The Thorax - Immature Stages - Insectomania (2024)

The thorax is composed of three segments: the first or prothorax, the second or mesothorax, and the third or metathorax. Primitively, and in apterygotes (bristletails and silverfish) and immature insects, these segments are similar in size and structural complexity. In most winged insects the mesothorax and metathorax are enlarged relative to the prothorax and form a pterothorax, bearing the wings and associated musculature. Wings occur only on the second and third segments in extant insects although some fossils have prothoracic winglets (Fig. 8.2) and homeotic mutants may develop prothoracic wings or wing buds. Almost all nymphal and adult insects have three pairs of thoracic legs - one pair per segment. Typically the legs are used for walking, although various other functions and associated modifications occur (section 2.4.1). Openings (spiracles) of the gas-exchange, or tracheal, system (section 3.5) are present laterally on the second and third thoracic segments at most with one pair per segment. However, a secondary condition in some

Fig. 2.18 Diagrammatic lateral view of a wing-bearing thoracic segment, showing the typical sclerites and their subdivisions. (After Snodgrass 1935.)

insects is for the mesothoracic spiracles to open on the prothorax.

The tergal plates of the thorax are simple structures in apterygotes and in many immature insects, but are variously modified in winged adults. Thoracic terga are called nota (singular: notum), to distinguish them from the abdominal terga. The pronotum of the prothorax may be simple in structure and small in comparison with the other nota, but in beetles, mantids, many bugs, and some Orthoptera the pronotum is expanded and in co*ckroaches it forms a shield that covers part of the head and mesothorax. The pterothoracic nota each have two main divisions - the anterior wing-bearing alinotum and the posterior phragma-bearing postno-tum (Fig. 2.18). Phragmata (singular: phragma) are plate-like apodemes that extend inwards below the antecostal sutures, marking the primary intersegmental folds between segments; phragmata provide

Fig. 2.19 The hind leg of a co*ckroach, Periplaneta americana (Blattodea: Blattidae), with enlargement of ventral surface of pretarsus and last tarsomere. (After Cornwell 1968; enlargement after Snodgrass 1935.)

attachment for the longitudinal flight muscles (Fig. 2.7d). Each alinotum (sometimes confusingly referred to as a "notum") may be traversed by sutures that mark the position of internal strengthening ridges and commonly divide the plate into three areas - the anterior prescutum, the scutum, and the smaller posterior scutellum.

The lateral pleural sclerites are believed to be derived from the subcoxal segment of the ancestral insect leg (Fig. 8.4a). These sclerites may be separate, as in silverfish, or fused into an almost continuous sclerotic area, as in most winged insects. In the pterothorax, the pleuron is divided into two main areas - the anterior episternum and the posterior epimeron - by an internal pleural ridge, which is visible externally as the pleural suture (Fig. 2.18); the ridge runs from the pleural coxal process (which articulates with the coxa) to the pleural wing process (which articulates with the wing), providing reinforcement for these articulation points. The epipleurites are small sclerites beneath the wing and consist of the basalaria anterior to the pleural wing process and the posterior sub-alaria, but often reduced to just one basalare and one subalare, which are attachment points for some direct flight muscles. The trochantin is the small sclerite anterior to the coxa.

The degree of ventral sclerotization on the thorax varies greatly in different insects. Sternal plates, if pre sent, are typically two per segment: the eusternum and the following intersegmental sclerite or interster-nite (Fig. 2.7c), commonly called the spinasternum (Fig. 2.18) because it usually has an internal apodeme called the spina (except for the metasternum which never has a spinasternum). The eusterna of the prothorax and mesothorax may fuse with the spinasterna of their segment. Each eusternum may be simple or divided into separate sclerites - typically the prester-num, basisternum, and sternellum. The eusternum may be fused laterally with one of the pleural sclerites and is then called the laterosternite. Fusion of the sternal and pleural plates may form precoxal and postcoxal bridges (Fig. 2.18).

