Attachment forces of the hemelytra-locking mechanisms in aquatic bugs (Heteroptera: Belostomatidae)

Abstract

Combined hemelytra-locking system of Heteroptera, consisting of several locking mechanisms, aids the mechanical stabilisation of the body at rest, resists external loads, and keeps air stored with the option to easily unlock hemelytra prior to flight. The resistance to unlocking of the hemelytron was measured (in mN) with the aid of a load cell force transducer combined with a three-axial micromanipulator. It is shown that macro- and microstructural features of several submechanisms are responsible for their directionality. The highest resistance to unlocking was measured in lateral and dorsal directions. Summarised force of separately measured submechanisms was considerably lower than the force measured in the combined mechanism. Each submechanism is optimised for achieving high resistance to the hemelytron uncoupling in particular direction(s) and to be easily unlocked in another direction. It was demonstrated in the high-speed videorecordings that hemelytra uncoupling is promoted by their short anterior displacement.

Introduction

There are two functionally different groups of wing-locking mechanisms in Heteroptera: the first one serves for interlocking hindwings to hemelytra in flight and the second one for coupling both wing pairs to the body in the resting position. The first mechanism is responsible for the functional diptery, whereas the second one provides an additional mechanical stability to the body keeping hindwings dry and clean, and also preventing water loss.

Because of the importance of the above mentioned functions, an ability to lock their wings to the body developed convergently in diverse insect groups (Gorb, 2001). Such wing-to-body locking devices have different designs and working principles in various insect groups. Many species can fix their wings to the thorax by the use of co-opted fields of cuticular outgrowths, such as spines, chetae, acanthae, microtrichia (Richards and Richards, 1979), located on the functionally corresponding surfaces of the thorax and wings (Richards, 1951). In Dermaptera, corresponding structures of two medio-dorsal thoracic fields and medial folds at the margins of covering wings consist of setae (Haas, 1995). Thoracic and wing fields may also consist of microtrichia, as those in representatives of Symphyta (Hymenoptera) (Schrott, 1986) and Chironomidae (Diptera) (Rodova, 1980). Wing-locking system based on microtrichia fields, also occurs in beetles (Coleoptera) (Baehr, 1980, Hammond, 1989, Samuelson, 1994, Samuelson, 1996). The beetle system is probably the most complex one regarding the shape of co-opted macrostructures, the number and directionality of fields of microtrichia involved (Gorb, 1998, Gorb, 1999, Haas and Beutel, 2001).

In aquatic Heteroptera, wings have an additional function: the space under the wings is used for air storage (Schuh and Slater, 1995). The hemelytra-locking mechanism takes part in sealing the space under the wings thus preventing air loss (Parsons, 1972). The mechanism can be divided into two types: those locking both hemelytra with each other and those locking the hemelytra to the body. Hemelytron-to-hemelytron locking mechanisms are (1) brush-to-brush system and (2) clavus–clavus clamp. Hemelytra-to-body locking mechanisms are (3) subcostal border of the hemelytra—lateral mesepimeron, (4) knob-and-socket system, (5) scutellum groove—clavus (Gorb and Perez Goodwyn, 2003).

The wing-locking mechanisms have been morphologically described (Poisson, 1924, Poisson, 1951, Cobben, 1957, Gorb and Perez Goodwyn, 2003), but the mechanical properties and functional significance of the single submechanisms have not been analysed previously. The objective of the present study was to measure the unlocking force of each wing-locking mechanism and that of the complete system of several locking mechanisms working together in aquatic bugs from the family Belostomatidae, in order to understand functional significance of morphological structures at the local and on global scales. We asked the following questions: Are there differences in the resistance among different unlocking directions? Is locking easier than unlocking? Do different locking mechanisms use different working principles? Is there combined working between mechanisms?