Functional Morphology of the Flight Apparatus of Ephemera vulgata (Ephemeroptera: Ephemeridae) and Implications for the Evolution of Flight in Insects

The mesothoracic flight apparatus of preserved and freshly killed specimens of Ephemera vulgata is examined in detail by means of functional morphological examinations. The basic drive mechanism of the forewing of Ephemera vulgata constitutes a four-bar kinematic chain, which encompasses the whole mesothorax. Based on this reassessment, the operating principles of the different subsystems of the flight apparatus of the mayfly were newly analysed. As in the Neoptera, the indirect dorsolongitudinal downstroke and dorsoventral upstroke muscles rotate the wing around an axis which runs through the main wing joint of the fulcrum. In contrast to the Neoptera, the wing is driven via the posterior notal wing process and fourth axillary. A strong direct subalar muscle is able to move the subalare (together with the pleural ridge) inwards and to affect the wingstroke via a bistable click mechanism in this unusual way. The axillary sclerites, two frontal first axillaries and one caudal fourth axillary, permit the radioanal plate to rotate forward–backward on the fulcrum. This motion system, which is superimposed on the kinematic chain mechanism, permits alterations of the wingstroke plane. A short muscle of the radioanal plate allows adjusting the passive pronation of the wing during the downstroke and achieving an increase of the aerodynamical angle of attack. The step-by-step derivability of basal flight mechanisms of the main groups of Pterygota indicates a paranotal origin of the wings. The possibility that the Ephemeroptera and Neoptera constitute sister groups is discussed.

MESOTHORACIC SKELETOMUSCULATURE AND MECHANICS OF FLIGHT AND JUMPING IN EUPELMINAE (HYMENOPTERA, CHALCIDOIDEA: EUPELMIDAE)

Mesothoracic skeletomusculature of male and female Eupelminae is described and compared with that of other Eupelmidae, Chalcidoidea, and Hymenoptera. Various external mesopleural features and structural dimorphism between the sexes are explained by differences in muscle form and placement. A set of terms for mesothoracic structure is proposed that is equally applicable to male and female eupelmines and to other chalcidoids. Mechanics of flight and jumping in male eupelmines, and of jumping in females, is also described. The flight mechanism of males is similar to that previously described in other hymenopterans and is structurally independent of the jumping mechanism. Contraction of large mesotergal-mesotrochanteral muscles, originating from the axillae and axillar phragmata, act directly to retract the mesotrochanters into the mesocoxae for jumping. Females have coadapted the flight and jumping mechanisms into a single mechanism to improve jumping greatly. The mesotergal-mesotrochanteral muscles are reduced to slender, tendon-like muscles originating from the anteroventral angle of each lateral axillar surface. Jumping in females results from contraction of large mesopleural-mesotergal muscles that insert into anterolateral processes of the mesoscutum by pads of resilin. The pads are stretched during contraction of the mesopleural-mesotergal muscles and the potential energy thus stored is subsequently released to flex the mesonotum along the transscutal articulation. The first and second axillary sclerites are modified to function as a hinge to control mesonotal flexing for jumping. Flexing the mesonotum rotates the lateral axillar surfaces anteriorly and dorsally, thereby pulling up on the mesotergal-mesotrochanteral muscles and changing a horizontally directed force into a vertical force that is used to retract the mesotrochanters for jumping. A mesothoracic lock mechanism to prevent initial mesonotal flexing is proposed, but is not documented. “Contortion” of female eupelmines is described, and is a consequence of the increased degree of mesonotal flexing required for their jumping mechanism. The modified mesocoxal articulation of females is hypothesized to function in rotating the middle legs cephalad to protect the head and antennae during landing. It is questioned whether female eupelmines can fly, and the adaptive significance of enhancement of jumping at the expense of flight in females, and of sexual dimorphism in the subfamily, is discussed.

How Diptera move their wings: a re-examination of the wing base articulation and muscle systems concerned with flight

The accepted mechanical model for dipteran wing movements is not compatible with the observations of unanaesthetized flies in tethered flight. In this paper a new model is proposed together with previously undescribed morphological features in support of it. A direct coupling is demonstrated between the indirect flight muscles, consisting of a wing base stop mechanism and a locking of the first axillary sclerite onto the parascutal shelf during the downstroke. We propose that wingbeat amplitude is controlled by axillary muscles which alter the downward force on the wing. We also describe the anatomical basis of the automatic changes in the wing stroke during the wingbeat. These are shown to be a direct consequence of the attachment between the fourth axillary sclerite and the scutellar lever arm. By virtue of their attachments to the sclerite, the fourth axillary muscles are shown to have potential control over the tonic characteristics of these automatic changes. The calypters are connected to the third and fourth axillary sclerites and this suggests an aerodynamic function for these structures which has not previously been proposed. A detailed analysis of the anatomy of the non-fibrillar muscles has allowed a complete functional characterization to be made which is consistent with our new model. The functions given to many of the muscles in this analysis are supported by physiological evidence from other studies. Muscular control of a switch mechanism comprising the radial stop and pleural wing process is discussed, as is the control of a wing base proprioceptor.

Wing Folding in Lepidoptera

The wing folding mechanism was investigated after a detailed study of the wing base morphology had been made (Sharplin, Canad. Ent. 95: 1024; 1121). Living moths were observed with a binocular microscope equipped with a micrometer eyepiece.The first and second axillary sclerites do not move anteroposteriorly; only the distal half of the wing base is involved in wing folding. The folding muscle originates on the pleural ridge and inserts on the third axillary sclerite. The movement of the third axillary is communicated to the bases of the anterior veins through the median plates. The radial plate rotates around the ventral second axillary sclerite which lies underneath the radial bridge at point p, (Fig. 1). Bending cuticle allows the radial bridge to buckle when the wing is folded. The first median plate ( Ml ) rotates about its articulation ( f ) with the dorsal second axillary sclerite. The distal median plate (M2) passes underneath the second cubitus and is fused to the radius. This connection to the radius restricts the backward movement of the second median plate so that point e instead of following the wider arc eg of a circle with its centre at f, must follow the arc cegd drawn about pivot p. The median plates are bent upwards during wing folding and their effective length is shortened so that they can follow the shallow arc epg. When point e is in position g the posterior margin of the median plates is straight, although the anterior margin remains arched causing the median plates to be buckled, (Fig. 2).