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

Abstract

AbstractMesothoracic 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.