Silk may have been a staple of human haute couture for thousands of years, but the gossamer thread is more than just a fashion accessory for the moths that spin it. The robust cocoons constructed from these prized fibres provide a durable refuge for the moth pupa developing within, come rain, wind, snow or shine. While many silk moths specialise in one densely woven design of cocoon, robin moth (Hyalophora cecropia) caterpillars produce double-skinned cocoons in one of two designs – a looser baggier structure and a more fitted compact form – which intrigued Patrick Guerra from the University of Cincinnati, USA. Knowing that some of the pupae endure harsh icy conditions while others experience less-severe winters, Guerra wondered whether the caterpillars might make different cocoons to match where they'll spend the winter: a waterproof design better suited to freezing conditions or one that absorbs water to keep the pupa within well hydrated.
First, Guerra and postdoc Adam Parlin needed to discover whether the caterpillars oriented their cocoons in particular directions depending on their design. ‘Cocoons can be hard to spot since they blend rather well with their environment. The best time to go hunting is during the late fall or early spring when they are less hidden by dense foliage’, says Guerra. However, after comparing the cocoons gathered by colleagues in Massachusetts and New York state and online images of the cocoons, they found no difference in the orientation of the looser baggy cocoons and the more compact structures. They all tended to be positioned with the exit valve – through which the adult insects emerge – angled upward. Then, Parlin simulated a 5 min downpour over each cocoon – positioned vertically, horizontally or at a 45 deg angle – and discovered that the compact cocoons barely absorbed any water, even when horizontal, while the baggy cocoons soaked it up, absorbing up to 5 g when lying flat, but only gaining 2 g when pointing upward.
Next, Parlin scanned the outer and inner layers of both types of cocoon, discovering that the baggy cocoon's outer layer was the finest (0.09 mm), in contrast to its inner layer and both layers of the compact cocoon, which were 0.2 mm thick. In addition, the cavity separating the baggy cocoon's layers was almost 3.5 times larger (45 cm3) than that of the more compact cocoons (13 cm3) and the external area of the baggy cocoon was approximately twice (99.6 cm2) that of the compact cocoons (49.6 cm2). Then, the duo tested the water repellence of each layer of the cocoon, by dripping water onto the surface held at different angles and measuring how proud the droplets were from the surface over time. The outer layer of the compact cocoons was more water repellent than that of the baggy cocoons, while the inner layer of both cocoons was equally as waterproof as the water-resistant compact cocoon's external layer. Hence, the pupae housed in the compact cocoons are more likely to remain dry, as water vapour is lost more easily through the outer layer of their cocoons than from the baggier cocoons.
The duo suspects that the baggy cocoons may be more advantageous in dry winters, allowing the cocoon to hold on to water, while the compact cocoons offer more protection in damp situations, saving the pupa from waterlogging. And they suggest that the caterpillars may almost flip a coin when deciding which cocoon to invest in, to hedge their bets and ensure that some of the developing pupae make it through to spring, regardless of the weather thrown their way.