Tarantulas grow by periodically shedding their hard outer layer, a process called ecdysis or molting, which allows them to increase in size. This physical change is a series of instars, or developmental stages, separated by molts. Hobbyists categorize these stages using specific terms: “sling” for the newly hatched spiderling, “juvenile” for the rapidly growing middle stage, and “adult” once sexual maturity is reached. The journey from a tiny sling to a fully grown adult involves a dramatic shift in appearance, including changes in size, color, and structural morphology.
Appearance During the Spiderling Stage
The initial stage of a tarantula’s life begins as a spiderling, commonly called a “sling,” characterized by its small size and delicate structure. Many terrestrial species start with a leg span of less than a quarter of an inch, while some arboreal species may be slightly larger. Newly emerged slings are frail, lacking the hardened exoskeleton and robust build of their older counterparts.
Coloration at this stage is often pale, translucent, or a dull brown or gray, which can make species identification difficult. Slings often lack the vibrant, species-specific markings that define them as adults. For instance, species that are brilliant blue or green as adults may appear as simple pinkish-white or tan slings. The abdomen is often disproportionately large relative to the cephalothorax and legs, especially after a recent feeding.
The sling’s soft appearance is due to the lack of dense pigmentation and complex hair structure. Their small size and softness make them particularly vulnerable. With each subsequent molt, the exoskeleton becomes thicker, and the first hints of the adult appearance begin to consolidate.
Juvenile and Sub-Adult Transition
After the fragile sling stage, the tarantula enters the juvenile phase, where its appearance rapidly transforms with each successful molt. The leg span increases noticeably, and body proportions align more closely with the adult form. This period is marked by the emergence of the species-specific color palette and pattern that was absent or muted in the sling.
The shift in coloration is a gradual process where pigments intensify and patterns become defined across the carapace and legs. Complex markings, such as the striped knees of ornamental tarantulas, solidify during this stage. Vibrant colors like blue or green may begin to show through the drab juvenile brown or black. The exoskeleton itself hardens and darkens, moving from the sling’s semi-translucent appearance to a more opaque, established look.
The term “sub-adult” refers to the stage where the tarantula is visually close to its maximum size and has adopted its full adult coloration. At this point, the spider’s growth rate and molting frequency typically slow down significantly. A sub-adult is essentially a large juvenile, differentiated by its near-final size and fully expressed coloration, but it has not yet undergone the final molt necessary to become sexually mature.
Adult Morphology and Sexual Dimorphism
The final molt to maturity results in the definitive adult appearance, often characterized by sexual dimorphism. Adult females are typically larger and more robust, possessing a heavier build and a larger abdomen designed to carry eggs. Females continue to molt periodically throughout their long lives, allowing for slight increases in size even after reaching maturity.
Mature males undergo their “ultimate” or final molt, after which they stop growing and take on a different, more slender morphology. They are generally smaller and “leggier” than females, with longer legs relative to their body size, built for mobility in searching for a mate. The most distinguishing visual characteristics of a mature male are specialized structures used for reproduction.
Furthermore, many mature males feature tibial hooks (tibial apophyses) on the first pair of legs, which are small spurs used to hold the female’s fangs during mating. The male’s pedipalps, the small appendages near the fangs, develop enlarged, bulbous tips called emboli or copulatory bulbs. These structures, often described as looking like boxing gloves, are used to transfer sperm. Males may also exhibit a different, sometimes more muted or drab, final coloration compared to the vibrant hues displayed by mature females.