Insect cuticular hydrocarbon composition influences their interaction with spider capture threads

The sticky truth: how spider predation success depends on their prey's body surface

Spiders are prominent predators for insects, with which they have a close co-evolutionary history. Manifold capture techniques have evolved, with spider webs being one of most well-known traps in the world. Many webs include specialised threads, bearing either glue or cribellate nanofibres as adhesive to capture prey. Some webs, such as the sheet webs of Tarantulae, have no such intricate threads. The adhesion of gluey threads has been extensively studied already, but often on artificial surfaces. However, recent studies discovered that adhesion of cribellate nanofibres increases massively after contact with insect cuticular hydrocarbons (CHCs). This raises the question whether insect CHCs generally influence prey capture. We compared the adhesion of cribellate, ecribellate gluey and ecribellate non-specialised threads to either uncoated or CHC-coated foil, or native prey body surfaces. We found an influence of CHCs on all silken threads, but with different outcomes. CHC presence, its composition as well as the surface structure can impact the final adhesion force positively or negatively, depending on the thread type. In extreme cases, the adhesion was reduced to nearly zero (e.g. for gluey capture threads in contact with real prey). Thus, prey influence on adhesion is not limited to cribellate capture threads, but is a universal influence on adhesion of spider silken capture threads. Future studies should consider both insect surface chemistry and surface structure when assessing the effectiveness of capture thread types in an ecological and evolutionary context.

Lipid Metabolism in Diapause

Organisms living in temperate and polar environments encounter seasonal fluctuations that entail changes in temperature, resource availability, and biotic interactions. Thus, adaptations for synchronizing the life cycle with essential resources and persisting through unfavorable conditions are critical. Diapause, a programmed period of developmental arrest and metabolic depression, is widely used by insects to survive winter and synchronize the life cycle. In some cases, insects spend over half the year (or in some cases, multiple years) in a nonfeeding diapause state. Thus, diapause is energetically challenging, and insects accumulate surplus energy stores and/or suppress metabolism to make it through the winter. As the most energy-dense, and often most abundant, energy reserve in insects, lipids play a central role in diapause energetics. In this chapter, we provide an overview of lipid metabolism in the context of diapause. First, as this is the only chapter in this book that covers diapause, we present some of the general features of diapause. We then discuss the role of lipids as an essential energy store during diapause, focusing on patterns of lipid accumulation before diapause and patterns of utilization during diapause. In the next section, we outline some other roles of lipids during diapause in addition to their role as an energy store. Finally, we end the chapter by discussing the molecular regulation of lipid metabolism in diapause, which has received increased attention in recent years.

Physico-chemical properties of functionally adhesive spider silk nanofibres

Currently, synthetic fibre production focuses primarily on high performance materials. For high performance fibrous materials, such as silks, this involves interpreting the structure-function relationship and downsizing to a smaller scale to then harness those properties within synthetic products. Spiders create an array of fibres that range in size from the micrometre to nanometre scale. At about 20 nm diameter spider cribellate silk, the smallest of these silks, is too small to contain any of the typical secondary protein structures of other spider silks, let alone a hierarchical skin-core-type structure. Here, we performed a multitude of investigations to elucidate the structure of cribellate spider silk. These confirmed our hypothesis that, unlike all other types of spider silk, it has a disordered molecular structure. Alanine and glycine, the two amino acids predominantly found in other spider silks, were much less abundant and did not form the usual α-helices and β-sheet secondary structural arrangements. Correspondingly, we characterized the cribellate silk nanofibre to be very compliant. This characterization matches its function as a dry adhesive within the capture threads of cribellate spiders. Our results imply that at extremely small scales there may be a limit reached below which a silk will lose its structural, but not functional, integrity. Nano-sized fibres, such as cribellate silk, thus offer a new opportunity for inspiring the creation of novel scaled-down functional adhesives and nano meta-materials.

Change of mechanical characteristics in spider silk capture threads after contact with prey

Most spiders rely on specialized capture threads to subdue prey. Cribellate spiders use capture threads, whose adhesion is based on thousands of nanofibers instead of specialized glue. The nanofibers adhere due to van der Waals and hygroscopic forces, but the adhesion is strengthened by an interaction with the cuticular hydrocarbons (CHCs) covering almost all insects. The interaction between CHCs and cribellate threads becomes visible through migration of the CHCs into the thread even far beyond the point of contact. In this study, we were able to show that the migrated CHCs not only influence adhesion but also change the mechanical characteristics of the thread. While adhesion, extensibility and total energy decreased in threads treated with CHCs from different insects, we observed an increasing force required to break threads. Such mechanical changes could be beneficial for the spider: Upon the first impact of the insect in the web, it is important to absorb all the energy without breaking. Afterwards, a reduction in extensibility could cause the insect to stay closer to the web and thus become additionally entangled in neighboring threads. An increased tensile force would additionally ensure that for insects already in the web, it is even harder to free themselves. Taken together, all these changes make it unlikely that cribellate spiders reuse their capture threads, if not reacting rapidly and removing the prey insect before the CHCs can spread across the thread. STATEMENT OF SIGNIFICANCE: Cribellate spiders use capture threads that, unlike other spiders, consist of nanofibers and do not rely glue. Instead, prey adheres mainly because their surface compounds, so-called cuticular hydrocarbons (CHCs), interact with the thread, this way generating strong adhesion forces. Previous studies on biomechanics and adhesion of cribellate threads only dealt with artificial surfaces, neglecting any interaction with surface compounds. This study examines the dramatical mechanical changes of a cribellate thread after interaction with prey CHCs, showing modifications of the thread's extensibility, tensile force and total energy. Our results highlight the importance of studying mechanical properties of silk not only in an artificial context, but also in real life.