Zygaena larvae may look cute, but do not be fooled by their harmless appearance. Poisonous defence mechanisms are only skin deep. Or cuticle deep in this case. After having their fingers “glued” a few times when collecting these larvae in the field, a team of researchers from the Max Planck Institute for Chemical Ecology in Germany decided to test just how well these defences work.
It turns out larvae of Zygaena filipendulae (Lepidoptera) have many little pockets along their cuticle filled with an extremely viscous and toxic fluid which, when deployed at the right time, can glue the mandibles and legs of any attackers that dare to come close. “Each time you touch a larva, for example when collecting them in the field, they release droplets from segmentally arranged cuticular cavities. Droplets glue your fingers, then harden and finally you can feel the crystalline-like precipitates. These defence droplets are a really special ‘substance’", says Stefan Pentzold, first author in a recent study in Scientific Reports looking at Zygaena’s types of ammunition.
The secret behind these droplets’ stickiness is a complex solution rich in glycine peptides, alongside significant amounts of glucose and proteins. “These characteristics” explains Pentzold, “seem to decrease the surface tension and allow the droplets to wet and glue the hydrophobic structures of predatory arthropods”.
As if having their legs and mandibles “tied up” wasn’t enough, within minutes of touching the larvae, the droplets have hardened. Almost like sandpaper, the crystals can be incredibly abrasive to the predators, and may cause serious damage to their bodies. The result may be bad news for the attacker, but the explanation is actually very simple: “Hardening of the droplets was mainly due to water evaporation and formation of antiparallel β-sheets of the peptide oligomers”, says Pentzold.
The type of proteins present in the droplets has also not been left to chance, with an eerie feeling it was designed to inflict the most harm and destruction onto potential attackers. The list includes defensive protease inhibitors, proteases, oxidases and hydrolases. For example, if a predator tries to bite and accidentally ingests the droplets, hydrolases and oxidases will start to degrade any food in its gut which can affect growth and development. To complete this gory picture, proteases will slowly start to digest the predators’ body, while protease inhibitors keep the larvae safe and protected.
Digging a little deeper, the team also found β-cyanoalanine in the droplets. In addition to being a potent neurotoxin in its own right, this compound can easily be converted to hydrogen cyanide (HCN) in the presence of a specific β-glucosidase. However, the only β-glucosidase spotted in the droplets was inadequate to make this conversion.
At first glance, it seems Zygaena larvae missed a trick by not using HCN as one of the weapons in their defence arsenal. However, it turns out these “sneaky” larvae have one last trick up their sleeve: they can make sure they become their attackers’ final meal as HCN is produced when the sticky droplets present on the outside mix with the right β-glucosidases in the Zygaena haemolymph (present inside the body).
This actually came somewhat as a surprise for the researchers. “We hypothesised that the HCN is generated in the droplets by the action of a β-glucosidase towards cyanogenic glucosides (linamarin, lotaustralin), activated upon droplet release”, explains Pentzold. However, “characterisation of the β-glucosidase gene showed that the enzyme was surprisingly not capable of hydrolysing linamarin and lotaustralin”. This HCN-based defence mechanism occurs only, adds Pentzold, “in case of severe damage and tissue rupture, mainly caused by larger predators, and consequently only when droplets and exuding haemolymph mix onto the cuticle”. In other words, HCN can only be generated when the larva is actually being chewed alive by an (unlucky) attacker.
With a particular interest in cyanogenic glucosides as defence compounds in Zygaena, Pentzold is planning to delve further into the gene controlling expression of the cyanogenic β-glucosidase in the haemolymph and to characterise this enzyme. “The experiments are currently under way”, concludes the researcher.
Photo: Harald Süpfle/CC-CY-SA-2.5