Adult Specialization

It is impossible to argue for or against the proposition that possession of elytra has helped beetles' evolutionary success because possession of elytra is a defining character of "beetle-

FIGURES 5-6 Protopaussuspristinus (Carabidae), described from Dominican amber. (5) Reconstruction of adult, dorsal view. (6) Distribution of Protopaussus: F, P. pristinus fossil; • localities of extant Protopaussus. [From Nagel, P. (1997) New fossil paussids from Dominican amber with notes on the phylogenetic systematics of the paussine complex. Syst. Entomol. 22, 345-362. © Blackwell Science Ltd.]

FIGURES 5-6 Protopaussuspristinus (Carabidae), described from Dominican amber. (5) Reconstruction of adult, dorsal view. (6) Distribution of Protopaussus: F, P. pristinus fossil; • localities of extant Protopaussus. [From Nagel, P. (1997) New fossil paussids from Dominican amber with notes on the phylogenetic systematics of the paussine complex. Syst. Entomol. 22, 345-362. © Blackwell Science Ltd.]

ness," and beetle families vary so dramatically in their diversity. For example, one of the earliest evolving beetle groups, the Cupedidae, is currently represented by only 26 species worldwide. Possession of elytra thus is only one step among many leading to successful diversification of Coleoptera. Nonetheless, functional study of the beetle body plan illustrates many instances in which "beetleness" has predisposed lineages to enter and proliferate in particular habitats.

Most generally, the organization of beetle bodies that has permitted entry into confining, laminar microhabitats involves (1) thick hard cuticle on the head and prothorax, (2) a prothorax flexibly articulating with the pterothorax, and (3) a pterothorax topped by elytra that cover folded flight wings and soft, expansible abdominal tergites. Carabid beetles utilize wedge-pushing locomotion to move through leaf litter and under loose tree bark. In these beetles, a rounded projection on the base of the hind femur impinges on the meta-trochanter, which articulates only in a horizontal plane with the immobile hind coxa. Pulling the hind leg forward pushes the apex of the femur away from the body, thereby elevating the carabid's dorsum (the wedge). This upward motion is then followed by a thrust of the hind legs, forcing the beetle body forward (the push). Using this mechanism, rhysodine carabid beetles can move through dying or even living wood without leaving a trace; the wood simply closes up behind them! Their goal in this unlikely activity is foraging on the amoeboid plasmodia of slime molds (Myxomycetes).

For beetles to both fly and move through confining spaces, their wings must be stowed under the elytra while walking or wedge pushing, yet quickly unfurled for flight. All wing folding is controlled through muscles attached to the wing base; as long as tension is applied so that the radial and cubital veins are pulled apart, the wing surface remains flattened. However, relaxation of this tension brings natural folds into play, so that, with the wing apex folding in upon itself (Fig. 7), the medius comes to lie above the radius posterior (Fig. 8).

Numerous variations on wing folding have evolved depending on elytral configuration. In the archostematan Cupedidae, the wing apex rolls up longitudinally (Fig. 9). Wing folding can proceed even given the evolutionary reduction of wing venation observed in tiny beetles such as Microsporidae or Ptiliidae (Figs. 10-11). In addition to the folding characteristics of the wings, setose binding patches occurring on the wing surface, inner elytral surface, and abdominal terga are used to manage the wing folding, thus ensuring safe stowage of the wing membranes (Fig. 12).

The generalized thickening of cuticle characteristic of archostematans, adephagans, and many polyphagans results in an adult insect that is highly constrained in internal volume. Abdominal sutures between ventrites allow these segments to move against one another so that well-fed or gravid individuals will exhibit an abdomen extending beyond the elytral apex. Nonetheless, longitudinal abdominal extension can be minimized and external structural integrity maintained if the body is allowed to expand in another

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