Weta morphology is classical of the general insect body plan. As with all insects, weta possess three body segments; head, thorax, and abdomen. Weta lack the obvious presence of wings. Figure 2.1 from Gibbs 1998 shows the basic weta body plan. The weta hatches as a smaller form of the adult (nymph), undergoing up to nine stages of incomplete metamorphosis (exopterygote/ hemimetabolous development), before reaching maturity.
Normally adulthood in insects is reached with the external development of wings but in the wetas case this is indicated by sexual activity. Because the weta is flightless it has lost the need for wings, this feature is what allows weta to grow to such proportions.
The weta possesses a strong exoskeleton constructed from the polysaccharide chiton and proteins bound together to form a complex glycoprotein. This layer or cuticle functions to prevent the loss of water without which the weta would dehydrate. The tough chitinous shell gives the insect some protection from smaller predators and aids defense of territories, especially with male tree weta who possess large mandibles. The exoskeleton covers the entire body and extends internally to line much of the digestive system, as well as the air transporting tubules of the trachea (Dorrit et al, 1991).
The degree of cephalisation within the different weta species is diverse, determined mainly by dietary requirements but also defensive needs. See Figure 2.2 (Gibbs, 1998). The mandibles of the tree weta and the addition of elephant-like tusks on the tusked weta best represent this. The main purpose of these features seems to be for battles between rival males who might be fighting over territory or for mating privileges. In the case of the tree weta, it has been shown that the weta with the largest head receives preference over a small head for mating privileges (Gibbs, 1994).
Like most insects the weta has a pair of compound eyes from which it is likely to be able to see an object in colour but out of focus unless within a few centimeters. They are excellent for picking up movement and would operate well under low intensity lighting (Gibbs, 1998).
Between the two compound eyes lies a triangular formation of three photoreceptors called ocelli. These simple eyes only detect the presence or absence of light (Dorrit et al, 1991). Therefore these ocelli are able to detect such changes as day length which allows determination of the seasons.
The feeding appendages of the weta consist of a pair of mandibles and two pairs of maxillae. The second pair of maxillae form three-jointed palps – the labium (or lower lip), while the upper lip (or labrum), formed by a shelf-like projection of the head, covers the mandibles anteriorly (Dorrit et al, 1991).
The labrum is the only unpaired feeding structure and can move to hold food between the jaws while being chewed and has, on its inner surface, a number of taste organs (Gibbs, 1998).
The mandibles are the first paired structures and dominate the features of the head. They are hinged to the lower front edges of the head capsule and are operated by large jaw muscles, which virtually fill the inside of the head. These blunt mandibles are used to tear off leaf fragments or chew up insects taken as prey. Weta can inflict a painful bite if provoked (Gibbs, 1998).
The palps of the weta are external sensory appendages used to taste and smell its environment, food, or mates. They are very mobile, delicate paddle-like projections that are held back and protected during any conflicts or during feeding (Gibbs, 1998).
The thorax of all insects is divided into three sections, to each of which is attached a pair of walking legs. Most insects carry a pair of wings on each of the last two segments but the weta is an exception to this rule. Most insects bear wings for the purpose of migration and mating. Weta mating occurs on the ground and they do most of their traveling by foot, sometimes up to 50 meters in one night.
The first three plates along the back behind the head belong to the thorax segments. These are called notal plates and the first one, the pronatum, is the largest and is often referred to as the thoracic shield or prenatal shield. This feature is used to identify common weta species (Gibbs, 1998).
The legs are attached to the bottom of the thorax via a complex joint or coxa. The coxa is the uppermost joint of the leg of an insect, which articulates with the thorax proximally, and with the trochanter (second joint) distally. The coxa is usually a stout, truncated cone of cuticle, and is freely mobile with respect to the thorax (Allaby, 1999). On the hind legs the trochanter articulates distally with the femur. Large extensor muscles give movement to the femur. The femur is attached to the tibia. See figure 2.3 (Gibbs, 1998) below.
The spines of the tibia are used for defense from predators. The Mahoenui giant weta raises its hind legs towards the predator when threatened, while the Banks Peninsula weta uses a play dead method attacking with its legs and jaws if provoked further. The newly discovered weta from the Paparoa Ranges uses the standard weta defense method of burrowing headfirst in a hole with the thorny spines forming a barricade across the entry (Gibbs, 1994).
The leg of the weta ends with a four-jointed foot or tarsus with a double claw at the tip. All but the cave weta have pale-coloured pads on the undersurface of the tarsus for the purpose adhesion to smooth surfaces (Gibbs, 1998).
The abdomen, the largest part of the body, is composed of 11 segments, which lack any appendages, with the exception of the reproductive organs at the rear. The purpose of the abdomen is to house vital digestive and sex organs. These organs are what enable differentiation of the sexes. The cerci of the male and female are different and the female weta possesses an ovipositor. The ovipositor enables the female weta to lay her eggs deep within the soil. Some weta such as the ground weta do not have long ovipositors, making it difficult to distinguish the sexes. They instead choose to lay their eggs in a brood chamber (Gibbs, 1998).
A female weta deposits her eggs into the soil.
Weta breathe, like all insects, through the use of a tracheal system of gas exchange. The tracheal system is diffusion facilitated, with air entering via the tracheal spiracles, and diffusing into the body through a network of tracheae