The Thorax

The thorax is the middle tagma of insects and is the main unit for locomotion because it bears the legs and, in pterygotes, the wings. The thorax primitively consists of three metameres although the first abdominal segment is closely associated with the thorax in Pterygota and is completely fused to it in the Apocrita (Hymenoptera). Each thoracic segment has one pair of legs. Anteriorly, there is a membranous region where the head attaches to the thorax. This is the cervix (Latin, meaning "neck"). This neck region may not be a true intersegmental boundary, and the region as a whole may be composite in origin. The evidence for the composite origin of the neck derives from the fact that there is no antecosta on the pronotum, the dorsolongitudinal muscles go directly from the antecosta of the mesothorax (i.e., from the first phragma) to the back of the head (to the postoccipital ridge), and the ventral longitudinal muscles go directly from the sternal apophysis of the prothorax to the posterior arm of the tentorium. Several cervical sclerites form a fulcrum on which the head rotates. The head is protruded when the muscles attaching to these sclerites contract. When the muscles relax, the head is pulled back to the thorax.

The dorsal structure of the thorax is very similar to abdominal terga (i.e., with typical secondary segmentation, see preceding discussion). In apterygotes and nymphal pterygotes the terga do not overlap each other; however, the terga overlap in adult pterygota. In winged insects the terga of the winged segments typically divide into a postnotum (which bears the phragma) and the alinotum (which bears the wing sclerites). In several lineages the alinotum becomes divided by a scuto-scutellar sulcus (or completely divided, typically called a suture but more appropriately called a fissure) and forms an anterior scutum and a posterior scutel-lum. In the mesothorax this separates the mesoscutum from the scutellum (more appropriately called the mesoscutellum) and in the metathorax it is the metascutum and metascutel-lum (these modifications are only rarely present in the metathorax). This separation allows for specific changes in the thoracic structure for flight.

The pleuron is the side of the thorax and is where the legs join the body (Figure 4.3). These sclerites are the least like the abdomen of any sclerites and have been interpreted in dramatically different ways by different authors. The most robust theory is the subcoxal theory, wherein the the pleura are composed of the subcoxa of the appendages. In this theory the subcoxa was perhaps primitively a podite (the basic units of a jointed leg) that became incorporated as the lateral wall of the body. After incorporation into the body wall, the subcoxa was primitively divided into three sclerites: the anapleurite, the coxopleurite (also called the katapleurite), and the sternopleurite. The former two are obvious in groups like Plecoptera. The sternopleurite is perhaps fused into the lateral portions of the eusternum (there is no distinct sternopleurite in Hexapoda, although it does occur in Chilopoda). The coxopleurite (katapleurite) is involved in the articulation with the coxa of the leg (the other articulation is hypothesized to have been on the sternopleurite), and when present in Arthropoda, it is there; otherwise in Hexapoda there is primitively no ventral articulation. Instead, a secondary articulation is present. The anapleurite and coxopleurite typically fuse to form the pleural wall of the body. In winged insects a pleural wing process is formed dorsally to make up the other part of the fulcrum in the wing articulation (see the discussion of Pterygota and wings later in this chapter). A pleural sulcus is formed running from the pleural wing process to the coxal articulation (at the pleural coxal process). The pleural sulcus corresponds to an interior ridge to strengthen the pleuron during the contraction of the flight muscles. The pleural sulcus divides the pleuron into anterior and posterior regions. The area anterior to the pleural sulcus is the episternum; the area posterior to the pleural sulcus is the epimeron (Figure 4.3). Thus the two main pleural sclerites (recall that the sternopleurite is perhaps fused into the eusternum in Hexapoda or lost altogether) can be divided into an anepisternumlanepimeron (from the anapleurite) and a katepisternumlkatepimeron (from the coxopleurite/ katapleurite). Sometimes these regions can be further subdivided, like a distinct anterior portion of the episternum, called the preepisternum. Typically the pleural sclerites are completely fused making any distinction between the anapleurite and coxopleurite impossible.

The mesothoracic and metathoracic pleura possess spiracles (also present laterally on the abdomen), which are openings for respiration. The spiracle is situated in a sclerite called the peritreme and attaches internally to tracheae, the main branches of the tracheal system. Sometimes the spiracle first opens into a small chamber, called the atrium, which can bear on its sides a series of folds or spines that serve dual functions - preventing the entrance of foreign particles and simultaneously catching water vapor before gases depart from the body. The tracheae are invaginations of the exoskeleton and are shed during molting.


