The Thorax
The skeleton of the thoracic segments is modified to give efficient support for the legs and wings, and the musculature is adapted to produce the movements of these appendages.
Morphology of the thorax
Tergum
The tergum of the prothoracic segment is known as the pronotum. It is often small serving primarily for attachment of the muscles of the first pair of legs, but in Orthoptera, Blattodea and Coleoptera it forms a large plate affording some protection to the pterothoracic segments. The meso- and meta-nota are relatively small in wingless insects and larvae, but in winged insects they become modified for the attachment of the wings. In the majority of winged insects, the downward movement of the wings depends on an upwards distortion of the dorsal wall of the thorax. This is made possible by a modification of the basic segmental arrangement.
Various strengthening ridges develop on the tergum of a wing-bearing segment. These are local adaptations to the mechanical stresses imposed by the wings and their muscles. The ridges appear externally as sulci which divide the notum into areas. Often a transverse sulcus divides the notum into an anterior prescutum and a scutum, while a V-shaped sulcus posteriorly cuts off the scutellum. The various elements of the tergum as a result of modifications leading to subdivisions include acrotergite, prescutum, scutum, scutellum along an anterior to posterior plane. The sulci of the tergum include the antecostal sulcus separating the acrotergite and prescutum; transverse sulcus separating the prescutum from the scutum; and the scutoscutellar sulcus separating the scutum from the scutellum.
Sternum
The primary sclerotizations on the ventral side are segmental and inter segmental plates which often remain separate in the thorax. The intersegmental sclerite is produced internally into a spine and is called the spinasternum, while the segmental sclerite is called the eusternum.
The sternum of the pterothoracic segments does not differ markedly from that of the prothorax, but usually the basisternum is bigger, providing for the attachment of the large dorsoventral flight muscles. The sternum is attached to the pleuron by pre- and post-coxal bridges.
Arising from the eusternum are a pair of apophyses, the so-called sternal apophyses. The origins of these on the sternum are marked externally by pits joined by a sulcus so that the eusternum is divided into a basisternum and sternellum, while in higher insects the two apophyses arise together in the midline and only separate internally, forming a Y-shaped furca.
Distally the sternal apophyses are associated with the inner ends of the pleural apophyses, usually being connected to them by short muscles. This adds rigidity to the thorax, while variation in the degree of contraction of the muscles makes this rigidity variable and controllable. The sternal apophyses also serve for the attachment of the bulk of the ventral longitudinal muscles.
Pleuron
The pleural regions are membranous in many larval insects, but typically become sclerotized in the adult. Basically there are probably three pleural sclerites, one ventral and two dorsal, which may originally have been derived from the coxa. Above the coxa the pleuron develops a nearly vertical strengthening ridge, the pleural ridge, marked by the pleural sulcus externally. This divides the pleuron into an anterior episternum and a posterior epimeron. The pleural ridge is particularly well developed in the wing-bearing segments, where it continues dorsally into the pleural wing process which articulates with the second axillary sclerite in the wing base.
In front of the pleural process in the membrane at the base of the wing and only indistinctly separated from the episternum are one or two basalar sclerites, while in a comparable position behind the pleural process is a well-defined subalar sclerite. Muscles concerned with the movement of the wings are inserted into these sclerites.
Typically, there are two pairs of spiracles on the thorax. These are in the pleural regions and are associated with the mesothoracic and metathoracic segments. The mesothoracic spiracle often occupies a position on the posterior edge of the propleuron, while the smaller metathoracic spiracle may similarly move on to the mesothorax. Diplura have three or four pairs of thoracic spiracles.
Heterojapyx, for instance, has two pairs of mesothoracic and two pairs of metathoracic spiracles.
Muscles of the thorax
The longitudinal muscles of the thorax, as in the abdomen, run from one antecostal ridge to the next. They are relatively poorly developed in sclerotized larvae, in adult Odonata, Blattodea and Isoptera which have direct wing depressor muscles, and also in secondarily wingless groups such as Siphonaptera. In these cases, they tend to telescope one segment into the next, while the more lateral muscles rotate the segments relative to each other. In unsclerotized insects, contraction of the longitudinal muscles shortens the segment.
In most winged insects, however, the dorsal longitudinal muscles are the main wing depressors and they are well developed, running from phragma to phragma so that their contraction distorts the segments. The ventral longitudinal muscles run mainly from one sternal apophysis to the next in adult insects, producing some ventral telescoping of the thoracic segments. Dorso-ventral muscles run from the tergum to the pleuron or sternum. They are primitively concerned with rotation or compression of the segment, but in winged insects they are important flight muscles. In larval insects an oblique intersegmental muscle runs from the sternal apophysis to the anterior edge of the following tergum or pleuron, but in adults it is usually only present between prothorax and mesothorax.
