Tarantulas possess an exoskeleton, a rigid external covering that provides both support and protection. Unlike animals with internal bone structures, this hard outer shell dictates the tarantula’s growth.
Understanding the Exoskeleton
An exoskeleton is a rigid, external covering that provides structural support and protection for the soft tissues of many invertebrates, such as arthropods and mollusks. This differs significantly from an endoskeleton, an internal skeletal framework like the bones found in vertebrates. Unlike an endoskeleton, an exoskeleton does not expand once formed.
The primary component of an arthropod exoskeleton is chitin, a complex carbohydrate derived from glucose. Chitin provides strength and flexibility, often combined with proteins to form a composite material.
The Tarantula’s External Armor
A tarantula’s exoskeleton is composed mainly of chitin and proteins. This outer layer, known as the cuticle, is secreted by the underlying epidermis. It consists of several sub-layers, including a thin, waxy outer epicuticle that helps prevent desiccation, and a thicker procuticle beneath it, which provides the primary structural strength. The procuticle is further divided into an exocuticle and an endocuticle. The exocuticle undergoes a hardening process called sclerotization, where protein chains cross-link to increase rigidity.
The tarantula’s body is divided into two main parts: the prosoma (or cephalothorax) and the opisthosoma (or abdomen), both encased in this segmented armor. While the exoskeleton is generally rigid, particularly in areas like the carapace, it also features flexible joints that allow for movement. Additionally, the exoskeleton supports numerous bristles or hairs, known as setae, which are also made of chitin.
Exoskeleton Functions
The exoskeleton serves multiple important functions. It acts as a protective barrier, shielding internal organs from physical impacts and reducing water loss, which is essential for survival in dry environments. This outer shell also provides a framework for muscle attachment, enabling the tarantula to move its legs and appendages efficiently.
The exoskeleton integrates various sensory structures, including chitinous setae covering the tarantula’s body. These specialized bristles are highly sensitive, allowing the tarantula to detect vibrations, air currents, and chemical cues. Some New World tarantulas also possess urticating hairs on their abdomen, specialized setae used as a defense mechanism against predators.
The Molting Process
Because the tarantula’s rigid exoskeleton cannot expand, growth requires molting, or ecdysis. As the tarantula grows, a new, soft exoskeleton forms underneath the old one, and enzymes separate the layers. The tarantula then sheds its old outer shell, often by lying on its back to push out of the old skin.
The period during and immediately after molting is a vulnerable time for the tarantula. The newly exposed exoskeleton is soft and pliable, leaving the spider susceptible to injury, dehydration, and predation. The new exoskeleton gradually hardens over several days as it expands to accommodate the tarantula’s increased size. This shedding process allows for growth, repair of minor injuries, and even regeneration of lost limbs over successive molts.