During the lifetime of an insect, many parts of its exoskeleton are subject to external stresses and mechanical impacts, eventually causing small cracks by fatigue, wear and tear. If not repaired or at least stopped, cracks can eventually grow bigger, ultimately reducing the exoskeleton’s biomechanical function and as a consequence the insect’s fitness.
One part of the insect body which needs to resist repeated high mechanical stresses are the wings; in particular those of long-distance flying insects such as the desert locust Schistocerca gregaria.
In their migratory stage, these insects can fly for days over several thousand kilometres in search of new habitats 
. During this time their wings are subject to deformation, torsion and bending for millions of cycles. How do locust hind wings cope with damage, often resulting from interactions (antagonistic or sexual), collisions or fatigue 
In most biological materials small defects due to wear and tear are inevitable and, rather than trying to prevent cracks, many organisms have adapted to either repair or withstand small defects in their structural tissues, such as plant stems, bone and skin 
. However, due to the histological structure and morphogenetic development of the wing membrane as part of the locust’s exoskeleton, the repair of cracks is not possible 
. This leaves only the option to minimize the effect of small defects by stopping them as soon as possible. How do locust wings prevent small defects from growing into large cracks?
So far the only indication of a crack inhibiting morphological adaptation in locust wings has been proposed by Wootton et al.
. A small crimped band around the edge of the wing could help to distribute stress, thus possibly preventing tearing of the membrane 
. However, this mechanism would only reduce the effect of tearing from the edge, and would be of no avail for defects starting within the wing. To prevent cracks from growing inside the wing, one can thus imagine two possible toughening mechanisms: i) a very tough membrane materials and/or ii) the wing veins as crack-inhibiting barriers.
Properties of the Wing Membrane
A very tough wing membrane cuticle could minimize the risk of cracks developing in the first place and inhibit their propagation through the material. Recently we were able to show that the toughness of the locust hind leg cuticle is amongst the highest of any biological composite material, minimizing the risk of cracks developing during the jump. Is the cuticle of the locust wing membrane particularly tough, too?
In a previous study Wootton et al.
have characterised the histology, morphology and stiffness of the S. gregaria
wing membrane in great detail 
. It is extremely thin (1.7 to 3.7 µm) and in the main consists of epicuticle only. This cuticle contains amorphous cross-linked proteins, no traceable amounts of chitin and only very little water. Tensile experiments with isolated sections of the locust wing membrane showed a mean isotropic stiffness of 9.89±3.47 GPa for the remigium and 3.70±2.71 GPa for the anal fan. However, there was no clear pattern in the distribution of stiffness along the wing, which lead subsequent studies to simplify the properties of the membrane and veins when modelling the wing 
. It was also shown that in respect to its biomechanical function, the locust wing membrane acts as a “stressed skin”, with the cells playing an important role in combining structural stiffness with flexibility during the wing movement. Nothing is known about the fracture toughness or strength of the locust wing membrane, or of any other insect wing membrane.
Properties of the Veins
A network of longitudinal veins and cross-sectional veins divides the wing-surface into characteristic numerous smaller membrane cells (see and 
). In locusts, the longitudinal veins are hollow cuticular tubes with a diameter of approx. 100 to 150 µm at the base, thinning towards the edge of the wing 
. Most of the longitudinal veins contain trachea, nerves and hemolymph 
. They branch distally and their beam-like structure provides stiffness and rigidity across the span of the wing and increases the resistance to torsion 
. In a previous study it was shown that the arrangement of the longitudinal veins within the anal fan very closely follows a truncated logarithmic spiral around the base of the wing 
. Using finite element modelling, it has been shown that this arrangement allows the hind wing to be mechanically unfolded like an umbrella during the downstroke 
. The cross-veins however have a relatively diverse shape and contain little or no hemolymph 
. In locusts, the typical annulated structure of the cross-veins is thought to increase the compliance of the cross-vein when in flexion (see and 
). Their role during insect flight has been far less studied than that of the longitudinal veins, however one of their main functions is believed to be to constrain the lateral buckling of the longitudinal veins and support the wing camber 
. There is no experimental data on the material stiffness, strength or fracture toughness of any of the veins 
Morphology of S. gregaria hind wings.
The aims of this study were to measure the fracture toughness of wing material and to investigate the role of the veins in preventing crack propagation.