips of the pedipalps to the end of the metasoma, or tail. This relatively small size allows them to easily navigate tight crevices and cracks, which is a major factor in their ability to enter homes.
Adult males tend to be slightly longer than females, reaching the maximum length of up to 3.1 inches (8 cm). This longer length in males is primarily due to a more elongated metasoma, which is the segmented portion commonly called the tail. Adult females, in contrast, generally reach a maximum length of about 2.8 inches (7 cm).
The body structure of C. sculpturatus is noticeably slender when compared to the more robust, stout bodies of other North American scorpions. This slender build can make them appear smaller than their actual length suggests. The slim profile is characteristic of the genus Centruroides and contributes to their climbing ability.
Factors Influencing Growth
The range in size observed among adult bark scorpions is a result of several interacting biological and environmental factors. One of the most important determinants is sexual dimorphism, the physical difference between sexes. While males are longer overall due to an extended metasoma, females are often heavier and more massive in the cephalothorax and mesosoma (the main body sections).
Females require a larger body mass to support the physiological demands of gestation, as they are viviparous (giving birth to live young). Gravid, or pregnant, females gain substantial body mass, but this increased weight does not appear to reduce their foraging efficiency. However, once they are brooding and carrying young on their backs, their ability to hunt is significantly reduced.
Growth in all scorpions is dependent on the process of molting, where they shed their rigid exoskeleton to allow the underlying soft tissue to expand. The frequency and success of these molts are highly influenced by resource availability and temperature. Scorpions that consistently find ample food and hydration will grow faster and reach their maximum size sooner.
Environmental temperatures dictate the scorpion’s metabolic rate and activity levels, as they are cold-blooded. Bark scorpions are generally active only when nighttime temperatures are consistently above 70°F (21°C). Cooler temperatures slow their growth and trigger periods of inactivity, such as overwintering, which extends the time required to reach adulthood.
Molting also requires specific environmental conditions, including sufficient humidity, and often involves climbing onto a vertical surface. If conditions are not right, a scorpion may delay molting, which delays growth and leads to a smaller adult size. The combination of gender-specific body structure, resource allocation for reproduction, and environmental influence on molting cycles explains why a bark scorpion’s size falls within a defined range.