2.4.1 Legs

In most adult and nymphal insects, segmented fore, mid, and hind legs occur on the prothorax, mesotho-rax, and metathorax, respectively. Typically, each leg has six segments (Fig. 2.19) and these are, from proximal to distal: coxa, trochanter, femur, tibia, tarsus, and pretarsus (or more correctly post-tarsus) with claws. Additional segments - the prefemur, patella, and basitarsus (Fig. 8.4a) - are recognized in some fossil insects and other arthropods, such as arachnids, and one or more of these segments are evident in some

Ephemeroptera and Odonata. Primitively, two further segments lie proximal to the coxa and in extant insects one of these, the epicoxa, is associated with the wing articulation, or tergum, and the other, the subcoxa, with the pleuron (Fig. 8.4a).

The tarsus is subdivided into five or fewer components, giving the impression of segmentation; but, because there is only one tarsal muscle, tarsomere is a more appropriate term for each "pseudosegment". The first tarsomere sometimes is called the basitarsus, but should not be confused with the segment called the basitarsus in certain fossil insects. The underside of the tarsomeres may have ventral pads, pulvilli, also called euplantulae, which assist in adhesion to surfaces. Terminally on the leg, the small pretarsus (enlargement in Fig. 2.19) bears a pair of lateral claws (also called ungues) and usually a median lobe, the arolium. In Diptera there may be a central spine-like or pad-like empodium (plural: empodia) which is not the same as the arolium, and a pair of lateral pulvilli (as shown for the bush fly, Musca vetustissima, depicted on the right side of the vignette of this chapter). These structures allow flies to walk on walls and ceilings. The pretarsus of Hemiptera may bear a variety of structures, some of which appear to be pulvilli, whereas others have been called empodia or arolia, but the hom*ologies are uncertain. In some beetles, such as Coccinellidae, Chrysomelidae, and Curculionidae, the ventral surface of some tarsomeres is clothed with adhesive setae that facilitate climbing. The left side of the vignette for this chapter shows the underside of the tarsus of the leaf beetle Rhyparida (Chrysomelidae).

Generally the femur and tibia are the longest leg segments but variations in the lengths and robustness of each segment relate to their functions. For example, walking (gressorial) and running (cursorial) insects usually have well-developed femora and tibiae on all legs, whereas jumping (saltatorial) insects such as grasshoppers have disproportionately developed hind femora and tibiae. In aquatic beetles (Coleoptera) and bugs (Hemiptera), the tibiae and/or tarsi of one or more pairs of legs usually are modified for swimming (natatorial) with fringes of long, slender hairs. Many ground-dwelling insects, such as mole crickets (Ortho-ptera: Gryllotalpidae), nymphal cicadas (Hemiptera: Cicadidae), and scarab beetles (Scarabaeidae), have the tibiae of the fore legs enlarged and modified for digging (fossorial) (Fig. 9.2), whereas the fore legs of some predatory insects, such as mantispid lacewings (Neuroptera) and mantids (Mantodea), are specialized for seizing prey (raptorial) (Fig. 13.3). The tibia and basal tarsomere of each hind leg of honey bees are modified for the collection and carriage of pollen (Fig. 12.4).

These "typical" thoracic legs are a distinctive feature of insects, whereas abdominal legs are confined to the immature stages of holometabolous insects. There have been conflicting views on whether (i) the legs on the immature thorax of the Holometabola are develop-mentally identical (serially hom*ologous) to those of the abdomen, and/or (ii) the thoracic legs of the holome-tabolous immature stages are hom*ologous with those of the adult. Detailed study of musculature and innervation shows similarity of development of thoracic legs throughout all stages of insects with ametaboly (without metamorphosis, as in silverfish) and hemimetaboly (partial metamorphosis and no pupal stage) and in adult Holometabola, having identical innervation through the lateral nerves. Moreover, the oldest known larva (from the Upper Carboniferous) has thoracic and abdominal legs/leglets each with a pair of claws, as in the legs of nymphs and adults. Although larval legs appear similar to those of adults and nymphs, the term prolegs is used for the larval leg. Prolegs on the abdomen, especially on caterpillars, usually are lobelike and each bears an apical circle or band of small sclerotized hooks, or crochets. The thoracic prolegs may possess the same number of segments as the adult leg, but the number is more often reduced, apparently through fusion. In other cases, the thoracic prolegs, like those of the abdomen, are unsegmented outgrowths of the body wall, often bearing apical hooks.