The Insect Legs and Locomotion
Insects typically have three pairs of legs, one pair on each of the thoracic segments. From this, the alternative name for insects, the ‘hexapods’, is derived, although not all hexapods are now regarded as insects.
Fig.1. Leg and articulations. Points of articulation shown by bold arrows. (a)Typical insect leg.
(b) Dicondylic articulation of trochanter with coxa and the apodemes of muscles moving the trochanter. Notice that the femur is united with the trochanter; there is no moving joint. (c), (d) dicondylic articulation of tibia and femur, (c) side view, (d) end view. (e) Monocondylic, ball articulation of tarsus with tibia.
Fig 2. Coxa, oblique lateral view: (a) typical insect ; (b) coxa with a large meron.
Fig. 3. Pretarsus
Basic Structure of the Legs
Each leg consists typically of six segments, articulating with each other by mono- or di-condylic articulations set in a membrane, the corium. The six basic segments are:
- Coxa – the coxa is often in the form of a truncated cone and articulates basally with the wall of the thorax. The part of the coxa bearing the articulations is often strengthened by a ridge indicated externally by the basi-costal sulcus which marks off the basal part of the coxa as the basicoxite (Fig. 2a). The basicoxite is divided into anterior and posterior parts by a ridge strengthening the articulation, the posterior part being called the meron (Fig. 2b).
- Trochanter – the trochanter is a small segment with a dicondylic articulation with the coxa such that it can only move in the vertical plane (Fig. 1b). In Odonata there are two trochanters and this also appears to be the case in Hymenoptera, but here the apparent second trochanter
is, in fact, a part of the femur. - Femur – the femur is often small in larval insects, but in most adults it is the largest and stoutest part of the leg. It is often more or less fixed to the trochanter and moves with it.
- Tibia – the tibia is the long shank of the leg articulating with the femur by a dicondylic joint so that it moves in a vertical plane. In most insects, the head of the tibia is bent so that the shank can flex right back against the femur (Fig. 1a).
- Tarsus – the tarsus, in most insects, is subdivided into from two to five tarsomeres. These are differentiated from true segments by the absence of muscles. The basal tarsomere, or basitarsus, articulates with the distal end of the tibia by a single condyle (Fig. 1e), but between the tarsomeres there is no articulation; they are connected by flexible membrane so that they are freely movable.
- Pretarsus – the pretarsus, in the majority of insects, consists of a membranous base supporting a median lobe, the arolium, which may be membranous or partly sclerotized, and a pair of claws which articulate with a median process of the last tarsomere known as the unguifer. Ventrally there is a basal sclerotized plate, the unguitractor, and between this and the claws are small plates called auxiliae (Fig. 8.4a). The development of the claws varies in different insect groups. Commonly they are more or less equally well-developed, while in other groups, the claws develop unequally.
All legs are equipped with an extensive arrangement of sensory structures that allow the insect to feel, hear, and taste, providing the insect with its initial assessment of the environment. Chemoreceptors, which are especially prevalent on the basitarsus and eutarsus, provide sensory input on environmental substances and can be used to determine the acceptability of food, ovipositional substrates, and perhaps mates. Mechanoreceptors, most often in the form of hair organs, but also campaniform, chordotonal, and plate organs, provide sensory information on position, movement, and vibrations borne by air and substrate.
Fig. 4. Adaptations of legs. (a) Digging. Foreleg of a larval cicada (b) Grasping. Leg of Haematopinus (Phthiraptera). (c) Grooming. Foreleg of a mutillid (Hymenoptera). (d) Hind tibia and tarsus of a worker honeybee showing the pollen-collecting apparatus.
MODIFICATIONS OF THE BASIC LEG STRUCTURE
The majority of insectan legs are either elongate, slender, and designed for walking and climbing. Various modifications allow the legs to be used in other forms of locomotion and non-locomotory functions. Amongst these are:
- Cursorial, i.e., adapted for running, as in the cockroach. During walking, the legs form alternating triangles of support, with the fore and hind legs of one side and the middle leg of the opposite side contacting the substrate as the other three legs move forward.
- Enlarged hind legs (metathoracic legs) of many Orthoptera, fleas, and other insects are saltatorial, meaning they are designed for jumping. The jump of insects such as fleas is aided by a rubber-like protein called resilin in the cuticle that stores and subsequently releases energy for the jump.
