In: R. Singer, (ed.), 1999. Encyclopedia of Paleontology, volume 1 (A-L), pp. 603-624. Fitzroy Dearborn, London. INSECTS AND OTHER HEXAPODS riexapods are a distinctive group of six-legged arthropods that Hexapod Structure possess tracheae (branching tubes) as breathing organs and bod- Hexapods primitively consist of probably 21 or 22 segments. These ies that are subdivided into an identifiable head, thorax, and segments are organized during early development into three body abdomen. They are differentiated from other arthropods by the regions, or tagmata: the head, responsible for sensory perception of presence on the head of a single pair of antennae, three pairs of the environment and procurement and ingestion of food; the tho- externally visible, mouthpart appendages, a three-segmented rax, bearing the organs of locomotion; and the abdomen, housing thorax, an abdomen of 12 or fewer evident segments, and an the reproductive organs and much of the digestive tract (Figure 1). elongate terminal filament originating from the last abdominal These regions have been externally altered in diverse ways, princi- segment. Hexapods inhabit virtually all terrestrial and freshwater pally by embryonic rearrangement of the fundamental exoskeletal environments and have been the most species-rich and ecologi- elements, the scierites, into a shifted, secondary segmenration cally wide-ranging group of macroscopic life since the spread of (Gillott 1995). Because hexapod internal anatomy is rarely pre- Carboniferous forests. The origin and early evolution of hexa- served as fossils, this discussion will emphasize external structure. pods is poorly understood, principally because of uncertain rela- tionships to other major clades of arthropods, and a poorly documented Devonian and Early Carboniferous record Head shrouded by the absence of forms connecting ancestral marine The hexapod head is a sclerotized, or stiffened, spheroidal capsule and subsequent terrestrial lineages. Traditionally thought to be whose anatomically ventral aspect houses the mouthparts (Figure 2). closely related to myriapods such as centipedes and millipedes The head capsule typically has an internal skeleton, the tentorium, (Snodgrass 1935; Boudreaux 1987), recent biomolecular studies possessing rigid processes that attach to numerous muscles involved indicate that hexapods are derived from an unspecified lineage of in movement of mouthparts and other structures. These tentorial crustaceans (Wägele 1993). This proposed placement reestab- processes originate on the head capsule at important anatomic land- lishes an older view of a close Crustacea + Hexapoda relationship marks•the anterior and posterior tentorial pits. In a generalized and the erection of a broader clade, the Mandibulata, perhaps mandibulate (chewing) insect such as the grasshopper, compound with inclusion of the Myriapoda as an early evolutionary lineage. eyes occur dorsolaterally on the top and to the side of the head, and Fossil evidence indicates that myriapods are an older mandibu- three ocelli face anteriorly and frontally, two between the compound late group whose similarities to hexapods may have arisen in par- eyes and a median ocellus located on an anterior triangular sclerite, allel by convergent evolution resulting from the similar the frons. The single pair of segmented sensory antennae are functional demands of a land existence. Nevertheless, the issue of inserted typically below the ocelli, adjacent to the compound eyes. hexapod origins remains unsettled; several basic features of the The area between the compound eye and the mouthparts is the head are shared by myriapods and hexapods, providing evidence gena. The head capsule is often divided by posterior, dorsal, and for descent from a non-crustacean ancestor. frontal sutures, which are lines of juncture between scierites. The 604 ENCYCLOPEDIA OF PALEONTOLOGY h nd wing Figure 1. Major structural divisions and landnnarks of a generalized female insect, the common locust. Key: S, sternum; T, tergum, Modified from Lawrence et al. (1991). Copyright © Commonwealth Scientific and Industrial Research Organization 1991. Used by per- mission of the American Publisher, Cornell University Press. medial epicraniaJ suture, which splits during molting, may bifurcate the head capsule. This structure is divided into proximal and a dis- anteriorly and terminate at the transverse clypeaJ suture. The grass- tal sclerites, the latter which supports two pairs of lobes•mesal hopper has mouthparts that are ventrally attached, whereas insects glossae and lateral paraglossae•and a pair of lateral, one- to four- such as cicadas have mouthparts angulated posteriorly; other insects segmented palps. such as ground beetles have mouthparts that are forwardly