2.4.2 Wings

Wings are developed fully only in the adult, or exceptionally in the subimago, the penultimate stage of Ephemeroptera. Typically, functional wings are flaplike cuticular projections supported by tubular, scler-otized veins. The major veins are longitudinal, running from the wing base towards the tip, and are more concentrated at the anterior margin. Additional supporting cross-veins are transverse struts, which join the longitudinal veins to give a more complex structure. The major veins usually contain tracheae, blood vessels, and nerve fibers, with the intervening membranous areas comprising the closely appressed dorsal and ventral cuticular surfaces. Generally, the major veins are alternately "convex" and "concave" in relation to the surface plane of the wing, especially near the

Fig. 2.20 Nomenclature for the main areas, folds, and margins of a generalized insect wing.

wing attachment; this configuration is described by plus (+) and minus (-) signs. Most veins lie in an anterior area of the wing called the remigium (Fig. 2.20), which, powered by the thoracic flight muscles, is responsible for most of the movements of flight. The area of wing posterior to the remigium sometimes is called the clavus; but more often two areas are recognized: an anterior anal area (or vannus) and a posterior jugal area. Wing areas are delimited and subdivided by fold-lines, along which the wing can be folded; and flexion-lines, at which the wing flexes during flight. The fundamental distinction between these two types of lines is often blurred, as fold-lines may permit some flexion and vice versa. The claval furrow (a flexion-line) and the jugal fold (or fold-line) are nearly constant in position in different insect groups, but the median flexion-line and the anal (or vannal) fold (or fold-line) form variable and unsatisfactory area boundaries. Wing folding may be very complicated; transverse folding occurs in the hind wings of Coleoptera and Dermaptera, and in some insects the enlarged anal area may be folded like a fan.

The fore and hind wings of insects in many orders are coupled together, which improves the aerodynamic efficiency of flight. The commonest coupling mechanism (seen clearly in Hymenoptera and some Trichoptera) is a row of small hooks, or hamuli, along the anterior margin of the hind wing that engages a fold along the posterior margin of the fore wing (hamulate coupling).

In some other insects (e.g. Mecoptera, Lepidoptera, and some Trichoptera), a jugal lobe of the fore wing overlaps the anterior hind wing ( jugate coupling), or the margins of the fore and hind wing overlap broadly (amplexiform coupling), or one or more hind-wing bristles (the frenulum) hook under a retaining structure (the retinaculum) on the fore wing (frenate coupling). The mechanics of flight are described in section 3.1.4 and the evolution of wings is covered in section 8.4.

All winged insects share the same basic wing venation comprising eight veins, named from anterior to posterior of the wing as: precosta (PC), costa (C), subcosta (Sc), radius (R), media (M), cubitus (Cu), anal (A), and jugal (J). Primitively, each vein has an anterior convex (+) sector (a branch with all of its subdivisions) and a posterior concave (-) sector. In almost all extant insects, the precosta is fused with the costa and the jugal vein is rarely apparent. The wing nomenclatural system presented in Fig. 2.21 is that of Kukalova-Peck and is based on detailed comparative studies of fossil and living insects. This system can be applied to the venation of all insect orders, although as yet it has not been widely applied because the various schemes devised for each insect order have a long history of use and there is a reluctance to discard familiar systems. Thus in most textbooks, the same vein may be referred to by different names in different insect orders because the structural hom*ologies were not recognized