The legs articulate directly with the pleural sclerites and are composed of a series of segments, or podites. The hexapod leg consists of six podites: coxa, trochanter, femur, tibia, tarsus, and pretarsus (Figure 4.4). The basalmost podite is the coxa. The coxa of the leg primitively has a single (mono-condylic) articulation with the pleuron; however, dicondylic articulations have evolved in many insect lineages. The secondary articulation is either with the sternum (perhaps the sternopleurite?) or the trochantin, which is visible in generalized Pterygota but lost or fused in most higher lineages. The trochantin is a precoxal sclerite perhaps derived from the

4.3. Basic thoracic structure of a grasshopper.

anterior portion of the primitive coxopleurite (i.e., the katepisternum) and forms the secondary articulation with the coxa along its anterior margin. The coxa is divided into proximal and distal portions by the basicostal sulcus, which is an external indication of the internal basicosta (a specialized costa) and the insertion point for extrinsic muscles (ones where the origins lie outside the appendage) that move the coxa. The portion of the coxa proximal to the basicostal sulcus is the basicoxite. The posterior part of the basicoxite is frequently developed into a large lobe called the meron (e.g., Neuroptera, Mecoptera, Trichoptera, Lepidoptera; but fused with the pleural wall in many Diptera). A coxal sulcus is often present and runs from the base of the coxa to the anterior trochanteral articulation. As noted, muscles operating the coxa are extrinsic muscles, while those operating the other podites are typically intrinsic muscles (where origins and insertions are within the appendage). Flexion and extension of the podites is accomplished by antagonistic sets of muscles (flexors and extensors, also called depressors and lev-ators). Flexors are used to flex a section of an appendage; flexors bend an appendage by moving portions of it toward the body (somewhat synonymous with depressors, which lower an appendage). Extensors are used to extend a section of an appendage; extensors straighten an appendage by moving portions of it away from the body (somewhat synonymous with levators, which raise an appendage). The trochanter is the second podite of the leg (the basalmost segment of the telopodite) and is generally rigidly attached to the base of the femur so that the articulation no longer functions; however, it is sometimes completely fused to the femur. There are two trochanters in Odonata, the first and second trochanters. A reductor muscle originates at the base of the trochanter and inserts on the femur. In the Odonata the second trochanter is a true trochanter because the reductor attaches to the base of the second trochanter. In some lineages a second trochanter (called the trochantellus in some groups of Hymenoptera) is developed from the base of the femur and is, therefore, not a homologue of the second trochanter seen in Odonata or other arthropod lineages. The reductor muscles attach internally at the base of the true trochanter and attach to the base of the trochantellus, so the trochantellus is merely the base of the femur demarcated by an outwardly visible sulcus. The femur is typically the largest podite of the leg and contains muscles that originate near its base and insert on the tibia (the tibial extensors above and the tibial flexors below: sometimes called the tibial levators and the tibial depressors, respectively). Highly developed tibial levators occur in several "jumping" groups such as grasshoppers. It also contains the origin of the pretarsal depressor (= pretarsal flexor). The tibia is typically a slender podite and the second largest (and often the longest) in the leg, with its basal end slightly bent toward the apex of the femur so

4.4. Basic external morphology of insects, based on orthopterans. The large ovipositor is from a katydid (Tettigoniidae); all other parts are from the acridid grasshopper shown in full. Not to the same scale.

that it forms the major joint of the hexapod leg. This bent head allows the tibia, when depressed (i.e., flexed) to be closely appressed to the undersurface of the femur, which is particularly important in jumping insects.

The tarsus is primitively one-segmented but is frequently subdivided into two to five subunits called tarsomeres (in Pro-tura, some Collembola, and most larvae it retains the primitive condition of only one unit) (Figure 4.4). The tarsomeres are typically movable, but the tarsus never has muscles intrinsic to itself and is operated entirely by muscles that originate in the tibia and insert on the base of the tarsus, the tarsal levators, and tarsal depressors. The basal tarsomere is typically enlarged and called the basitarsus. Externally the tarsomeres sometimes possess small pads called tarsal pul-villi or euplantulae. The pretarsus is the apicalmost podite of the leg and, despite its minute size, is quite complicated. The pretarsus consists of lateral claws (ungues, which are erroneously and frequently called the "tarsal claws") and a median arolium (rarely entirely sclerotized) (Figure 4.4). The claws are attached via membranes to the unguifer, a small dorsal process of the last tarsomere. There can sometimes be minute, lateral sclerites near the base of the claws called aux-iliae. Ventrally the pretarsus consists of a sclerite called the unguitractor, which is typically partially invaginated into the apex of the last tarsomere. The unguitractor can be further subdivided at times or have a distal sclerite called a planta. The pretarsus is moved via a long tendon that originates in the tibia and femur and inserts on the unguitractor plate of the pretarsus. This tendon is sometimes called the retractor of the claws. This muscle is a depressor (= flexor); there is no levator (= extensor) for the pretarsus, and elevation is done entirely by touching the substrate. The tendon is attached to the unguitractor plate of the pretarsus by a thin apodeme that extends back through the tarsus and tibia, and the actual flexor is in the base of the tibia or the femur.