- Fossorial, digging legs, are best known in the Scarabaeoidea and the mole cricket, Gryllotalpa. Powerful, spade-like forelegs (prothoracic legs) of mole crickets, scarab beetles, burrowing mayflies, and other insects are or adapted for digging and rapid burrowing (Fig. 4a). In Gryllotalpa, the forelimb is very short and broad, the tibia and tarsomeres bearing stout lobes which are used in excavation. In the scarab beetles, the femora are short, the tibiae are again strong and toothed, but the tarsi are often weakly developed. Larval cicadas are also burrowing insects. They have large, toothed fore femora, the principal digging organs, and strong tibiae which may serve to loosen the soil (Fig. 4a). The tarsus is inserted dorsally on the tibia and can fold back. In the first stage larva it is three-segmented, but it becomes reduced in later instars and may disappear completely.
- Grasping The ability to hold on is important in ectoparasitic insects. These usually have welldeveloped claws and the legs are frequently stout and short as in Hippoboscidae, Ischnocera
and Anoplura. In the latter two groups, the tarsi are only one or two segmented and often there is only a single claw which folds against a projection of the tibia to form a grasping organ (Fig. 4b). - Flattened, fringed legs of aquatic insects such as dytiscid and gyrinid beetles and notonectid backswimmers serve as oars for paddling or swimming (natatorial legs), while long legs with hydrophobic tarsal hairs and ante-apical claws, as seen in water striders (Gerridae), are for skating on the surface of water.
- Raptorial legs, i.e., those designed to seize prey, have arisen independently in many insectan lineages. Either of the three sets of thoracic legs can be raptorial, but the trait is probably most often expressed in the forelegs (e.g., in Mantodea and Reduviidae) and less frequently in the middle legs (e.g., in some Empididae) and hind legs (e.g., in some Mecoptera).
- Grooming Insects commonly use the legs or mandibles to groom parts of the body, removing particles of detritus in the process. The eyes and antennae are often groomed, and so are the wings. In Apis and other Hymenoptera there is a basal notch in the basitarsus lined with spinelike hairs forming a comb. Here, the metathoracic legs are modified. A flattened spur extends down from the tip of the tibia in such a way that when the metatarsus is flexed against the tibia the spur closes off the notch to form a complete ring (Fig. 4c). This ring is used to clean the antenna. First it is closed round the base of the flagellum and then the antenna is drawn through it so that the comb cleans the outer surface and the spines on the spur scrape the inner surface.
- Pollen collection, The hind legs of Apoidea are modified to collect pollen from the hairs of the body and accumulate it in the pollen basket. Pollen collecting is facilitated by pectinate hairs which are characteristic of the Apoidea. In the honeybee, Apis, pollen collected on the head region is brushed off with the forelegs and moistened with regurgitated nectar before being passed back to the hind legs which also collect pollen from the abdomen using the comb on the basitarsus (Fig. 4d). The pollen on the combs of one side is then removed by the rake of the opposite hind leg and collects in the pollen press between the tibia and basitarsus. By closure of the press, pollen is forced outwards and upwards on to the outside of the tibia and is held in place by the hairs of the pollen basket. On returning to the nest, the pollen is kicked off into a cell by the middle legs.
Silk production Insects in the order Embioptera are unique in having silk glands in the basitarsus of the front legs in all stages of development of both sexes. The basal tarsomere is greatly swollen, and within it are numerous silk glands each with a single layer of cells surrounding a reservoir (Fig. 5). There may be as many as 200 glands within the tarsomere, each connected by a duct to its own seta with a pore at the tip through which the silk is extruded.
Fig. 5. Silk production in the foreleg of an embiid. (a) Basitarsus seen in transparency to show the silk glands. (b) A single silk gland showing its connection to a seta.
Other forms of modifications
Legs also play an important defensive role, not only in permitting escape by running, jumping, burrowing, and swimming, but also in ways such as kicking and slashing. The spines on the legs of many insects, when used in defense, effectively deter predators and competitors and can inflict considerable damage. Insects such as stink bugs and treehoppers deliver powerful kicks at parasitoids and predators that attempt to attack their young.
Autotomy, or the loss of legs at predetermined points of weakness, often at the level of the trochanter, occurs in insects such as crane flies, leaving a predator with only a leg in its clutches as the insect escapes. Legs, whether lost through autotomy or accident, often can be regenerated to various degrees if one or more molts follow the amputation.
In many insects, the legs are used in sound production. Familiar examples include the shorthorned grasshoppers, which have a stridulatory mechanism on the hind femur, involving a series of pegs— the scraper—that is rubbed across a ridged wing vein. Some larval hydropsychid caddisflies have a scraper on the prothoracic femur that is rubbed against a file on the venter of the head. Legs also can be used to produce sound for intraspecific communication by drumming them against a substrate, as in some Orthoptera.
Insects such as Chironomidae seem to use the legs much as a second set of antennae. In Protura, which lack antennae, the forelegs probably have assumed an antennal (i.e., sensory) function.