Fig. 2.21 A generalized wing of a neopteran insect (any living winged insect other than Ephemeroptera and Odonata), showing the articulation and the Kukalovä-Peck nomenclatural scheme of wing venation. Notation as follows: AA, anal anterior; AP, anal posterior; Ax, axillary sclerite; C, costa; CA, costa anterior; CP, costa posterior; CuA, cubitus anterior; CuP, cubitus posterior; hm, humeral vein; JA, jugal anterior; MA, media anterior; m-cu, cross-vein between medial and cubital areas; MP, media posterior; PC, precosta; R, radius; RA, radius anterior; r-m, cross-vein between radial and median areas; RP, radius posterior; ScA, subcosta anterior; ScP, subcosta posterior. Branches of the anterior and posterior sector of each vein are numbered, e.g. CuA^^. (After CSIRO 1991.)

Fig. 2.21 A generalized wing of a neopteran insect (any living winged insect other than Ephemeroptera and Odonata), showing the articulation and the Kukalovä-Peck nomenclatural scheme of wing venation. Notation as follows: AA, anal anterior; AP, anal posterior; Ax, axillary sclerite; C, costa; CA, costa anterior; CP, costa posterior; CuA, cubitus anterior; CuP, cubitus posterior; hm, humeral vein; JA, jugal anterior; MA, media anterior; m-cu, cross-vein between medial and cubital areas; MP, media posterior; PC, precosta; R, radius; RA, radius anterior; r-m, cross-vein between radial and median areas; RP, radius posterior; ScA, subcosta anterior; ScP, subcosta posterior. Branches of the anterior and posterior sector of each vein are numbered, e.g. CuA^^. (After CSIRO 1991.)

correctly in early studies. For example, until 1991, the venational scheme for Coleoptera labeled the radius posterior (RP) as the media (M) and the media posterior (MP) as the cubitus (Cu). Correct interpretation of venational hom*ologies is essential for phylogenetic studies and the establishment of a single, universally applied scheme is essential.

Cells are areas of the wing delimited by veins and may be open (extending to the wing margin) or closed (surrounded by veins). They are named usually according to the longitudinal veins or vein branches that they lie behind, except that certain cells are known by special names, such as the discal cell in Lepidoptera (Fig. 2.22 a) and the triangle in Odonata (Fig. 2.22b). The pterostigma is an opaque or pigmented spot anteriorly near the apex of the wing (Figs. 2.20 & 2.22 b).

Wing venation patterns are consistent within groups (especially families and orders) but often differ between groups and, together with folds or pleats, provide major features used in insect classification and identification. Relative to the basic scheme outlined above, venation may be greatly reduced by loss or postulated fusion of veins, or increased in complexity by numerous cross-veins or substantial terminal branching. Other features that may be diagnostic of the wings of different insect groups are pigment patterns and colors, hairs, and scales. Scales occur on the wings of Lepidoptera, many Trichoptera, and a few psocids (Psocoptera) and flies. Hairs consist of small microtrichia, either scattered or grouped, and larger macrotrichia, typically on the veins.

Usually two pairs of functional wings lie dorsolater-ally as fore wings on the mesothorax and as hind wings on the metathorax; typically the wings are membranous and transparent. However, from this basic pattern are derived many other conditions, often involving variation in the relative size, shape, and degree of sclerotization of the fore and hind wings. Examples of fore-wing modification include the

Fig. 2.22 The left wings of a range of insects showing some of the major wing modifications: (a) fore wing of a butterfly of Danaus (Lepidoptera: Nymphalidae); (b) fore wing of a dragonfly of Urothemis (Odonata: Anisoptera: Libellulidae); (c) fore wing or tegmen of a co*ckroach of Periplaneta (Blattodea: Blattidae); (d) fore wing or elytron of a beetle of Anomala (Coleoptera: Scarabaeidae); (e) fore wing or hemelytron of a mirid bug (Hemiptera: Heteroptera: Miridae) showing three wing areas - the membrane, corium, and clavus; (f ) fore wing and haltere of a fly of Bibio (Diptera: Bibionidae). Nomenclatural scheme of venation consistent with that depicted in Fig. 2.21; that of (b) after J.W.H. Trueman, unpublished. ((a-d) After Youdeowei 1977; (f) after McAlpine 1981.)