Joints are formed by regions of membranous cuticle called arthrodial membranes formed between adjacent podites. The joints make movement between podites possible, while the type of articulation controls what kind of movement is possible. The main coxal articulation is situated at the ventral terminus of the pleural sulcus. Dicondylic coxae come in two forms; the first has a secondary articulation formed anteriorly with the trochantin (the trochantin is lost in many higher orders), while another secondary articulation is formed ventrally with the eusternum. All other joints in adult insects are dicondylic within the appendage. Larvae of holometabolous insects, however, frequently have monocondylic articulations even though the adults are dicondylic. Articulations are typically formed of an anterior and posterior point of articulation except at the trochanteral-femoral articulation, where (if present) it is sometimes composed of dorsal and ventral points of articulation.


While it is understood how vertebrate wings evolved from forelimbs, homologues and origins of insect wings have been confusing and controversial. Although often depicted in general texts as relatively simple structures with a few veins running through them, the insect wing is a structure of daunting complexity. Here, we provide just a basic account. Functional wings are present only in adult insects, the mayflies (Ephemeroptera) being the only insects where the last nymphal instar (the subimago) primitively possess functional wings. Wings begin to develop in earlier instars, either externally or internally, but do not become functional until after the final molt in all other pterygotes. Insect wings occur exclusively on the middle (i.e., mesothoracic) and metatho-racic segments, together called the pterothorax, and they articulate to the body via a series of sclerites called pteralia. While numerous bones and muscles shape the foil of the vertebrate wing during flight, an insect wing is actively operated only at its base in the same way a lever hinges on a fulcrum. In the wing itself there are no muscles that allow the insect deliberate control over the movements of the wing beyond its base. This is not to indicate that insect flight is as simple as flapping up and down. Indeed, insects are capable of a greater range of movements than are the wings of vertebrates. Insects, particularly those that hover, are the most acrobatic fliers. These movements are a result of various muscles attaching at the wing base that pull on the pteralia, as well as how veins, folds, and flexion lines buttress and fold the wing.

As noted, however, the insect wing is similar to a lever acting on a fulcrum. As such, the structure of the pterothorax is also critical to wing movement. The wing extends between the dorsal plate of the insect thorax (the notum) and the side of the thorax (the pleuron), and from these are processes that function in the wing's articulation - the anterior and posterior notal wing processes, and the pleural wing process. The pleural wing process forms the fulcrum for the wing while two notal processes push down against the base of the wing on either end. Two additional plates, situated in membrane on either side of the pleural wing process and called the epi-pleurites, are the basalare and subalare. These plates provide insertion points for muscles that control the tilt of the wing during flight and assist in the downstroke of flight. In the Neoptera, the basalare also serves to extend the wing from its folded position over the abdomen.

The wing itself is formed of two epidermal layers, an upper layer and a ventral layer, which grow out from the body and fuse together. They are living structures, complete with hemolymph, tracheae, and nerves. Cavities form during this development, called lacunae, and form channels through which tracheae move along with some nerves. At eclosion to an adult, most of the epidermal cells die and form a cuticular wing membrane with cavities, called veins. This is an extremely durable structure that fossilizes much more readily than any other part of the body. The veins provide some structural support to form a more-or-less stable wing foil.

The veins are perhaps the most notable feature of the insect wing and a rich source of characters for understanding the evolutionary relationships of numerous groups and most insect fossils. They also provide information on flight biome-chanics. The insect wing is primitively fluted, like a Japanese fan, with the veins alternating between concave and convex (typically denoted as " + " for convex and " —" for concave). This corrugation provides strength to the wing as it experiences various flight stresses. Convex veins sit on an elevated ridge, while concave ones lie in a trough or depression. Several systems have been proposed for naming veins; they are based

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