directed. Mouthparts are modified into major, diverse, multielement Hexapods have five major mouthpart regions, the posterior organs used for consuming fluid and particulate food (Snodgrass three of which correspond to primitive segment divisions 1935). Piercing-and-sucking mouthparts are formed by transfor- (Snodgrass 1935). The anterior-most labrum, or "upper lip" (Fig- mation of various combinations of mandibles, maxillary laciniae, ure 2a), is a medially located, movable, and enclosing flap that hypopharynx, and other mouthpart elements into stylets, often bears gustatory (taste) and other sensory receptors on its inner sur- housed in a closed sheath or an open channel. Feeding on surface face, the epipharynx. The hypopharynx (Figure 2d) is a typically fluids is accomplished by modification of mouthparts into various fleshy, medially positioned (mesal) lobe that divides the preoral mechanisms such as a siphon formed by conjoined galeae, or a cavity into a dorsal mouth and a ventral salivarium; it usually bears sponging organ resulting from expansion of the labium into a a complement of sclerites in primitive lineages. The mandibles are fleshy structure. Relatively minor modifications of individual paired, generally triangular, and highly sclerotized jaws that gener- mouthpart elements can result in pronounced mouthpart asym- ally contact the clypeus anteriorly by an articulatory hinge, and metry, co-optation of adjacent regions such as the labium and the gena posteriorly by an exoskeletal process (condyle) and associ- hypopharynx in the formation of silk-producing spinneret organs ated head cavity (Figure 2b, d). Posterior and ventral to the mandi- in certain larvae such as caterpillars, and transformation of mandi- bles are the paired maxillae that consist of medial and lateral bles and maxillary lobes into rakes and brushes as filtering or siev- elements (Figure 2b, c). A basal sclerite attaches each maxilla to a ing devices for consumption of water-suspended detritus. head cavity, whereas a longer, distal segment bears all the append- ages. These include a mesal, often spinose (spiny) or toothed lobe, the lacinia, and a generally fleshy lateral galea, in addition to a Thorax one- to seven-segmented palp. The labium, or "lower lip," is a The thorax consists of three segments: an anterior prothorax, mid- medial structure representing fusion of two maxilla-like structures dle mesothorax, and posterior metathorax. Each thoracic segment membranously connected to the posterior and ventral margin of is divided into a dorsal tergum, two lateral pleura, and a ventral INSECTS AND OTHER HEXAPODS 605 occipital sulcus hypopharnx anterior tenlorial pit Figure 2. Head and mouthpart structures of a generalized pterygotan insect, a, anterior; b, lateral; c, posterior; d, ventral. Modified from Gillott (1995). sternum, which respectively bear tergites, pleurites, and sternites. femut; a femur that is the most conspicuous part of the leg; a gen- These sclerites are arranged in a complex series of exoskeletal erally elongate tibia; a slender tarsus, which is subdivided into a plates of varying sizes and shapes, connected by intersegmental musculated basitarsus and two to four nonmusculated subseg- membranes (Figure 1). In winged insects thoracic segments are ments known as the eutarsus, and the pretarsus, which in most structured to meet the biomechanical forces induced by wings, forms bears one to three lobes and a pair of lateral claws. and the segments can accommodate a significant volume of flight Apterygote hexapods are defined as primitively lacking muscle. The anchoring and movement of legs also determine scler- wings, whereas modern pterygotes bear two pairs of lateral wings ite size, shape, and robustness. Although the archetypal hexapodan that articulate to the meso- and metathoracic segments by a com- leg is used for walking or climbing, specializations include jump- plex system of articular sclerites (Gillott 1995). The prothoracic ing, swimming, and impaling prey. The number of fundamental segment of some Paleozoic insects bore movable, lateral lobes with segments in the hexapod leg is debated, but it is likely that Paleo- veins that indicate a parallel developmental path (serial homology) zoic insects bore additional free segments that later became fused with functional wings. In other insects the prothorax elongated as in more modern formal taxonomic groups, or taxa (Kukalová-Peck a feeding specialization, or became spinose to deter prédation. 1991). Modern taxa have, from proximal to distal, the following Both pairs of wings are primitively similar in general form and segments: a coxa, the functional leg base articulating with the tho- construction, and they are fluted into a system of alternating con- racic