Fig. 2.22 The left wings of a range of insects showing some of the major wing modifications: (a) fore wing of a butterfly of Danaus (Lepidoptera: Nymphalidae); (b) fore wing of a dragonfly of Urothemis (Odonata: Anisoptera: Libellulidae); (c) fore wing or tegmen of a co*ckroach of Periplaneta (Blattodea: Blattidae); (d) fore wing or elytron of a beetle of Anomala (Coleoptera: Scarabaeidae); (e) fore wing or hemelytron of a mirid bug (Hemiptera: Heteroptera: Miridae) showing three wing areas - the membrane, corium, and clavus; (f ) fore wing and haltere of a fly of Bibio (Diptera: Bibionidae). Nomenclatural scheme of venation consistent with that depicted in Fig. 2.21; that of (b) after J.W.H. Trueman, unpublished. ((a-d) After Youdeowei 1977; (f) after McAlpine 1981.)

thickened, leathery fore wings of Blattodea, Dermaptera, and Orthoptera, which are called tegmina (singular: tegmen; Fig. 2.22c), the hardened fore wings of Coleoptera that form protective wing cases or elytra (singular: elytron; Fig. 2.22d & Plate 1.2), and the hemelytra (singular: hemelytron) of heteropteran Hemiptera with the basal part thickened and the apical part membranous (Fig. 2.22e). Typically, the hetero-pteran hemelytron is divided into three wing areas: the membrane, corium, and clavus. Sometimes the corium is divided further, with the embolium anterior to R + M, and the cuneus distal to a costal fracture. In Diptera the hind wings are modified as stabilizers (halteres) (Fig. 2.22f) and do not function as wings, whereas in male Strepsiptera the fore wings form halteres and the hind wings are used in flight (Box 13.6). In male scale insects (see Plate 2.5, facing p. 14) the fore wings have highly reduced venation and the hind wings form hamulohalteres (different in structure to the halteres) or are lost completely.

Small insects confront different aerodynamic challenges compared with larger insects and their wing area often is expanded to aid wind dispersal. Thrips (Thysanoptera), for example, have very slender wings but have a fringe of long setae or cilia to extend the wing area (Box 11.7). In termites (Isoptera) and ants (Hymenoptera: Formicidae) the winged reproductives, or alates, have large deciduous wings that are shed after the nuptial flight. Some insects are wingless, or apterous, either primitively as in silverfish (Zygentoma) and bristletails (Archaeognatha), which diverged from other insect lineages prior to the origin of wings, or secondarily as in all lice (Phthiraptera) and fleas (Siphonaptera), which evolved from winged ancestors. Secondary partial wing reduction occurs in a number of short-winged, or brachypterous, insects.

In all winged insects (Pterygota), a triangular area at the wing base, the axillary area (Fig. 2.20), contains the movable articular sclerites via which the wing articulates on the thorax. These sclerites are derived, by reduction and fusion, from a band of articular sclerites in the ancestral wing. Three different types of wing articulation among living Pterygota result from unique patterns of fusion and reduction of the articular scler-ites. In Neoptera (all living winged insects except the Ephemeroptera and Odonata), the articular sclerites consist of the humeral plate, the tegula, and usually three, rarely four, axillary sclerites (1Ax, 2Ax, 3Ax, and 4Ax) (Fig. 2.21). The Ephemeroptera and Odonata each has a different configuration of these sclerites compared with the Neoptera (literally meaning "new wing"). Odonate and ephemeropteran adults cannot fold their wings back along the abdomen as can neopterans. In Neoptera, the wing articulates via the articular sclerites with the anterior and posterior wing processes dorsally, and ventrally with the pleural wing processes and two small pleural sclerites (the basalare and subalare) (Fig. 2.18).

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