Form and Function
Dorsal view of the head and thorax of a two-striped walking stick. Photo used courtesy of Alex Wild.

Unlike most species of mammals and birds, the female Anisomorpha buprestoides is consistently much larger than the male both in length and width in all three sections of their bodies: the head, thorax, and abdomen.  On average, the female body length ranges from 5.80 cm to 8.50 cm, while the male body length ranges from 3.80 cm to 5.20 cm (Conle et al. 2009).  In fact, the males are small enough in proportion to the females that they almost continuously ride on the females’ back. 

Because these insects do not have skeletons as we do, the entire body of both the male and female is encased in a rigid exoskeleton or cuticle comprised of chitin, a polymer of a glucose derivative.  The exoskeleton protects the insect, gives it shape and structure, and allows for muscle attachment to aid in movement.  Unfortunately, it does not grow as A. buprestoides does and must be molted periodically and then reformed.

The head, which is nearly oval in shape, encloses the walking stick’s brain and oral cavity.  The head also includes two compound eyes composed of many individual photosensitive lenses that absorb light from different directions (Littig 1942).  These lenses give the insect a greater viewing angle and improved detection of quick movement in comparison to the simple eyes of humans.  In addition, two antennae extend from the head of A. buprestoides that serve as sensors, detecting surroundings, odors, and temperature (Sharp 2014).  The antennae are therefore incredibly important for the walking stick’s survival and extend longer than two-thirds of the length of the body, reaching an average of 4.20 cm in the females and 3.56 cm in the males (Littig 1942). 
 Close-up of a two-striped walking stick's head. Photo used courtesy of Christopher Tozier.
The thorax of the walking stick has three segments—the pronotum, mesonotum, and metanotum—and connects the head and abdomen.  In the female, the thorax is rather stout and around 2.82 cm in length (Littig 1942).  The male thorax has a length of about 1.60 cm (Littig 1942).  The thorax also acts as the attachment point for all three pairs of legs, each of which are jointed and segmented into a femur, tibia, and tarsus to allow movement.  The tarsus has claws and suction pads, which allow them to climb different surfaces, such as tree trunks or plant stems, successfully.  The legs are all long and slender, thus meeting the needs for the walking stick’s unhurried and deliberate stride (Littig 1942).  The thorax also produces and stores an acidic, milky compound that can irritate and temporarily blind an attacker.  This secretion is released, when needed, through two prothoracic glands located behind the head on the dorsal side of the thorax.  A. buprestoides can immediately respond to advances from a predator and accurately hit its target from up to 30 to 40 centimeters away (Thomas 2003).
Close up of a two-striped walking stick and its prothoracic gland. Photo used courtesy of Christopher Tozier.

Finally, the abdomen of the walking stick makes up roughly half of the body length.  Again, the female’s abdomen, about 3.69 cm long, is thick and tapers off at the end, while the male’s abdomen, about 2.33 cm long, is thin and remains the same width throughout each segment (Littig 1942).  The abdomen includes the insect’s digestive and reproductive systems.  Both the abdomen and the thorax contain numerous spiracles, or small openings on the side of the body, which pass air into the body via tubes (Sharp 2014).  The abdomen then contracts and expands as the gases travel into and out of the body to facilitate breathing.

While the southern two-striped walking stick has been found in a relatively small range of states including Florida and those along the Gulf Coast, each of the three distinct color forms within the species has adapted to different environments, resulting in variations in both morphology and behavior.  The most prominent difference is the color of the two longitudinal dorsal stripes on the insect: white, orange, or brown.  Altogether, these stripes are striking in contrast to the black stripes that run down the center of the back and on the sides, and do not provide excellent camouflage in close proximity.  Instead, it is believed that the vivid stripes hint at the walking stick’s irritating defensive spray and serve to ward off predators (Conle et al. 2009).
A white form pair of two-striped walking sticks traveling on a sandy beach. Photo used courtesy of Don Filipiak.

However, an alternative hypothesis suggests that the colored stripes do aid in camouflaging A. buprestoides in specific settings (Conle et al. 2009).  The white form is normally well hidden from aerial predators, such as birds, when walking along white sand.  The orange form is often found resting on plants with yellow or orange flowers, effectively blending the bright orange stripes.  Similarly, the brown form spends most of its day hiding in the brown bark of pine or oak trees.  It is likely that such behaviors heavily influenced the success of each color form in their varying environments.

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